Laura Erickson's For the Birds

One of the most fascinating birds on the planet is the cowbird, unique among North American birds in being a nest parasite. Like European cuckoos and tropical cowbirds, our Brown-headed Cowbird lays its eggs in the nests of other, usually smaller species, and leaves caring for the nestlings to those foster parents. Female cowbirds probably expend just as much energy and time on reproduction as most birds, only in their case it’s in searching out nests of other birds and producing a huge number of eggs—reportedly occasionally as many as 40 eggs a season!

Brown-headed Cowbirds were once closely associated with American Bison, feeding on insects churned up by the bison and seeds and insects in the soil, which the cowbirds couldn’t reach without the bison’s heavy hooves cutting through the thick prairie sod. Bison are nomadic, and so if cowbirds built their own nests and raised their own babies, they and their young would be in big trouble if partway through a nesting cycle the bison suddenly wandered away. Laying their eggs in the nests of birds that can acquire food regardless of the presence of bison allowed cowbird babies to survive whether the bison remained in an area or not during the nesting season.

Female cowbirds apparently have three techniques for finding nests: they may perch in the tops of shrubs or trees and silently watch for nest building activity in surrounding open areas; they may walk on the ground, looking about and watching movements of other birds; or they may make noisy, short flights and land in leaves, flapping their wings as if intentionally trying to flush potential hosts from nests.

When a female cowbird finds a nest, she tosses out one host egg just before laying her own, meaning there’s about a 25% reduction in nest productivity right off the bat. Then, because cowbirds usually select nests from smaller species and because cowbirds have only a 9- or 10-day incubation period, and because the larger cowbird egg holds the mother a bit of a distance from her own eggs, so they aren’t kept quite as warm as they should, the cowbird generally hatches at the same time or earlier than the other nestlings. Cowbird nestlings aren’t particularly aggressive, and unlike European Cuckoos they don’t throw out eggs or nestlings in the nest with them. But they’re bigger than the other nestlings, and extremely effective at eliciting feeding behavior from their foster parents. When food is abundant, one or more natural nestlings can survive with a baby cowbird, but when food is scarce, the cowbird is usually the only survivor. Some cowbird hosts such as Chipping and Song Sparrows renest, but most neotropical migrants, like warblers, nest only once a season, so if they lose all their own young in one nest, that’s it for their reproduction for a whole year. Once the cowbird can find its own food, it wanders away from its exhausted foster parents and instinctively joins cowbird flocks.

When cowbirds were strictly associated with bison, few foster parent birds were likely to host baby cowbirds more than once in a lifetime because the bison roamed so much. There were also plenty of limiting factors preventing cowbirds from getting too abundant, including difficulties obtaining food in winter and limits of bison distribution, so the species that hosted cowbird young apparently did fine despite occasional parasitism. But when we wiped out the bison, we introduced cattle, and agriculture and development provided plenty of disturbed soil for cowbirds far away from large mammals anyway. Now, in the 21st century, we’ve eliminated virtually all of the natural restrictions on cowbird numbers even as we’ve made life increasingly difficult for the songbirds that serve as their hosts. Much as I love cowbirds for their lovely liquid musical notes and their exuberant mating rituals, when I was in Oklahoma this month I got some insights into just how huge the impact of cowbirds is on their host birds. Tomorrow I’ll talk about why cowbird control is so critical for several endangered songbirds, and why I think we need to initiate some serious studies to find out whether cowbird control won’t be of benefit for declining but not-yet endangered birds.