I recently posted this photo to Facebook – a yellow warbler feeding a cowbird baby. A good friend replied, “I disdain the parasitic cowbirds.”
I so get that. Cowbirds make me crazy, too.
For those who don’t know, cowbirds are “brood parasites.” Or deadbeat parents. That is they lay their eggs in the nests of other birds – about 220 different species – then leave the raising of their young to others.
Some bird species recognize cowbirds eggs and push them out of the nest, or puncture their shells with their beaks. But most species are unable to recognize cow bird eggs. Or, there is new evidence that suggests that some species recognize the foreign eggs but accept them in order to avoid having their nests being destroyed by cowbird parents as a form of punishment for not raising cowbird young.
In any event, cowbirds eggs hatch faster. The foster parents, for whatever reason, feed whichever gaping beak is in their nest. As the bigger cowbirds gain strength, they frequently push the other eggs out of the nest or smother their nest mates in the bottom of the nest.
To watch a tiny yellow warbler, which weighs about a third of an ounce, feed a young cowbird is to watch an over-worked parent fill the gaping maw of real-life Baby Huey that will grow to five times its size.
How did cowbirds evolve this way? Blame it on the buffalo. Cowbirds are native to North America, and co-evolved with the massive bison herds of yesteryear. Cowbirds would follow the herds and feast on the bugs stirred up by the grazing bison. But because bison are nomadic, when they moved on, the cowbirds were forced to move with them. Which meant that someone else would have to watch over their young. And so some clever bird figured to lay its eggs in the nest of some other mother bird.
Once the great nomadic bison herds were eliminated, cowbirds kept to their bad parenting ways, and pose a significant threat to song bird and grassland bird populations, which are facing numerous other threats to their long-term survival.
That’s another reason why Midewin is so important. One of the most effective ways of controlling cowbird parasitism is to restore large landscapes, which minimizes what is known as “edge habitat.” Cowbirds prefer forest edges, which provides them ready access to the nests of many grassland bird species. But restoring Midewin’s 19,000 acres to native tallgrass prairie, eliminating the old hedgerows and volunteer stands of weed trees, will greatly reduce the ability of cowbirds to prey on the nests of unsuspecting birds.
Especially with the recent reintroduction of bison to Midewin, there are certain to remain some native cowbirds as part of the prairie ecosystem. But Midewin is big enough – sometimes size really does matter – to provide balance among all of the many different kinds of birds, mammals and plants of the native tallgrass prairie.