Laura Erickson's For the Birds

Every year listeners send me letters and e-mails when they see a little bird, usually a warbler or Chipping Sparrow, feeding a huge bruiser of a baby that is almost twice as big as its parents. What they’re seeing isn’t a baby warbler or sparrow–it’s a cowbird.

Cowbirds, like European Cuckoos, are nest parasites. This means they lay their eggs in the nest of other species, and leave the incubation and child-rearing to the host. When the mother cowbird lays her egg, she tosses out one of the eggs already in the nest, so automatically the host mother’s nest productivity drops by 20- or 25%. Then because the cowbird selects nests of species that are smaller than herself, the cowbird baby ends up being larger than the host’s babies in the nest. This gives it an enormous advantage at feeding time, since birds cue in on the size of their babies’ mouths to decide which is the hungriest each time they return to the nest. Since the cowbird’s mouth is so huge to begin with, it has to be pretty stuffed before the mother bird will put food into a smaller mouth, so except in years of extreme food abundance, most or even all of her natural babies will die of starvation while the cowbird grows fat. A single cowbird egg in a nest is a significant detriment to that nest’s productivity.

Before people opened up America’s eastern forest, cowbirds were limited to the Great Plains, where they followed bison around, moving from place to place, and seldom parasitizing the same bird more than once in a lifetimes. They don’t live in deep forests, but do spend a lot of time in forest edges. Right now there is so much edge habitat for them now that they are far more numerous than in pre-settlement days. Kirtland’s Warblers and Black-capped Vireos would probably be extinct today if for the past few decades cowbirds hadn’t been aggressively trapped and removed in the small areas where these critically endangered species live.

Some species are more vulnerable to cowbirds than others. Cedar Waxwings nest in exactly the same edges that cowbirds invade, but their population has increased over the past 20 years despite cowbirds. Waxwings have two advantages. First, they’re excellent at recognizing cowbird eggs, especially early in their laying period, and if they detect a cowbird egg, they toss it out of the nest. Second, while almost all songbirds feed their nestlings a protein-rich diet of insects, baby waxwings eat mostly berries. If a cowbird does hatch, it simply can’t survive long on that low-protein, high carbohydrate diet. In the few documented cases of waxwings raising cowbird babies, most cowbirds died within a few days of hatching, and not one survived to become independent.

Yellow warblers are also good at detecting cowbird eggs, but their tiny bill is way too small to toss out a cowbird egg. So they often start over in the same spot, building up the sides of the old nest and placing a new nest floor right over the cowbird egg. This is like throwing the baby out with the bathwater, because their own eggs are also buried and lost, but they at least don’t have to start from scratch building a new nest, and they usually only lose a few day’s work.

Cowbirds will always be with us. They are interesting in their own right, and evolved their unique role due to unique evolutionary pressures. Little by little, other songbirds will evolve strategies to deal with them or they will decline, as always happens in nature, where change is the only constant.