WESTCHESTER COUNTY, N.Y. — Parents love all their children, even the annoying ones, and the same holds true for birds. Some birds make their way in life by conniving against their trusting fellows. To us, this may seem mean-spirited. But in the Darwinian scheme of things, they have simply found a profitable economic niche.
Take the brown-headed cowbird, commonly found in backyards. It doesn’t build a nest of its own. Instead, females watch for the unguarded nests of other species. When they find one, they zip in, lay their eggs, and fly away. The surrogate parents incubate all the eggs — their own and the cowbirds’ — and feed all the nestlings when they hatch. Cowbird chicks grow faster than their nestmates do. Typically, they grab most of the food the parents bring, often leaving the “real” babies to starve.
Cowbird eggs have been found in the nests of some 220 species. At least 140 of these species raised the nestlings as their own. This trick is contributing to the demise of some threatened birds, such as the Kirtland warbler. Other birds fight back. Yellow warblers and blue-gray gnatcatchers will sacrifice their existing eggs and start again with a new nest. Robins, gray catbirds and brown thrashers will kick out the cowbird egg.
Cowbirds became parasites by necessity. They fed on insects that lived on and around buffalo, and had to follow the herds to survive. They had no time raise their own young, so they figured out a way to let other species do it for them. Enough birds are fooled for the cowbirds to thrive.
Blue jays, beautiful as they are, behave badly, too. They’ve been known to eat the eggs and nestlings of other birds. Given the chance, crows and starlings will do the same.
Cooper’s hawks piggyback on bird enthusiasts who put up birdfeeders. They’re forest dwellers that can make startling, rapid bursts of flight. A Cooper’s hawk will find a perch from which it can watch a feeder without being seen. When the titmice and chickadees come for their breakfast, the hawk will suddenly swoop down for a breakfast of its own. You can bet there’s a Cooper’s hawk in the vicinity if the birds at your feeder suddenly vanish and silence reigns in your yard.
It’s the age-old evolutionary story. Animals learn how to use the resources at hand, while expending the least amount of energy. Strong individuals live while the weak are weeded out. Species that adapt will survive, while the others, unfortunately, may perish. Cruel as it may seem at the individual level, you are watching nature trying to maintain a healthy balance.
John Hannan is Audubon’s director of development for Audubon in Connecticut.