Illinois Natural History Survey - University of Illinois

Cowbird Parasitism in Different Habitats

The Brown-headed Cowbird is one of the most abundant, ubiquitous, and widespread birds in North America. Because cowbirds are brood parasites that lay 30 to 80 eggs per season in the nests of other species, their abundance can pose a severe problem for many of their 200 or more host species. In the forests of Illinois, for example, 30-90% of the nests of most migratory songbirds are parasitized, often with more than one cowbird egg. The combination of high levels of nest predation and cowbird parasitism reduces nesting success of many species below levels necessary for population maintenance. This well-publicized phenomenon is the basis for management recommendations aimed at enlarging forest tracts to reduce access to cowbirds, which feed in pastures and row crops rather than in the forest.

Mature Brown-headed cowbird.

When we expanded our studies of songbird nesting success to grasslands and shrublands, we expected to find similar problems with cowbird parasitism. Much to our surprise, however, we found virtually no parasitism in grasslands and low levels of parasitism in some but not all shrubland habitats. Less than 10% of the nests of most grassland species were parasitized across a wide variety of sites including the Midewin National Tallgrass Prairie, Goose Lake State Park, the Des Plaines Conservation Area, Nachusa Grasslands, and the Savannah Army Depot. Shrubland species that do not reject cowbird eggs were heavily parasitized in some sites (e.g., Lake Shelbyville and Middle Fork Wildlife Area), but not in others (e.g., Shawnee National Forest, parts of the old Joliet Army Ammunition Plant). Over 90% of Orchard Oriole (Icterus spurius) nests, for example, are parasitized with an average of 2.2 cowbird eggs per nest at Lake Shelbyville, whereas in Shawnee National Forest, less than 40% are parasitized with an average of only one cowbird egg per nest.

These results raise interesting questions for conservation and evolution. Cowbird parasitism is not a major conservation issue for grassland birds in Illinois; parasitism levels are significantly higher near shrubs that cowbirds can use as perches to search for nests, but even near woody vegetation, the majority of nests escape parasitism. Even in grazed grasslands in which cowbirds forage, only a few nests are parasitized. In contrast, the majority of nests of birds in adjacent forest habitats are parasitized.

For shrubland birds, parasitism levels appear to vary with the landscape context. In heavily forested landscapes, most shrubland species are parasitized only at moderate levels (10-50%) compared with forest species (30-90%). In landscapes with little forest cover, parasitism levels of shrubland birds are comparable to those in forest habitats.

Cowbird eggs.

Why do cowbirds appear to prefer forest hosts over shrubland hosts? Part of the answer may lie in the proportion of hosts in each habitat that have effective defenses against cowbirds. Many shrubland species eject cowbird eggs from their nests (e.g., Gray Catbird, Baltimore Oriole, Warbling Vireo, American Robin, Brown Thrasher), abandon most parasitized nests (e.g., Yellow Warbler, Bell's Vireo, Field Sparrow), or aggressively mob cowbirds that approach their territories (e.g., Eastern Kingbird, Willow Flycatcher, Red-winged Blackbird). Because cowbirds do not appear to discriminate among these unsuitable and suitable hosts, their nesting success is probably much lower than it is in forest habitats in which none of their hosts have effective defenses. These problems for cowbirds are further exacerbated by the much higher nest predation rates experienced by most shrubland birds compared with their forest counterparts. Cowbirds therefore may avoid shrubland habitats simply because their own eggs have much less chance of producing eggs in shrublands than in forests.

We know much less about the defenses of grassland birds against parasitism, and are just beginning to gather data on predation rates for comparisons with forest habitats. Grasslands with few perches simply may be too difficult for cowbirds to search, especially given the extremely cryptic behavior of grassland birds near their nests.

Cowbird fledgling.

The high proportion of unsuitable cowbird hosts in shrubland habitats suggests that these open-country hosts have a long co-evolutionary history of coping with cowbirds. The lack of comparable defenses in forest birds suggests a more recent exposure to cowbird parasitism. The evolution of defenses in forest species may be further slowed by the existence of large forest areas in which cowbirds are very rare and host nesting success is high. Any strong selection for host defenses in small woodlots in which nesting success is low and parasitism levels are high may be swamped by excess production of young from areas with little or no cowbird parasitism.

The next steps in our research program are to study the behavior of cowbirds to determine if they avoid shrubland habitats and to determine if grassland hosts have defenses against parasitism. An exciting possibility is that some grassland species may be in the process of evolving defenses against parasitism.

Scott K. Robinson and James L. Herkert, Center for Wildlife Ecology

Charlie Warwick, editor

Illinois Natural History Survey

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