All birds build nests, right? Well, no, that’s not always correct! Most megapodes, for example the ostrich, make rudimentary scrapes in the soil, or seabirds lay their eggs on bare rock ledges, letting the shape of the egg keep the young safe. Some ducks such as mergansers will sometimes lay their eggs in another merganser’s nest and let that female do all the work, while Emperor penguins are famous for using their feet as the “nest” to keep the eggs off the frozen ice.
But one species goes even further, and not only doesn’t build a nest but also doesn’t even try to raise its own young, relying on other species to keep its offspring safe and fed. I won’t keep you in suspense, it is the Brown-headed Cowbird. Why do they do this and risk the future of their species? Are they too lazy or just poor parents?
The cowbird was closely linked to plain’s animals such as the North American bison. Originally known as Buffalo birds, the cowbirds followed the wandering herds feeding on insects unearthed by the cumbersome beasts. It was hard to raise young when you were constantly on the move, so over time, as livestock replaced bison, they learned to rely on other species to raise their young! This is a common phenomenon known as brood parasitism, where one species lays its eggs in another’s nest and let’s the new host raise the baby.
Across North America, over 220 species have been used as hosts, with 144 of them successfully raising young cowbirds. The sad part is, the baby cowbirds hatch quickly, grow fast and eat a lot. So even if the natural nest inhabitant was allowed to hatch, the cowbird out-competes it for food and usually only the cowbird survives. Sometimes the eggs or nestlings of the host species are tossed out of the nest by the cowbird, increasing the chance of survival of the young cowbird even more!
But how do they find nests and make this happen? Recently, I observed a cowbird in our yard watching a nest-building house wren, and when the opportunity presented itself, the cowbird dashed in and tried to lay her egg in the wren’s nest when the wren was absent.
The female cowbird is a patient bird and will perch high in a tree, or walk slowly in suitable habitat, and watch what’s happening around her. If she sees signs of nest building she will wait until at least two eggs are laid by the host and then lay her own eggs in the nest. She may initiate noisy short flights, accompanied with wing flapping, to flush potential hosts from nests. The male may also distract the host female’s attention or chase her away, and the female cowbird then rushes in and lays her eggs.
Although a female can lay up to 40 eggs per year, she generally only lays one egg per nest, as the host is usually smaller than her and can’t raise many baby cowbirds.
The female cowbird will often eat land snails or the eggshells of the host species to get calcium, since she lays so many eggs! The most common species in our area affected by brood parasitism are Yellow Warbler, Song Sparrow, Red-eyed Vireo, Chipping Sparrow, Eastern Phoebe, Eastern Towhee, Ovenbird, Common Yellowthroat, American Redstart, Indigo Bunting, Red-winged Blackbird, Willow Flycatcher and Field Sparrow. Some species, like Kirtland’s Warbler and Black-capped Vireo, have been so negatively impacted by cowbirds they were pushed to edge of extinction and intensive efforts to control cowbirds within their breeding ranges were needed to save them.
Sometimes birds recognize the cowbird and take evasive action, such as trying to chase it away. But they are rarely successful. In some cases, as with the Yellow Warblers, once the host species recognizes the cowbird eggs in its nest, it will abandon the nest and immediately start a new one elsewhere or build a new nest directly on top of the old nest.
Nature can be a tough place to live!
Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line, at www.avocetnatureservices.com and on LinkedIn and Facebook.