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Cowbird Parasitism in Grassland and Cropland
in the Northern Great Plains

Discussion


As the region with the highest concentrations of breeding cowbirds, the northern Great Plains might be expected to show parasitism that has a major influence on the reproduction of host species. Increases in the number of cowbirds in the northern Great Plains between 1966 and 1991 (Peterjohn et al. Chapter 2, Wiedenfeld Chapter 3) may be related to increased coverage of trees and brush. Current data are insufficient to evaluate the importance of brood parasitism as a factor contributing to population declines of grassland birds. Cowbirds have coexisted with their grassland hosts in this region for a long time. Mayfield (1965) reviewed the scanty data available in the mid-1960s and found that the only species characteristic of the north-temperate grassland with over 30% parasitism was the Dickcissel (Spiza americana, 31%).

Recent data, however, indicate that about half of grassland species in the Great Plains have moderate frequencies in the 10-30% range, and nearly half have values above 30% (Tables 1 and 2). All grassland birds in Illinois, in contrast to the general results from the Great Plains, had parasitism frequencies below 20% (see Robinson et al., Chapter 33, this volume). Two of the lightly parasitized populations were in southwestern North Dakota (George et al. 1992, Table 1). Red-winged Blackbirds can be moderately or heavily parasitized in uplands but are usually lightly (under 10%) parasitized in wetlands (Friedmann 1963, Fleischer 1986).

The long period of coexistence of Brown-headed Cowbirds and their grassland hosts suggests that host species may have evolved tactics, such as removal of cowbird eggs or desertion of nests, that reduce the effects of parasitism on host populations (Mayfield 1965). Among grassland birds, only the Lark Bunting has been reported to remove cowbird eggs (Hill 1976), but the overall frequency of this behavior seems to be low. Nest desertion because of parasitism has been reported in Dickcissels (Elliott 1978, Zimmerman 1983), Lark Buntings (Hill 1976), and Clay-colored Sparrows (Fox 1961, Knapton 1979), but its frequency also seems to be low. Furthermore, desertion may have multiple causes and is not an effective response to parasitism in all situations (Grzybowski and Pease, Chapter 16, this volume). The moderate or heavy parasitism among these grassland species suggests that their productivity is often reduced, but further analysis and different data are needed to estimate the magnitudes of these effects (see Grzybowski and Pease Chapter 16).

We documented moderate or high frequencies of parasitism in south central North Dakota. The effects of moderate or heavy parasitism on reproductive rates of most grassland species are poorly known, largely because they are difficult to determine (Smith 1981, May and Robinson 1985). It is thus possible that parasitism is contributing to population declines of the Dickcissel, Lark Bunting, Clay-colored Sparrow, Grasshopper Sparrow, Bobolink, and Eastern Meadowlark (Sturnella magna)(Peterjohn and Sauer 1993). If this is confirmed, land managers may want to explore options to reduce the frequency of parasitism. Habitat management is one option that could be used to reduce parasitism of grassland birds. We found variation in the frequency of parasitism across the three habitats we examined. We speculate that much of this variation is related to the cowbird/host ratio and to two habitat features, nest visibility and availability of perch sites for cowbirds (see also Davis and Sealy, Chapter 26, this volume). These habitat features may affect frequency of parasitism directly or indirectly through the cowbird/host ratio (Figure 1).

Figure 27.1
Figure 1.   Schematic diagram of factors that may affect frequency of Brown-headed Cowbird parasitism.

The high cowbird/host ratio in the southern study of natural grassland fields may be due to the prevalence of cattle pastures in this area. The preferred foraging habitat of Brown-headed Cowbirds is short grass, which is often in pastures (Mayfield 1965, Rothstein et al. 1984). Cowbirds are capable of commuting long distances between laying sites and foraging sites if these sites are limited (Rothstein et al. 1984). In the southern area, however, laying sites were often in grassland next to or including feeding sites, making short commuting trips possible. Cowbirds may be attracted to pasture-rich landscapes and may reach higher abundance in these than in nearby landscapes with fewer pastures.

Visibility of nests and host females to female cowbirds is probably related to the height and density of the vegetation at or near the nest site. Of the habitats we examined, seeded grassland generally had the tallest and densest vegetation. Natural grassland fields were generally similar except in the southern study, in which some fields were grazed in some years. The most open habitat we examined was cropland, in which several species nested before the crops grew tall. Horned Larks, which were heavily parasitized, use open areas for nesting. Thus, a ranking of nest visibility in the three habitats corresponds to a ranking of the mean frequencies of parasitism. Furthermore, in natural grassland, birds in fields that probably had the highest visibility (that is, the grazed fields in the southern study) experienced a higher frequency of parasitism than the birds in the northern study.

The availability of suitable perch sites for female cowbirds searching for host nests (Norman and Robertson 1975) was presumably lowest in seeded grassland, which had little brush but had some sweet clover and tall forbs. The birds in seeded grassland had the lowest overall frequencies of parasitism. Availability of perch sites was highest in natural grassland, the only habitat with a substantial amount of brush. The cropland fields generally had few perches, except for the fallow fields that were farmed organically. These fields consisted entirely of sweet clover, which is sturdy enough to be used for perching and was the primary nesting habitat of Lark Buntings. This species was heavily parasitized (61%, Table 2).

The ratio of cowbirds to hosts, visibility of nests, and availability of perches are probably not the only factors that affect frequencies of parasitism. High nesting density of a colony of host species (e.g., Red-winged Blackbirds in wetlands) may constitute a defense against cowbird parasitism (Robertson and Norman 1976; Carello and Snyder, Chapter 11, this volume). Nesting phenology can reduce the likelihood of cowbird parasitism in some species (Middleton 1977), as indicated by our data on Horned Larks. Host-species preferences of cowbirds may also have an effect (Fleischer 1986). With the possible exception of the first, however, these factors are not amenable to management.

The correspondence between the cowbird/host ratio and frequency of parasitism suggests those effects of habitat on parasitism act in part by affecting local cowbird abundance (Table 2). Other studies have found that cowbirds respond to changes or differences in habitat structure. Wiens (1963) reported higher frequencies of parasitism in a study area after several years of growth of low, brushy vegetation. He suggested that cowbird numbers increased relative to hosts as a result. Lowther and Johnston (1977) concluded that the prairie-shrub successional stage provided better breeding conditions for cowbirds in Kansas than do forests or extensive grasslands or prairie.

Results from these studies suggest that local densities of cowbirds in grassland habitats can be reduced by managing brush. Wiens (1963) found a lower frequency of parasitism in areas far from brushy thickets. Johnson and Temple (1990), who studied remnant tracts of tallgrass prairie, found lower frequencies of parasitism far from wooded edges and suggested that prairie fragments be as large as possible to minimize edge and that woody vegetation be removed. Managers should realize, however, that reducing brush probably affects more than cowbird densities. Reducing brush may reduce the population density of Clay-colored Sparrows (Knapton 1979) and other species that commonly nest in low brush.

Managers could also increase or maintain the amount of the landscape with seeded grassland, the habitat in which we observed the lowest frequency of parasitism. Many seeded grassland fields we studied were CRP fields. When this cropland retirement program ends, landowners and managers will have the option of returning these seeded fields to crop production. Our data suggest that such a conversion of habitat may increase the overall frequency of parasitism in the northern Great Plains, even if cowbird abundance does not change.

Another management option is to reduce the intensity of grazing by livestock. Knapton (1979) observed that parasitism declined in one study area after cattle were removed and suggested that visibility of nests decreased as vegetation gained height. In addition to decreasing visibility of nests, lower numbers of livestock may reduce cowbird abundance in the landscape by reducing the amount of short grass that cowbirds require for foraging. In addition, fewer cows will reduce foraging opportunities afforded directly by the presence of livestock (Mayfield 1965; Rothstein et al. 1980, 1984). However, reduced cowbird abundance after a reduction in livestock numbers has yet to be demonstrated, and it may only occur under extreme levels of stock reduction. A small amount of short grass in the landscape may meet the needs of many cowbirds (Mayfield 1965).

All management options have costs as well as benefits. Increased understanding of the relative importance of factors that affect parasitism in the northern Great Plains may require an experimental approach at the local and at the landscape scales.


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