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Nest functions

What does a female look for in a nest site? Evolution has favored those females selecting sites where the chances of producing offspring (and perpetuating their genes) are maximized. First, a female has to get fertilized so the nest site must be accessible to males on a booming ground. The actual nest site must provide shade, concealment cover from predators but, in the event of discovery by a predator (usually mammals), an unobstructed escape route must allow upward flushing from the nest. The nest site must provide some protection from weather factors such as hard rains, hailstorms and flooding. It must be reasonably close to high protein and energy food resources needed by the female during the preincubation, laying, and incubation periods. Nests must also be situated so that vulnerable chicks have ready access to their specialized habitat needs upon hatching. Nests are generally located within 2 miles (3.2 km) of booming grounds which apparently serve as an orientation point since some hens return to a given booming ground in subsequent years (Svedarsky 1988). Females select nest sites within their preincubation home ranges of about 200 acres (81 ha); often in the same vicinity as the preincubation range surrounding their successful nests of the preceeding year (Svedarsky 1988). Robert Eng (personal commun.) suggests that prairie chicken females may not be as opportunistic as waterfowl in exploiting new habitats after they become associated with a favored nesting area and/or display ground (a familiar area).

Are nests oriented around booming grounds or vice versa? Both are probably involved to an extent but males are probably more adaptable as to display sites than females are to successful nest sites. Males are only able to perpetuate their genes through females so, evolutionarily speaking, they should maximize the likelihood of encountering and mating with females, and that should be in the vicinity of good nesting cover. Meanwhile, males should display in a setting where their visibility (to females) is maximized along with their security from mammalian and avian predators; that is, being seen and be able to see. In sharptail habitat restoration after strip-mining in southeastern Montana, a square mile of good nesting and brooding cover was established and males established dancing grounds on closely grazed pastures radiating out from this core of cover (Robert Eng, personal commun.).

Brown (1966:220) commented further on the relationship between sharptail display grounds and the quality of residual cover (used for nesting, brooding, and roosting) in Montana: "A clear cut relationship appears to exist between the establishment of new breeding grounds and the estimated cover density... Without exception, the largest breeding grounds have been located in areas surrounded by extensive, heavy stands of residual herbage." In Wisconsin, Westemeier (1971:34) found a significant correlation between the number of prairie chicken cocks on a booming ground and the amount of certain habitats (generally grassland/cropland cover types) within a 1 mile (1.6 km) radius of the booming ground, leading him to suggest that, "The number of birds using a booming ground may be a useful measure of the productivity of the surrounding cover." John Toepfer (personal commun.) has found that most young cocks are recruited to booming grounds near their brooding area.

Measurements

Reported measurements (Table 3) of prairie chicken nest characteristics are quite variable due to the broad geographical range of the species and different measuring procedures (Buhnerkempe et al. 1984). Commonly, "height", is given in the literature but it is not clear if that indicates the maximum height of any part of the vegetation, usually the flowering stems, or the height of the vegetation which is most functional in concealing a nesting female. Newell (1987:16) used "height" to essentially indicate "maximum height" and "effective height" as that level below which all dots on a cover board were obscured by vegetation; similar to the 100% visual obstruction reading (VOR) of Robel et al. (1970a). Buhnerkempe et al. (1984) used a rod marked off in cm intervals and used "maximum height" as the highest interval touched by standing vegetation and "effective height" as the height which included the lower 90% of the vegetation strikes on the sampling rod; also, somewhat comparable to the 100% VOR. They also used the rod to measure "litter depth" as the height of the first cm interval seen on the rod from the ground upwards.

In order to compare structural characteristics of prairie chicken nesting covers it is important to use consistent terminology and techniques as well as measure or estimate conditions at the time of nest site selection. Growth or cover removal activities can substantially alter conditions if measurements are taken later in the season.

Table 3. Summary of greater prairie chicken nest characteristics.

¹"Effective height" approximately equals 100% VOR, "average height" probably from 0-50% VOR.

Figure 3 indicates the variation in growth form of 4 general cover types used by nesting prairie chickens. Consider the variation in levels at which one would record 0%, 50%, and 100% Visual Obstruction Readings (VOR) in the four types. Westemeier (Westemeier and Edwards, 1987) working first in Wisconsin and for the last 26 years in Illinois has examined over 1,000 prairie chicken nests and believes (and Svedarsky concurs) that "ideal" nesting cover should be relatively dense close to the ground, about 40 cm (16 in.) tall, and have a growth form similar to smooth brome. This would have a 100% VOR reading of about 2.7 dm (Svedarsky, 1979). Brome has the majority of leaves positioned on stems as opposed to the base of the plant and withstands snow flattening if maintained in a vigorous condition. Interestingly, prairie chickens recently transplanted from Kansas and Minnesota, to Illinois showed similar nesting cover preferences (brome, redtop, timothy) as Illinois birds. Fields where 11 birds successfully nested had late March 100% VOR's of 2.0 (Westermeier, et al. 1995). Svedarsky (1988) working in a Minnesota study area having a variety of native and non-native cover types, found prairie chickens strongly preferred brome with 22% of 36 nests found in this cover which comprised only 2.9% of the study area. This is not to suggest that smooth brome (an exotic) is favored in an overall sense over native species in ecosystem management, since it can invade and degrade native prairies in some settings. The structural features of brome however might be used as a reference in managing whatever cover a manager has to work with in order to maximize prairie chicken nesting values.

Figure 3. Characteristics of four nesting covers

Plant species composition in itself is not important so long as density requirements are met, but obviously a given species has a characteristic life form. Prairie chickens have existed for many years in Wisconsin on "substitute prairie" dominated by Kentucky bluegrass (Hamerstrom, et al. 1957:12) and on redtop and timothy seed fields in Illinois (Westermeier, 1980). Quackgrass is currently a dominant species in the Wisconsin range (John Toepfer, personal commun.).

Westermeier (1973) found prairie chickens avoided tall, (>1 m) native vegetation in Illinois. He believed that a female is most secure in a cover setting where she can stand and have surveillance of the surrounding area and readily flush, not run, if threatened. Svedarsky (1979), Yeatter (1943), Toepfer (1988) and Hamerstrom, et al. (1957) also believed that vegetation can be too tall and dense for nesting prairie chickens.

Nest success

Litter is generally not separated from "residual vegetation" in the literature but would generally be that part of the residual cover which is horizontally oriented and is more than one growing season old. Residual cover is all accumulated growth from past years but usually the growth from the previous year provides the majority of the concealment cover. Svedarsky (1979) and McKee (1995) found litter amount to be inversely related to nesting success. Of several nest characteristics, McKee found litter cover the best single predictor of nest success; nest sites having >25% failing at twice the rate of nests with <25% litter cover. She related increased litter to decreased grass and forb cover at nests. Buhnerkempe, et al. (1984) found litter depths of 5.6 cm in fields containing only unsuccessful nests whereas fields with only successful nests had litter depths of 4.2 cm. John Toepfer (personal commun.), in a sample of 57 nests in 1994, found those in Minnesota CRP habitats (presumably with deeper litter due to no-disturbance management) experienced lower nesting success (29.7%) compared to those in native habitats (50.0%); a pattern observed from 1992-1996. Increasing litter depths may have been one of the factors contributing to prairie chickens declining on Soil Bank lands in North Dakota, 5-7 years after seeding (Kirsch, et al., 1973). Excessive litter could indirectly reduce nest success by providing better small mammal habitat and thus attracting red foxes and/or better conditions for June beetle larvae and thus attracting striped skunks (Svedarsky, 1979).

In Minnesota, Kimmel et al. (1994) evaluated cover characteristics of warm-season vs cool-season grasses in CRP plantings after establishment. They found percent litter cover increased with stand age for cool-season grass (r² = 0.82) but not so in warm-season plantings (r² = 0.34). Litter depth was not related to stand age in either type. VOR's of cool-season grasses declined with stand age (r² = 0.71) but increased in warm-season grasses (r² = 0.82). This suggests the greater importance in rejuvenation management of cool-season grasses to maintain optimum residual cover features in comparison to warm- season cover.

In Illinois, Westemeier (1985) in a sample of 810 prairie chicken nests found the highest prairie chicken nesting productivity (1.1 successful nests/4 ha) in brome and the lowest in planted prairie grass (0.3). Brome tends to maintain cover in a more vertical position than prairie grass (in this case, mostly switchgrass, Indian grass and big bluestem). At least in Illinois, Westemeier (1985:35) believed brome comes closer to being a "seed it and leave it" cover than any type used on their sanctuaries. Westemeier (1988) found nest densities of prairie voles (Microtus orchrogaster) and southern bog lemmings (Synaptomys cooperi) highest in prairie grass and lowest in brome, 20.5 and 8.4 nests/10 ha, respectively, and suggested an attraction for mammalian predators.

Svedarsky (1979) in Minnesota and McKee (1995) in Missouri also found the extent of woody vegetation lowered prairie chickens nest success. McKee recorded 9 of 11 nests hatched when there was no woody cover but only 9 of 32 when it was present. Perhaps, woody vegetation adds to the amount of "edge" and encourages mammals to hunt in an area. It could also serve as perch sites for potential avian nest predators such as crows and magpies.

Dense vegetation is thought to offer a barrier to predators as nests placed close to cattle trails experienced higher predator losses than those at some distance (Capel, 1965). Kirsch (1969) found red fox readily used vehicle trails as access routes into idle cover. Westemeier (1973:333) however, felt that redtop seed harvesting operations in Illinois "creates a desirable patchwork of holes in an otherwise too-thick stand of redtop by leaving, in addition to the wheel tracks, wads of chaff and stems scattered throughout the stubble." This was desirable from a prairie chicken nesting use standpoint because of the "too-thick" stand but Westemeier did not evaluate the wheel tracks from a predator access standpoint. Duebbert (1969) believed that dense cover reduced mammalian predator scenting conditions because of minimizing horizontal movement of air currents.

Management considerations

When do optimum prairie chicken nesting conditions occur after a new seeding or disturbance management (burning, grazing, mowing) of an established cover? Westemeier (1973) found the highest prairie chicken nest densities (1 nest per 7.7 acres or 3.4 ha) in redtop during the second growing season. Kirsch (1974) suggested that prairie chickens probably do not begin nesting in new seedings until 2-3 years when residual cover is established but that stands lose their value between 5 and 7 years after seeding (Kirsch, et al. 1973). Recent field studies by John Toepfer (personal commun.) and co-workers indicate optimum nesting values of CRP plantings in years 2-4, thus supporting Kirsch, et al. (1973).

The early availability of good residual cover provides a decided advantage to birds nesting in an area because it permits early nest initiation and increases the total period available for nesting which may be important to accommodate 1 or 2 renesting attempts if needed (Kirsch, 1969). Early nests have the potential to produce more offspring due to larger clutches of first nests and also broods could hatch before the rainy season, at least in Minnesota (Svedarsky, 1979). Furthermore, chicks hatching earlier in the season will be older going into the fall and may have higher survival because of it (John Toepfer, personal commun.)

Covers vary with regard to their ability to maintain structure due to compacting effects of winter snow. This is one of the primary reasons why cover rejuvenation is critical in the North where substantial amounts of snow occur. Covers dominated by unmanaged Kentucky bluegrass, for example, tend to flatten more than native cover dominated by bunch grasses such as little bluestem. Little bluestem is commonly mentioned as a principal species at prairie chicken nest sites (Table 3). The importance of bunch grasses being resistant to snow flattening was emphasized by Brown (1966) in Montana.

Summary

1. Residual nesting cover is considered one of the key limiting factors throughout the range of the greater prairie chicken, but it must be viewed in the total habitat perspective. In order to nest, females must survive the 8-9 months between nesting seasons!
   

2. Nesting cover is variously described but vegetation that is dense close to the ground with a 100% VOR of 2.5 dm and structure similar to managed smooth brome is probably optimum. Some litter is desirable, perhaps 5 cm, but depths greater than 10 cm or coverage >25% and maximum vegetation heights over 50 cm are not desirable.
   

3. Woody vegetation should be minimized within prime nesting areas.
   

4. Nesting cover should be rejuvenated by disturbance management every 3 to 5 years with prescribed burning probably the best practice, whenever possible.
   

5. Nesting cover preferences throughout the range of prairie chickens are probably quite similar with reported differences more a matter of measurements selected by researchers rather than regional preferences of the birds.
   

6. Due to the greater production potential of early nests, management should focus to make them as secure as possible.
   

7. Research in Minnesota suggested that cool-season grass plantings lose nesting cover values sooner than warm-season grass plantings, in terms of litter coverage and VOR measurements.