Effects of Management Practices on Wetland Birds:

Black Tern

This report is one in a series of literature syntheses on North American grassland birds. The need for these reports was identified by the Prairie Pothole Joint Venture (PPJV), a part of the North American Waterfowl Management Plan. The PPJV adopted a goal, to stabilize or increase populations of declining grassland- and wetland-associated wildlife species in the Prairie Pothole Region. To further that objective, it is essential to understand the habitat needs of birds other than waterfowl, and how management practices affect their habitats. The focus of these reports is on management of breeding habitat, particularly in the northern Great Plains.

This resource is based on the following source:

This resource should be cited as:

Zimmerman, A. L., J. A. Dechant, D. H. Johnson, C. M. Goldade, B. E. Jamison, and B. R. Euliss. 2002. Effects of management practices on wetland birds: Black Tern. Northern Prairie Wildlife Research Center, Jamestown, ND. Northern Prairie Wildlife Research Center Online. http://www.npwrc.usgs.gov/resource/literatr/wetbird/blte/blte.htm (Version 30SEP2002).

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Effects of Management Practices on Wetland Birds:

Black Tern

Amy L. Zimmerman, Jill A. Dechant, Douglas H. Johnson,
Christopher M. Goldade, Brent E. Jamison, and Betty R. Euliss

Series Coordinator: Douglas H. Johnson
Series Assistant Coordinator: Jill A. Dechant

Reviewer: Erica H. Dunn

Range Map: Jeff T. Price

Illustration: Patsy Renz

Funding: Prairie Pothole Joint Venture
U.S. Fish and Wildlife Service
U.S. Geological Survey

Organization and Features of this Species Account

Information on the habitat requirements and effects of habitat management on wetland birds were summarized from information in more than 500 published and unpublished papers. A range map is provided to indicate the relative densities of the species in North America, based on Breeding Bird Survey (BBS) data. Although the BBS may not capture the presence of elusive waterbird species, the BBS is a standardized survey and the range maps, in many cases, represent the most consistent information available on species' distributions. Although birds frequently are observed outside the breeding range indicated, the maps are intended to show areas where managers might concentrate their attention. It may be ineffectual to manage habitat at a site for a species that rarely occurs in an area. The species account begins with a brief capsule statement, which provides the fundamental components or keys to management for the species. A section on breeding range outlines the current breeding distribution of the species in North America, including areas that could not be mapped using BBS data. The suitable habitat section describes the breeding habitat and occasionally microhabitat characteristics of the species, especially those habitats that occur in the Great Plains. Details on habitat and microhabitat requirements often provide clues to how a species will respond to a particular management practice. A table near the end of the account complements the section on suitable habitat, and lists the specific habitat characteristics for the species by individual studies. The area requirements section provides details on territory and home range sizes, minimum area requirements, and the effects of patch size, edges, and other landscape and habitat features on abundance and productivity. It may be futile to manage a small block of suitable habitat for a species that has minimum area requirements that are larger than the area being managed. The section on brood parasitism summarizes information on intra- and interspecific parasitism, host responses to parasitism, and factors that influence parasitism, such as nest concealment and host density. The impact of management depends, in part, upon a species' nesting phenology and biology. The section on breeding-season phenology and site fidelity includes details on spring arrival and fall departure for migratory populations in the Great Plains, peak breeding periods, the tendency to renest after nest failure or success, and the propensity to return to a previous breeding site. The duration and timing of breeding varies among regions and years. Species' response to management summarizes the current knowledge and major findings in the literature on the effects of different management practices on the species. The section on management recommendations complements the previous section and summarizes recommendations for habitat management provided in the literature. The literature cited contains references to published and unpublished literature on the management effects and habitat requirements of the species. This section is not meant to be a complete bibliography; a searchable, annotated bibliography of published and unpublished papers dealing with habitat needs of wetland birds and their responses to habitat management is posted on the main page under the section Searchable Bibliography.

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Keys to management are maintaining wetlands within large wetland complexes that contain nearly equal proportions of well-interspersed emergent vegetation and open water, maintaining stable water levels of >30 cm throughout the breeding season, and providing abundant nest substrates.

Breeding Range:

Suitable habitat:

Black Terns commonly occupy wetlands that are within wetland complexes (Powell 1991; Naugle 1997; Naugle et al. 1999a, 2000; D. H. Johnson, unpublished data). Presence of Black Terns in South Dakota was positively related to the area of semipermanent wetlands and grassland within a 25.9-km² block around individual wetlands (Naugle 1997; Naugle et al. 1999a, 2000, 2001). Similarly, preliminary data from D. H. Johnson (unpublished data) indicated that Black Terns in North Dakota and South Dakota were more common in wetlands that had semipermanent wetlands within 0.4 km than in wetlands without semipermanent wetlands nearby. In another South Dakota study, the presence of Black Tern was positively related to the area of surface water within the wetland, wetlands that contained central expanses of open water composing >5% of the wetland area and surrounded by a peripheral band of emergent vegetation cover averaging ≥1.8 m in width, vegetation height, shoreline distance, and the presence of semipermanent wetlands within a 400-ha area of surveyed wetlands (Weber 1978). The presence of Black Terns in wetlands is negatively affected by percent woody vegetation present along wetland margins (Naugle et al. 1999b, Shutler et al. 2000).

Black Tern densities are highest on seasonal or semipermanent wetlands that have roughly equal proportions of well-interspersed emergent vegetation and open water (Weller and Spatcher 1965; Weller and Fredrickson 1973; Kantrud and Stewart 1984; D. H. Johnson, unpublished data). Preliminary analysis of data from D. H. Johnson (unpublished data) showed that in North Dakota and South Dakota, number of breeding pairs of Black Terns was highest in seasonal and semipermanent wetlands and was lowest in alkali wetlands. In Iowa, Black Tern densities peaked from 1 to 4 yr after a dry wetland reflooded, when the proportions of vegetation cover and open water were about equal (Weller and Spatcher 1965, Weller and Fredrickson 1973). Populations then steadily declined during the fifth and sixth years postflood as open water cover increased. In a South Dakota study comparing waterbird densities on wetlands modified for waterfowl production versus unmodified wetlands, Brady (1983) found that Black Tern densities were higher in modified wetlands than in unmodified wetlands. Modified wetlands consisted of a system of channels, ponds, and spoil islands that provided deep, open water and upland nesting cover for waterfowl. In Alberta, Prescott et al. (1993) reported that Black Tern densities were significantly higher in wetlands within cropland plots than in wetlands within plots of dense nesting cover.

Within suitable wetlands, nesting occurs in sparse or moderately dense stands of emergent vegetation or in open water with no emergent cover, where nests are placed on floating mats of vegetation (Tout 1902, Bowman 1904, Rockwell 1911, Harris 1931, Allen 1934, Cuthbert 1954, Bent 1963, Bergman et al. 1970, Stewart 1975, Bailey 1977, Beule 1979, Dunn 1979, Burger 1985, Powell 1991, Dunn and Agro 1995, Hickey 1997, Hickey and Malecki 1997, Mazzocchi et al. 1997). Nests in dense emergent vegetation are rare (Cuthbert 1954, Dunn 1979). Density of emergent vegetation or nest substrate availability appear to be more important factors in nest-site selection than plant type (Bergman et al. 1970, Dunn and Agro 1995, Naugle et al. 2000). Common emergent vegetation surrounding nests includes cattail (Typha spp.), bulrush (Scirpus spp. and Schoenoplectus spp.), or bur-reed (Sparganium eurycarpum) (Eifrig 1919, Provost 1947, Cuthbert 1954, Hoffmann 1954, Campbell 1970, Tilghman 1980, Chapman Mosher 1986, Firstencel 1987, Einsweiler 1988, Mossman et al. 1988, Manci and Rusch 1989, Faber 1990, Powell 1991, Dunn and Agro 1995, McCollough and McDougal 1996, Hickey 1997, Hickey and Malecki 1997, Mazzocchi et al. 1997, Naugle et al. 2000). Nesting also occurs among sedges, reed canary grass (Phalaris arundinacea), reeds (Phragmites sp.), water horsetail (Equisetum fluviatile), rushes (Juncus spp.), hairgrass (Deschampsia spp.), common threesquare (Schoenoplectus pungens), pond lilies (Nuphar sp.), waterlilies (Nymphaea sp.), pickerelweed (Pontederia cordata), broad-leaf arrowhead (Sagittaria latifolia), knotweed (Polygonum sp.), sweetflag (Acorus americanus), wild rice (Zizania palustris), and cultivated rice (Hoffman 1926, Harris 1931, Provost 1947, Cuthbert 1954, Hoffmann 1954, Eddy 1961, Parmelee 1961, Bent 1963, Salt and Salt 1976, Chapman Mosher 1986, Stern 1987, Faber 1990, Knutson 1991, Dunn and Agro 1995, McCollough and McDougal 1996, Mazzocchi et al. 1997). In South Dakota, vegetation density at nest sites decreased with increasing vegetation height, indicating that Black Terns nested in vegetation that was either short and dense or tall and sparse, affording overhead protection for chicks and aerial access for adults defending the nest (Naugle et al. 2000). In British Columbia, Black Terns nested in areas where the proportion of emergent vegetation was 10-70% (Chapman Mosher 1986). Sites with >70% vegetation were probably too dense to allow access to the surface of the water and sites with <10% vegetation were probably too sparse to provide cover from wind and wave action or from predators.

In New York, the factors best predicting the presence of a nest were density of dominant vegetation, horizontal cover 0.5 m above water, vegetation cover to water ratio, and water depth (Hickey 1997, Hickey and Malecki 1997). Of 26 nests, 85% were located in sparse (stems widely scattered with water visible through stem bases) to moderately dense (stems closer than sparse and water still visible through stem bases) vegetation, 85% were placed in areas where the mean horizontal cover 0.5 m above water was ≤50%, and 65% of nests were placed where the vegetation cover to water ratio ranged from 40:60 to 60:40. Although mean water depth at nest sites was not different from random points, mean water depth remained in the model due to its influence on vegetation and its importance to wetland management. Water depth at 26 nests ranged from 40 to 60 cm and averaged 48.2 cm. Knutson (1991) compared nest-site characteristics of 25 nest sites and 25 random sites in New York. Nest sites were surrounded by shallower water (mean of 40 cm vs. 60 cm), shorter vegetation (mean of 100 cm vs. 160 cm), and greater percent mud cover (mean of 6% vs. 1%) than random sites.

The most common nest substrate is a floating mat of residual vegetation (algae, submerged aquatic plants, or leaves and stems of emergent vegetation), but nests also are placed on abandoned muskrat (Ondatra zibethicus) houses or on muskrat feeding platforms, uprooted stalks of emergent vegetation, floating logs or boards, deserted nests of other wetland bird species (e.g., Pied-billed Grebe [Podilymbus podiceps], Red-necked Grebe [Podiceps grisegena], or American Coot [Fulica americana]), mudflats, sandbars, or artificial nest platforms (Job 1902; Bowman 1904; Rockwell 1911; May 1923; Hoffman 1926; Pittman 1927; Harris 1931; Peters 1941; Provost 1947; Cuthbert 1954; Hoffmann 1954; Boyer and Devitt 1961; Parmelee 1961; Bent 1963; Weller and Spatcher 1965; Bergman et al. 1970; Campbell 1970; Doane 1972; Stewart 1975; Salt and Salt 1976; Bailey 1977; Dunn 1979; Tilghman 1980; Faanes 1979, 1981, 1982; Burger 1985; Eichhorst and Reed 1985; Firstencel 1987; Skadsen 1987; Carroll 1988; Einsweiler 1988; Mossman et al. 1988; Faber 1990, 1992b, 1996; Knutson 1991; Powell 1991; Delehanty and Svedarsky 1993; Dunn and Agro 1995; Teeuw 1995; McCollough and McDougal 1996; Hickey 1997; Hickey and Malecki 1997; Mazzocchi et al. 1997). Some studies suggest that Black Terns avoid nesting on muskrat lodges that are being actively used by muskrats (Bergman et al. 1970, Dunn 1979). Black Terns may move chicks into an auxiliary nest if the primary nest has been disturbed (Cuthbert 1954, Dunn and Agro 1995).

Nests are generally 2-6 cm high, 2-20 cm above water, placed in water 0.05-1.2 m deep, located in stands of emergent vegetation either adjacent to or within 0.5-2 m of open water, and are rarely placed near shore (Bowman 1904, Provost 1947, Cuthbert 1954, Eddy 1961, Parmelee 1961, Bent 1963, Campbell 1970, Stewart 1975, Bailey 1977, Dunn 1979, Firstencel 1987, Skadsen 1987, Stern 1987, Einsweiler 1988, Manci and Rusch 1989, Delehanty and Svedarsky 1993, Dunn and Agro 1995). Nest height above water varies with substrate, but nests on floating mats of vegetation are generally 2-5 cm above water (Bergman et al. 1970, Dunn and Agro 1995). Nests on muskrat lodges in Iowa averaged 7 cm above water (Weller and Spatcher 1965) and two nests on mud mounds in New York were 20 cm above water (Firstencel 1987). Width of nest substrates (e.g., old muskrat lodges, muskrat feeding platforms, cattail rootstalks, and floating mats of dead vegetation) vary from 28 cm to 2.8 m (Bergman et al. 1970, Dunn 1979, Dunn and Agro 1995). In Kansas, Black Terns nested in a moist soil impoundment; nests were not placed near (distance not specified) dikes (Parmelee 1961).

The main factors influencing nest success are nest substrate type and water depth (Weller and Spatcher 1965, Bergman et al. 1970, Chapman Mosher 1986, Dunn and Agro 1995, Teeuw 1995, Faber 1996). Nests placed near emergent vegetation or on taller, more stable nest substrates (e.g., muskrat lodges or muskrat feeding platforms) are less likely to be damaged from wind and wave action (Weller and Spatcher 1965, Bergman et al. 1970, Chapman Mosher 1986, Dunn and Agro 1995, Teeuw 1995). In British Columbia, nests that were completely surrounded by vegetation or that were on artificial nest platforms were more successful than nests in open water (Chapman Mosher 1986). In Minnesota, Faber (1996) also found that 23 nests on artificial platforms were more successful (65%) than 185 nests on natural substrates (44%). Also in Minnesota, unsuccessful nests had lower minimum water depths than successful nests (Faber 1992a, 1996). Only four of 21 nests with a minimum depth <30.5 cm hatched (Faber 1996).

Black Terns nest singly or in semicolonial groups ranging in size from two to hundreds (Job 1902, Bowman 1904, May 1923, Hoffman 1926, Pittman 1927, Provost 1947, Cuthbert 1954, Hoffmann 1954, Eddy 1961, Parmelee 1961, Bergman et al. 1970, Doane 1972, Bailey 1977, Beule 1979, Dunn 1979, Chapman Mosher 1986, Stern 1987, Einsweiler 1988, Mossman et al. 1988, Faber 1990, Powell 1991, Delehanty and Svedarsky 1993, Dunn and Agro 1995, McCollough and McDougal 1996, Barrett and Kay 1997, Cooper and Campbell 1997, Mazzocchi et al. 1997). Most nests either are found in small groups of 2-5 or are scattered throughout a wetland (Cuthbert 1954, Bent 1963, Bergman et al. 1970, Bailey 1977, Beule 1979, Dunn 1979, Chapman Mosher 1986). Multiple colonies composed of 20-30 nests each are occasionally found within a single large wetland (Pittman 1927, Provost 1947). Most nests are 3-30 m apart but some nests are occasionally ≤1 m from one another (Job 1902, Cuthbert 1954, Hoffmann 1954, Bent 1963, Doane 1972, Bailey 1977, Dunn 1979, Firstencel 1987, Dunn and Agro 1995, Teeuw 1995). Nests in close proximity to one another are usually visually isolated from each other (Dunn 1979). Of 82 nests surveyed in Wisconsin, 6% were 0-1 m apart, 25% were 1-5 m apart, 37% were 5-20 m apart, 16% were 20-50 m apart, and 16% were >50 m apart (Bailey 1977). In British Columbia, 62% of 339 nests were <20 m apart, 19% were 20-30 m apart, 9% were 30-40 m apart, and 10% were >40 m apart (Chapman Mosher 1986). When two nests were within 20 m of one another, the next-nearest nest commonly was 100 m distant. Mean internest distances in British Columbia varied significantly with vegetation type; in areas of hardstem bulrush (Schoenoplectus acutus) and cattail interspersed with open water, terns nested more solitarily (mean internest distance of 54 nests was 35.7 m) than in areas of reed canary grass (mean internest distance of 239 nests was 19.7) or water horsetail (mean internest distance of 46 nests was 16.8 m) (Chapman Mosher 1986). Provost (1947) found that nests on small (not defined) wetlands were more isolated than nests on large wetlands. Black Terns sometimes nest in association with Forster's Terns (Sterna forsteri) (Hoffmann 1954, Bergman et al. 1970). A table near the end of the account lists the specific habitat characteristics for Black Terns by study.

Area requirements:

Brood parasitism:
Only one author reported conspecific brood parasitism among Black Terns (Rockwell 1911). In Colorado, clutches with >3 eggs were thought to have been laid by more than one female; two nests contained five eggs and one nest contained six eggs (Rockwell 1911). Hoffmann (1954) observed a Forster's Tern nest that contained two Forster's Tern eggs and three 2-day old Black Tern young; the nest was most likely the result of inappropriate laying by the Black Tern.

Breeding-season phenology and site fidelity:

Most first clutches are initiated in late May or early June (Hoffmann 1954, Boyer and Devitt 1961, Stewart 1975, Chapman Mosher 1986, Stern 1987, Dunn and Agro 1995, Teeuw 1995, Hickey 1997, Mazzocchi et al. 1997). Black Terns commonly renest after the failure of an initial nest (Hoffmann 1954, Bent 1963, Bergman et al. 1970, Bailey 1977, Eichhorst and Reed 1985, Dunn and Agro 1995, Mazzocchi et al. 1997). Renests may be constructed at the previous site or within about 40 km from the initial site (Bailey 1977, Dunn and Agro 1995). Black Terns typically raise one brood per season (Dunn and Agro 1995, but see Doane 1972).

Although Black Terns generally exhibit weak site tenacity, some terns may return to a nest site in consecutive years, provided that water and vegetation conditions remain favorable (Bowman 1904, Bailey 1977, Dunn 1979, Carroll 1988, Dunn and Agro 1995). Low site fidelity may be related to changes in water level, vegetation density, and availability of nest substrates (Faanes 1979, Dunn and Agro 1995, Cooper and Campbell 1997). In Wisconsin, seven of 35 banded adult birds were observed at the same breeding site in successive years (Bailey 1977). Shifts in nest-site location for four recaptured, banded adult Black Terns from one year to the next ranged from 500 to 4000 m. No nest sites were reused in successive years (Bailey 1977). In Oregon, 15% of 506 banded adult Black Terns were recaptured in successive years (Stern 1987). Fifty-four terns nested at the same or at an adjacent colony in successive years. The remaining 22 returning terns nested in completely different colonies than the previous year. Although terns generally do not exhibit mate fidelity between years, a pair may mate again if both return to the same breeding site (Stern 1987, Dunn and Agro 1995). In Oregon, Stern (1987) found that all five pairs returning to the same wetland from one year to the next retained the same mate.

Species response to management:
Little is known about the effects of burning, mowing, or grazing on Black Terns. Because wetland suitability varies with yearly fluctuations of water levels and subsequent changes in vegetation, Black Terns use different wetlands or different locations within wetlands from year to year (Bailey 1977, Carroll 1988, Dunn and Agro 1995). In South Dakota, Black Tern presence was positively correlated with the total area of semipermanent wetlands in the surrounding landscape (Naugle et al. 1999a, 2000, 2001). Thus, it is important to protect and maintain wetland complexes (Dunn and Agro 1995; D. H. Johnson, unpublished data). In Nebraska, successional changes (i.e., wooded vegetation encroachment) within the channel of the Platte River has reduced the quality of river edge vegetation formerly occupied by Black Terns (Faanes and Lingle 1995).

Black Terns prefer wetlands with stable water levels and roughly equal proportions of well-interspersed emergent vegetation and open water through the breeding season (Bowman 1904, Weller and Spatcher 1965, Weller and Fredrickson 1973, Stewart 1975, Carroll 1988, Svedarsky 1992, Dunn and Agro 1995, Barrett and Kay 1997, Mazzocchi et al. 1997, Naugle et al. 2000). Favorable water levels and proportions of vegetation cover and open water can be maintained by artificially manipulating water levels of wetlands or wetland complexes through the use of water control structures. A study in New York found that Black Terns recolonized artificially manipulated impoundments the year following reflooding, and peak populations occurred in the second and third years postflood; information on Black Tern use of wetlands >3 yr postflood was not given (Hickey 1997, Hickey and Malecki 1997).

Naugle et al. (2000) and Mossman et al. (1988) found that Black Terns avoided dense, monotypic stands of cattails. Following application of glyphosate (N-[phosphonomethyl] glycine) herbicide to control cattails in North Dakota wetlands, Black Tern abundance was positively correlated with area of open water and area of dead cattails (Linz and Blixt 1997). Experiments involving various methods of cattail control were described by Beule (1979). Methods used to create openings in stands of cattail included weather-resistant covers (black polyethylene tarps), mechanical crushing, herbicides (Amitrol T* [1H-1,2,4-triazole-3-ylamine], Radapon* or Dowpon* [both 2,2-dichloropropionic acid]), physical injury to plants, cutting cattail stems below the water's surface, scraping cattails and 5-10 cm of the soil surface, and wetland drawdowns. Amitrol T and Radapon were applied at rates ranging from 3.85 to 34 kg/ha. Application rates for Dowpon ranged from 5.6 to 10 kg/ha. Cutting mature cattails ≥8 cm below the water's surface stopped the flow of air to the roots and was an effective control measure. Scraping dried wetlands removed the residual cattail stems and rhizomes located in the top layer (5-10 cm) of the soil. All of the methods described by Beule (1979) created openings in cattail stands. Black Terns nested in openings where floating debris was abundant.

Restored wetlands can provide nesting habitat for Black Terns (Svedarsky 1992, Delehanty and Svedarsky 1993). Black Terns nested in a newly restored wetland in Minnesota each year of a 3-yr study (Svedarsky 1992, Delehanty and Svedarsky 1993). The rapid colonization of the restored wetland was attributed to the availability of flooded, dead vegetation (Delehanty and Svedarsky 1993). In Iowa, Black Terns were present, but did not nest in wetlands that had been restored for 1-4 yr; the study did not examine restored wetlands older than 4 yr (VanRees-Siewert 1993, VanRees-Siewert and Dinsmore 1996). Preliminary analysis of data from D. H. Johnson (unpublished data) indicated that in North Dakota and South Dakota, Black Terns were more common in natural wetlands than in restored wetlands.

No cases of direct mortality of Black Terns due to toxic chemicals have been reported in the literature. In North Dakota, no young were reared from any of eight Black Tern nests that were monitored in a wetland that was sprayed with toxaphene (chlorinated camphene, 67-69% chorine) and oil at a rate of 2.3 kg/ha; eight juvenile birds were found dead, but dead birds were not necropsied to determine the cause of death (Hanson 1952). In New York, no chick deformities were observed in seven eggs containing contaminant levels ranging from 2.07 to 6.39 parts per million (ppm) polychlorinated biphenyl (PCB), 1.14 to 4.83 ppm DDE (a metabolite of dichlorodiphenyltrichloroethane or DDT), 35.1 to 58.8 parts per billion (ppb) hexachlorobenzene, 19.2 to 90.7 ppb octachlorostyrene, and 0.01 to 0.04 ppm mirex (dodecachlorooctahydro-1,3,4-metheno-1H-cyclobuta[CD]pentalene) (Firstencel 1987). Organochlorine contamination of wetlands may be responsible for the thinning of Black Tern eggshells (Faber and Hickey 1973, Weseloh et al. 1997). Measurements of five Black Tern eggs collected from the Great Lakes region and from Louisiana during 1970 had significantly thinner eggshell indices (shell weight divided by the product of egg length times breadth) than measurements of 91 eggs collected before 1947; DDE, PCB's, dieldrin, and mercury levels from 107 eggs sampled from 18 avian species were all negatively correlated with eggshell indices (Faber and Hickey 1973). In Ontario, some degree of eggshell thinning occurred, but organochlorine levels were not significantly correlated with eggshell thickness (Weseloh et al. 1997).

A study conducted by Shealer and Haverland (2000) revealed that neither hatching success (proportion of nests from each treatment group that hatched >1 egg) nor fledging success (proportion of chicks fledged from all eggs that hatched) were negatively affected by investigator disturbance (repeated nest visits and trapping and banding adults). Black Tern mortalities due to utility wire collisions have been reported, but appear to occur infrequently (Thompson 1978, Dunn and Agro 1995).

* References to chemical trade names does not imply endorsement of commercial products by the Federal Government.

Management Recommendations:

The management recommendations that follow are based on the species' habitat requirements and may apply to the community of wetland bird species as a whole. Further wetland loss and degradation should be avoided (Faanes 1979; Brown and Dinsmore 1986; Chapman Mosher 1986; Carroll 1988; VanRees-Siewert 1993; Peterjohn and Sauer 1997; Naugle et al. 2000, 2001). The long-term protection of wetlands can be achieved through conservation easements and purchases of wetland basins (Faanes 1979, VanRees-Siewert 1993, VanRees-Siewert and Dinsmore 1996, Haig et al. 1998, Weller 1999). The ideal management strategy for waterbirds is to maintain wetland complexes and large wetlands or lakes (Brown and Dinsmore 1986, Fredrickson and Reid 1986, Hickey 1997, Weller 1999, Naugle et al. 2000). Due to variation in water levels over seasons or years, wetland complexes are more likely to have at least some wetlands with water and plant regimes favorable to a particular species, thus ensuring diverse species' representation in a geographical area (Colwell and Oring 1988, Weller 1999). For Black Terns, provide areas of habitat ≥10 ha in size that have about equal proportions of well-interspersed emergent vegetation and open water, stable water levels throughout the breeding season, and abundant nest substrates (Chapman Mosher 1986, Faber 1992a, Dunn and Agro 1995, Cooper and Campbell 1997, Hickey 1997).

If feasible, manage wetland flooding/drawdown regimes to preserve appropriate emergent vegetation coverages and nesting substrates, and to provide stable water levels throughout the nesting season (Carroll 1988, Faber 1992a, Dunn and Agro 1995, Hickey 1997). Maintaining stable water levels decreases the probability of nest destruction due to rapidly rising water levels and decreases the probability of nest depredation (Chapman Mosher 1986, Faber 1992a, Hickey 1997). During the nesting season, maintain water levels >30 cm (Faber 1992a, Hickey 1997) and use a 4- to 6-yr cycle of drawdown, with reflooding occurring during years 2-5 (Hickey 1997). Water levels should be maintained higher than normal in the first year following reflooding in order to allow muskrat populations to recover (Hickey 1997). Removal of vegetation by muskrat herbivory benefits Black Terns by improving the interspersion of vegetation cover and open water and by increasing the availability of nest substrates (e.g., muskrat lodges, muskrat feeding platforms, and floating dead vegetation) (Hickey 1997).

Water-level control measures, discing, prescribed burning, and good muskrat populations may be used to control dense cattail stands and promote good interspersion of vegetative cover and open water (Hickey 1997). Beule (1979) provided additional recommendations for cattail control for three water depth zones: deep water (>76 cm; herbicides, cutting stems below the water's surface); intermediate water (30-76 cm; cutting stems below the water or on ice, mechanical crushing); and shallow water (2.5-30 cm; mechanical crushing, applying herbicides). Herbicide application may be an effective way to manage cattail growth when water-level manipulation, biological methods (e.g., muskrat herbivory, livestock grazing, or prescribed burning), or physical methods (e.g., mowing, discing, crushing, excavating) are not logistically or economically feasible (Linz and Blixt 1997). Rotate vegetation treatments within and among wetland complexes to achieve varied successional stages of emergent vegetation and to maintain avian diversity at a regional scale (Linz and Blixt 1997). Weller (1999) recommended the use of water level manipulation and fire rather than artificial methods to influence nutrient dynamics and natural plant succession because these were more ecologically and economically sound.

Placing artificial nest platforms in a wetland may enhance Black Tern productivity (Chapman Mosher 1986; Faber 1990, 1992b, 1996; Dunn and Agro 1995; Weller 1999). Hickey (1997) suggested using artificial platforms in the first year following a drawdown cycle, when natural substrates were lacking. Platforms are used more often by Black Terns if dead vegetation is piled on the platforms, if platforms are placed in areas of emergent vegetation interspersed with open water, and if platforms are of the right size (Dunn and Agro 1995, Teeuw 1995). Chapman Mosher (1986) suggested that nesting platforms should be at least 12 cm by 20 cm. Black Terns in Minnesota nested on artificial platforms 81 cm by 81 cm in size more frequently than on platforms 61 cm by 61 cm in size (Faber 1992b).

If possible, utility wire lines should be placed several kilometers away from wetlands, waterfowl concentration areas, flyways, roosting areas, feeding areas, low passes, breeding areas, and especially paths between feeding and roosting or nesting areas (Thompson 1978).

Table. Black Tern habitat characteristics.

Author(s)	Location(s)	Habitat(s) Studied*	Species-specific Habitat Characteristics
Allen 1934	Northern U.S. (not specified)	Wetland	Preferred the open-water zone of wetlands
Bailey 1977	Wisconsin	Lake	Nested in and around large stands of hardstem bulrush (Schoenoplectus acutus); nests were constructed of cattail (Typha spp.), hardstem bulrush, or algae; 50% of 143 nests were placed on floating cattail rootstalks, 20% in live cattail stands (stands were 5-25 m in diameter), 14% on floating bulrush stems, 9% on mats of floating algae, and 7% on floating boards; the closest nest to shore was 25 m and all nests were within 1-2 m of open water; nested in loose groups of two to four pairs, but sometimes in groups of ≥10 pairs; two closest nests were 75 cm apart; of 82 nests surveyed in 1976, 6% were 0-1 m apart, 25% were 1-5 m apart, 37% were 5-20 m apart, 16% were 20-50 m apart, and 16% were >50 m apart
Barrett and Kay 1997	Northwest Territories	Wetland	Nested in hardstem bulrush 15 m from shore in a 3.75-km² wetland with an extensive (>300 m) sedge (Carex spp.) periphery, 50% open water, and water depth of 1 m
Bent 1963	Rangewide	Wet meadow, wetland	Occupied wetlands and wet meadows; nested in common reeds (Phragmites australis) that were either tall and thick or beaten down and partially open; nested on abandoned muskrat (Ondatra zibethicus) lodges, floating mats of reeds and sweetflag (Acorus americanus), old grebe (Podicipedidae) or American Coot (Fulica americana) nests, driftwood, or boards; nested in water 30-60 cm deep, but nests also were found in a wet meadow with only a few cm of water; although most nests were widely scattered, four nests were found in a radius of 4.5 m
Bergman et al. 1970	Iowa	Wetland	Of 197 nests, 53% were on floating cattail rootstalks, 25% on inactive muskrat lodges, 11% on muskrat feeding platforms, and 11% on dead floating emergent vegetation; individual substrates (e.g., muskrat lodge) never held more than one nest; nest bowls averaged 3.3 cm above water; nest substrates averaged 52.2 cm in diameter; nested in both open water and dense cattail stands; nests in open water were either on deteriorated muskrat lodges or on muskrat feeding platforms
Beule 1979	Wisconsin	Wetland	Nested in groups of three to five pairs in open areas of water with abundant floating dead vegetation; nests were not located in areas of live emergent vegetation
Bowman 1904	North Dakota	Wetland	Nested in wetlands ranging in size from 0.4 to 40 ha with good interspersion of open water and emergent vegetation; avoided wetlands that were overgrown with rushes (scientific name not given); nested in small (not defined) patches of open water surrounded by emergent vegetation; nests were placed on dead vegetation, piles of dead vegetation, or on abandoned muskrat lodges; water depth varied from 5 to >60 cm; colonies consisted of 12-50 pairs
Boyer and Devitt 1961	Ontario	Wetland	Nested on floating material such as logs, sticks, and mats of dead cattail stems
Brady 1983	South Dakota	Wetland, wetland (modified)	Densities were higher in dug-brood complexes (modified wetlands comprising a system of channels, ponds, and human-created islands to provide deep, open water and upland nesting areas for waterfowl) than in unmodified semipermanent wetlands
Brown and Dinsmore 1986	Iowa	Wetland	Preferred wetlands >5 ha; frequency of occurrence was >50% only in the two largest (11-20 ha and >20 ha) wetland size classes; occurred in smaller (<5 ha) wetlands when these were part of wetland complexes
Burger 1985	Minnesota	Wetland	Nested in areas of sparse (not defined) cattails, usually on floating nests that were loosely attached to vegetation stems, or less commonly on top of muskrat houses
Campbell 1970	British Columbia	Wetland	Nested among cattails on a floating mat of vegetation; water depth was 61 cm
Carroll 1988	New York	Lake, river, wetland, wetland complex	Nested on abandoned muskrat lodges, muskrat feeding platforms, floating cattail rootstalks, dead floating emergent vegetation, mats of floating algae, and floating boards; colonies occurred at mouths of rivers, wetlands, lake shorelines, and wetland complexes; nested in areas where open water and emergent vegetation were well interspersed
Chapman Mosher 1986	British Columbia	Lake, wetland complex	Nested in broad-leaved cattail (T. latifolia), hardstem bulrush, a mixture of water horsetail (Equisetum fluviatile) and beaked sedge (C. rostrata), and most frequently in reed canary grass (Phalaris arundinacea); nested in areas averaging 42% matted vegetation, 33% open water, and 25% standing vegetation (sample size not given); used areas where the proportion of standing vegetation was 10-70%; nested in areas where the area occupied by stalks of emergent vegetation was 10-50 cm²/m²; nested on floating boards, floating vegetation mats, or artificial nest platforms; nest success on artificial platforms was higher than nests placed on mats or boards; nest success was higher for nests in reed canary grass than for nests in hardstem bulrush due to greater protection from wind and wave action; hatching success was negatively affected by wind and wave action; fledging success was positively correlated with nesting in water horsetail; mean internest distances varied with vegetation type; terns nested more solitarily in areas of hardstem bulrush and cattail (mean internest distance of 54 nests was 35.7 m), more so than in areas of reed canary grass (mean internest distance of 239 nests was 19.7) or water horsetail (mean internest distance of 46 nests was 16.8 m); when two nests were within 20 m of one another, the next nearest nest commonly was 100 m distant; of 339 nests, distance to nearest neighbor was <20 m for 62%, 20-30 m for 19%, 30-40 m for 9%, and >40 m for 10%; one pair of nests was 1 m apart
Cooper and Campbell 1997	British Columbia	Wetland	Number of pairs per colony ranged from one to 60; most colonies contained 6-20 pairs
Cuthbert 1954	Michigan	Wetland	Nested mostly in hardstem bulrush and softstem bulrush (Schoenoplectus tabernaemontani), followed by cattail/bulrush mixture, cattail, cattail/yellow pond lily (Nuphar lutea) mixture, and bur-reed (Sparganium eurycarpum); nest substrates were floating dead plant material, floating logs or boards, abandoned muskrat lodges, and non-floating piles of dead bulrushes; of 27 nests, 85% were placed in water ≥0.6 m in depth; nested most commonly in thinly scattered bulrush ≤1 m from open water, although two nests were in dense cattails near a clearing made by muskrats; a colony of 17 nests were in an 8-ha tract and an additional 10 nests were scattered in five sets of two each; distances between nests ranged from 9 to 36 m; distances between the pairs of nests ranged from 90 to 200 m
Delehanty and Svedarsky 1993	Minnesota	Impoundment, wetland (restored)	Nested in a newly restored wetland in all 3 yr of study; used dead hardstem and softstem bulrush for nesting material; nested near the wetland edge in an area protected from the wind by emergent vegetation, trees, and a ridge; fledglings moved from the nests in the restored wetland to open, sandy points on the edge of the reservoir where they were fed by adults
Doane 1972	Wisconsin	Wetland	Colony contained 16 nests; one nest was on a mudflat; a few (actual number not given) nests were <1 m from one another
Dunn 1979	Ontario	Lake	Nested in live cattail stands of moderate density (described as standing ≥1 m tall and dispersed enough to allow a canoe to pass through); a few nests were located in thin, new cattail growth, but nests in dense, old cattail stands were rare; of 24 nests, 75% were built on mats of floating residual cattail, 17% on floating boards or logs, and 8% on cattail rootstalks; no nests were constructed on muskrat lodges or feeding platforms, although these substrates were abundant; the majority (actual percentages not given) of 25 nests were 2-5 cm tall, 25 cm wide, constructed of dead cattail over 1-1.2 m of water, on mats of dead vegetation averaging 8 m²; and located 4 m from open water (range 0.5-12 m); spatial layout of nests was usually only two or three nests per 25 m², although a few nests were ≤3 m apart
Dunn and Agro 1995	Range-wide	Island, lake, rice field, river, sewage lagoon, wetland	Nested in shallow freshwater wetlands with emergent vegetation, including prairie wetlands, lake margins, river or island edges, sewage lagoons, restored wetlands, and cultivated rice fields; preferred to nest in semipermanent wetlands; used snags or posts for copulating, resting, and feeding fledglings; density of emergent vegetation and nest substrate availability were more important to nest-site selection than plant type or water depth; vegetation at most nests was sparse to moderately dense (described as dispersed enough to allow a canoe to pass through); nested in areas with 25-75% emergent cover; nest-site vegetation included cattail, bulrush (Schoenoplectus spp.), bur-reed, sedges, reed canary grass, water horsetail, rushes (Juncus spp.), hairgrass (Deschampsia spp.), pond lily (Nuphar spp.), and cultivated rice; floating dead vegetation was common at nest sites; water at nests was generally 0.5-1.2 m deep and few nests were near (not defined) shore; nested adjacent to or within 0.5-2 m of open water; also nested on artificial platforms; nest substrates were usually floating dead vegetation, floating rootstalks, floating boards, or floating muskrat feeding platforms; less common substrates were muskrat lodges, small mud mounds, rooted and flattened vegetation, or abandoned nests of other waterbirds; width of nest platforms ranged from 28 cm to 2.8 m; nests were usually 2-6 cm tall and 2-5 cm above water colonies commonly contained 11-50 nests, and ranged from two to hundreds of nests; most nests were 5-20 m apart but some were reported as close as 1 m; preferred wetlands or wetland complexes >20 ha, although were found in a 5.3-ha wetland; area ≤2 m of the nest was defended; temporary feeding territories maintained during the fledging period; when a site became unsuitable for nesting, birds were more likely to move to more distant (>5 km) sites than to closer (<1 km) ones
Eddy 1961	Minnesota	Lake	Nested in bulrush, waterlily (Nymphaea spp.), and cattail; water depth at 51 nests was 15-79 cm and nests were located in a 5.1-ha area; defended the area ≤2 m from the nest
Eichhorst and Reed 1985	Wisconsin	Lake	Renested on a deserted Red-necked Grebe (Podiceps grisegena) nest
Einsweiler 1988	Michigan	Impoundment, lake, wetland	Nested in cattail and bulrush; mean water depth at 34 nests was 24 cm during incubation and decreased to 20.5 cm during the nestling stage; 17 of 34 nests were on mud mounds in shallow (depth not given) water, 14 nests were on floating grass/sedge mats in deep-water areas, 2 nests were on deserted Pied-billed Grebe (Podilymbus podiceps) nests, and 1 was on an artificial nest platform
Faanes 1979	Wisconsin	Wetland	Of 52 nests, 51 were on mats of floating vegetation in the deep-marsh zone of a wetland; one was on a muskrat lodge; nest substrates were cattail (17 nests), river bulrush (Schoenoplectus fluviatilis) (16 nests), hardstem bulrush (12 nests), submerged aquatic vegetation (6 nests), and muskrat lodge (1 nest)
Faanes 1981	Minnesota, Wisconsin	Wetland	Occurred on large seasonal and semipermanent wetlands that supported an abundance of emergent vegetation; preferred to nest on floating mats of vegetation composed of submerged plants and emergent plant leaves
Faanes 1982	North Dakota	Wetland	Occurred in about equal numbers on semipermanent and permanent wetlands; occasionally used seasonal wetlands; 16 nests were observed either on mats of floating vegetation or on decaying muskrat houses
Faanes and Lingle 1995	Nebraska	River channel island, wetland	Occurred in semipermanent and seasonal wetlands; suitability of river edge vegetation was reduced due to wooded vegetation encroachment within the channel of the Platte River
Faber 1990 1992a, 1992b, 1996	Minnesota	Wetland	Nested in shallow (<46 cm) water among bur-reed, common threesquare (Schoenoplectus pungens), and cattail; nested on larger (81 cm by 81 cm) artificial platforms more frequently than on smaller (61 cm by 61 cm) platforms; nest success was greater on artificial platforms (65% of 23 nests) than on natural substrates (44% of 185 nests); successful nests had greater water depths than unsuccessful nests (49.7 cm vs. 39.9 cm in one year and 53.6 vs. 46.6 cm in the second year); only four of 21 nests with a minimum water depth <30.5 cm were successful; colony sizes ranged from 2 to 56 pairs
Faber and Hickey 1973	Louisiana, Michigan, Minnesota, Wisconsin	Lake	Measurements from five eggs sampled in 1970 had significantly thinner eggshell indices (shell weight divided by the product of egg length times breadth) than measurements from 91 eggs collected before 1947
Firstencel 1987	New York	Wetland	Nested in small (not defined) clearings within cattail stands; nests were typically located on mats of floating cattail rootstalks adjacent to open water, although some (number not given) terns nested on logs and mud mounds; eggs in nests on mud mounds were about 20 cm above water; nests on mud mounds were typically ≤8 m from one another
Graetz and Matteson 1996	Wisconsin	River, wet meadow, wetland	Occurred in wetlands, river edges, and flooded sedge meadows; breeding sites were dominated by bulrush (Scirpus spp. and Schoenoplectus fluviatilis), cattails, bur-reed, sedges, grasses (Poaceae), water plantain (Alisma sp.), and arrowhead (Sagittaria spp.)
Harris 1931	Nebraska	Wetland	Nested on floating mats of rushes in a small (1.4 m²) open-water area in a stand of rushes; water was "chest deep" and four nests were 1.5 m apart
Hartman 1994	Indiana	Wetland, wetland (restored)	Occurred at one of 26 restored wetlands
Hickey 1997, Hickey and Malecki 1997	New York	Wetland complex	Most (97.5% of 50) nests were located in bur-reed or cattail and most (78% of 105) nests were placed on abandoned muskrat lodges or feeding platforms; 85% of 26 nests were located in sparse (stems widely scattered with water visible through stem bases) to moderately dense (stems closer than sparse and water still visible through stem bases) vegetation; horizontal cover 0.5 m above water at 85% of 26 nest sites was ≤50%, vegetation cover to water ratio ranged from 40:60 to 60:40 at 65% of 26 nest sites, and mean water depth at 26 nest sites was 48.2 and ranged from 40 to 60 cm; following drawdown of a wetland, terns recolonized impoundments the year following reflooding and peak populations occurred in the second and third years postflood; information on use of wetlands >3 yr postflood was not given
Hoffman 1926	Wisconsin	Lake	Nested on small hummocks or on floating mats of dead bulrush; nests were constructed of dead bulrush; six nests were found in a 21-m² area
Hoffmann 1954	Wisconsin	Lake	Nested in shallow (not defined) bays of inland lakes or shallow river widenings that contained cattails, bulrush, wild rice (Zizania palustris), pond lilies (Nuphar sp.), and pickerelweed (Pontederia cordata); nested on floating dead bulrush that gathered along the outer edges of cattail and bulrush stands, or on abandoned muskrat lodges or exposed mudflats; nested in loose colonies; most nests were ≥3 m apart, rarely 1.5 m apart
Job 1902	North Dakota	Lake	Nested on small mounds of floating vegetation; colonies contained about a dozen nests spaced 2-15 m apart
Johnsgard 1980	Nebraska	Impoundment, wetland	Nested in wetlands with stands of emergent vegetation interspersed with open water
D. H. Johnson, unpublished data	North Dakota,South Dakota	Wetland	Number of breeding pairs was highest in seasonal and semipermanent wetlands and was lowest in alkali wetlands; were more common in natural wetlands than in restored wetlands, and were more common in wetlands that had semipermanent wetlands within 0.4 km than in wetlands without semipermanent wetlands nearby
Kantrud and Stewart 1984	North Dakota	Wetland complex	Highest density was found in semipermanent wetlands, followed by seasonal, fen, temporary, and permanent wetlands
Knutson 1991	New York	Wetland	Nested on floating mats of broad-leaved arrowhead (Sagittaria latifolia), sedges, bulrush, cattails, knotweed (Polygonum sp.), or grasses; preferred to nest in shallow-marsh areas or in sedge meadows rather than deep-marsh areas; compared to 25 random sites, 25 nest sites had shallower water (mean of 40 cm vs 60 cm), shorter vegetation (100 cm vs. 160 cm), and greater mean percent mud cover (6% vs. 1%); mean percent vegetation cover (78% for nest sites) and mean percent open water (16% for nest sites) did not differ between nest sites and random sites; nest sites were dominated by broad-leaved arrowhead and sedges, whereas random sites were dominated by cattail
Linz and Blixt 1997	North Dakota	Wetland	Abundance was positively correlated with hectares of open water and with hectares of dead cattails following application of glyphosate herbicide to control cattails
Manci and Rusch 1989	Wisconsin	Wetland	Nested in cattail stands where the mean water depth was 29 cm; avoided water >50 cm deep
May 1923	New York	Lake	Nested on a sandbar on the shore of Lake Ontario; colony consisted of 6-7 pairs
Mazzocchi et al. 1997	New York	Wetland complex	Nested in stands of bur-reed and pickerelweed mixture and in stands of cattail; common submergent plant species at nests were duckweed (Lemna spp.), frogbit (Hydrocharis sp.), and algae; 54% of 37 nests were placed in emergent vegetation 26-50 cm tall; mean percent coverages at 37 nest sites were 43% emergent vegetation, 35% open water, 19% floating or submergent vegetation, and 3% shrubs; percent vegetation cover to water ratio ranged from 40:60 to 60:40 for 54% of 37 nests; 84% of 31 nests in both years were in sparse to moderately dense (not defined) vegetation; mean horizontal cover at 0.5 m above water was <21% at 81% of 37 nest sites; most common nest substrate was floating vegetation mats (58% of 151 nests), followed by artificial platforms (17%), abandoned muskrat houses (14%), uprooted pickerelweed stalks (10%), Pied-billed Grebe nests (1%), and floating logs (0.7%)
McCollough and McDougal 1996	Maine	Wetland	Colony sizes ranged from one to 23 pairs at 10 wetlands; nested in wetlands >25 ha in size
Mossman et al. 1988	Wisconsin	Wetland	Nested in areas with a mixture of emergent vegetation, mudflats, and shallow (not defined), open water; 71% of 173 nests were located on rhizomes of hardstem bulrush, 9% on artificial nest platforms, 9% on mats of residual bulrush or cattail stems, 7% on floating boards, 3% on mats of muskgrass (Chara spp.), and 1% on inactive nest structures of Red-necked or Pied-billed grebes; avoided dense (not defined) stands of cattails
Naugle 1997; Naugle et al. 1999a,b, 2000, 2001	South Dakota	Wetland	Preferred semipermanent wetlands over seasonal wetlands; presence was positively related to the area of semipermanent wetlands and grassland in the 25.9 km² area surrounding a wetland; dominant emergent vegetation at nest sites included cattail (65% of 20 sites), bulrush (20%), and bur-reed (15%); preferred wetlands that had about equal proportions of emergent vegetation and open water; vegetation height and density were lower at nest sites than at sites outside the colony; at nest sites, vegetation density decreased with increasing vegetation height; presence was negatively affected by the extent of woody vegetation along wetland margins
Parmelee 1961	Kansas	Wetland (diked)	Nested on floating mats of green rushes (species not specified) in "knee deep" water; did not nest near (not defined) dikes
Peters 1941	New Brunswick	Lake	Two nests found in a shallow (not defined) lake on mounds of vegetation
Pittman 1927	Saskatchewan	Wetland	Nested in groups of 20-30 on mats of floating residual vegetation
Powell 1991	Minnesota	Wetland, wetland complex	Nested on semipermanent wetlands 15-50 ha in size with 5-95% open water and patches of sparse to moderately dense (not defined) emergent cover; vegetation was dominated by bulrush; nested on mats of floating dead bulrush in the interior of the wetland; three colonies contained 4, 10, and 15 pairs; colonies occurred only on wetlands within wetland complexes
Prescott et al. 1993	Alberta	Cropland, dense nesting cover (DNC; idle seeded-native), mixed-grass pasture, tame pasture, wetland	Densities were significantly higher in cropland plots that contained wetlands than in DNC plots that contained wetlands
Prescott et al. 1995	Alberta	Wetland	Most abundant in large (>8 ha) fresh wetlands, followed by medium (1-8 ha) fresh wetlands, medium saline wetlands, small (<1 ha) fresh wetlands, small saline wetlands, and large saline wetlands
Provost 1947	Iowa	Idle tallgrass, tallgrass pasture, wetland,wet-meadow pasture	Nested in scattered groups of 10-20 at the edge of rushes and bur-reed in water >60 cm deep; nests were loose structures of floating dead vegetation or algal mats; commonly nested on small (not defined) wetlands, but the nests were more isolated than on large wetlands
Rockwell 1911	Colorado	Wetland	Nested on floating mats of dead cattails 3 m from shore in a patch of sparse (not defined) cattail cover amidst otherwise dense cattails in water ≤1 m deep; also nested on top of a wooden duck blind that was floating at the edge of the cattails and in 1-m deep open water
Salt and Salt 1976	Alberta	Wetland	Nested on a raft of reeds (Phragmites sp.) and grasses in shallow (<60 cm) water
Shutler et al. 2000	Saskatchewan	Cropland, DNC (idle seeded-native, idle seeded-tame), wet meadow, wetland	Were present in areas of conventional tillage (applications of herbicides and tillage [≥3 times per year] to control weeds), minimum-tillage (reduced tillage [<3 times per year] and direct seeding into previous year's crop stubble), and organic farming (cultivation and crop rotation); presence was negatively affected by percent woody vegetation present along wetland margin
Skadsen 1987	South Dakota	Wetland	Nested on floating mats of cattails in water 91-122 cm deep
Stern 1987	Oregon	Wetland	Nesting habitats included deep-water (40-60 cm) sites dominated by hardstem bulrush, intermediate water depths (15-30 cm) dominated by sedges and rushes, and shallow-water (<15 cm) sites dominated by tufted hairgrass (D. caespitosa)
Stewart 1975	North Dakota	Impoundment, lake, stock pond, wetland	Occurred in wetlands with stands of emergent vegetation with adjacent areas of open water; habitats included wetlands and lakes, shallow river impoundments, and occasionally large stock ponds; bred most frequently in fresh and slightly brackish semipermanent wetlands, followed by fresh seasonal wetlands, fresh permanent wetlands, and fen wetlands; nests were constructed of floating dead vegetation (algae, submerged aquatic plants, or leaves and stems of emergent vegetation) and were located in moderately dense (not defined) emergent vegetation or in open water with no cover; water depth at 41 nests averaged 43 cm and ranged from 10 to 86 cm
Stewart and Kantrud 1965	North Dakota	Wetland	Highest densities were found on seasonal wetlands with clumps of emergent cover interspersed with open water, and on fresh through brackish semipermanent wetlands with closed stands of emergent cover
Svedarsky 1992	Minnesota	Idle mixed-grass, idle mixed-grass/tame, idle tallgrass, idle tame, impoundment, wetland (restored)	Nested in a restored wetland in an area that had equal amounts of open water and emergent vegetation; foraged in areas of open water
Teeuw 1995	Ontario	Wetland	Nested in areas where the proportions of emergent vegetation and open water were about equal; occupied stands of emergent vegetation ranging in size from 45 by 30 m to 100 by 100 m; of 38 nests, 15 were on floating mats of vegetation, 15 on artificial nest platforms, 5 on mud mounds, and 3 on floating boards; 25 of 38 nests were located in small (ranged in size from 1.5 m by 1.5 m to 9 m by 12 m) open water pools within stands of emergent vegetation; mean water depth at nests ranged from 32 cm for 12 nests to 61 cm for 22 nests; 38 nests averaged 12 m from the open water of a large bay; internest distance of 38 nests averaged 11 m
Tilghman 1980	Wisconsin	River, wet meadow, wetland	Nested in wetlands, river edges, and flooded sedge meadows in areas dominated by cattail, bulrush, and sedges; emergent vegetation cover ranged from 51 to 75% in over 85% of 205 occupied sites; nests substrates were floating peat mats, muskrat feeding platforms, dead floating cattails, or floating cattail rootstalks
Tout 1902	Nebraska	Wetland	Nested in small areas of open, shallow (not defined) water
VanRees-Siewert 1993, VanRees-Siewert and Dinsmore 1996	Iowa	Wetland (restored)	Were present in wetlands that had been restored for 1-4 yr, but no nests were found; the study did not examine restored wetlands older than 4 yr
Weber 1978, Weber et al. 1982	South Dakota	Cropland, idle mixed-grass, idle shortgrass, idle tallgrass, mixed-grass pasture, shortgrass pasture, stock pond, tallgrass pasture, tame hayland, wetland, woodland	Preferred semipermanent wetlands with emergent vegetation; most frequently occurred in semipermanent wetlands, followed by stock ponds, seasonal wetlands, and dugouts; presence was positively related with hectares of surface water, presence of semipermanent wetlands within 400-ha of surveyed wetlands, wetlands with central expanses of open water composing >5% of the wetland area and surrounded by a peripheral band of emergent vegetation cover averaging ≥1.8 m in width, vegetation height, and shoreline distance
Weller and Fredrickson 1973	Iowa	Wetland	Density peaked the year of reflooding when the areas of open water and emergent vegetation were roughly equal; in the 7 yr following reflooding, populations declined steadily as emergent vegetation became flooded and open water cover increased
Weller and Spatcher 1965	Iowa	Wetland	Nested on muskrat feeding stations, floating mats of dead vegetation, floating rootstalks, or dense beds of submerged, rooted aquatic vegetation; nests in emergent vegetation were better protected from the wind; nests on muskrat feeding stations averaged 7 cm above water; numbers peaked 3-4 yr following reflooding, when open water and emergent vegetation were well interspersed

* In an effort to standardize terminology among studies, various descriptors were used to denote the management or type of habitat. "Idle" used as a modifier (e.g., idle tallgrass) denotes undisturbed or unmanaged (e.g., not burned, mowed, or grazed) areas. "Idle" by itself denotes unmanaged areas in which the plant species were not mentioned. Examples of "idle" habitats include weedy or fallow areas (e.g., oldfields), fencerows, grassed waterways, terraces, ditches, and road rights-of-way. "Tame" denotes introduced plant species (e.g., smooth brome [Bromus inermis]) that are not native to North American prairies. "Hayland" refers to any habitat that was mowed, regardless of whether the resulting cut vegetation was removed. "Burned" includes habitats that were burned intentionally or accidentally or those burned by natural forces (e.g., lightning). In situations where there are two or more descriptors (e.g., idle tame hayland),the first descriptor modifies the following descriptors. For example, idle tame hayland is habitat that is usually mowed annually but happened to be undisturbed during the year of the study.

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