In response to an email from my 9-year-old budding birder nephew, I prepared the following essay on cowbirds. I hope you find it informative:
Cowbirds get a lot of their food in the summer from snatching insects that get kicked up by the hooves of large mammals. Before Europeans settled North America there were no cows here, and “cow”birds followed herds of bison. (They probably also followed herds of mammoths, camels, and ground sloths a few thousand years ago!) Bison are always on the move, and to keep up with them, the cowbirds had to move too. They didn’t have the luxury of settling down in one spot for the 4 – 6 weeks it takes to find a mate, make a nest, lay some eggs, incubate the eggs, feed the babies in the nest, feed the babies out of the nest, etc. So they started doing something interesting – laying their eggs in the nests of other birds.
Female cowbirds at this time of year are expert nest-finders. They quietly watch other birds that are in the process of laying eggs in their own nests. Birds lay one egg per day and then start incubating when all their eggs have been laid. (That’s so all the babies hatch at the same time.) If a meadowlark, for example, normally lays 5 eggs in its nest, it’ll take her 5 days to do that. Watchful cowbirds are tricky. They find nests during that 5-day “laying” period and sneak in to make a switch. The mother cowbird will remove one of the meadowlark’s eggs and then lay one of her own in its place! If you occasionally see an egg lying around without a nest nearby, it’s probably an egg that a mother cowbird has stolen and discarded from some other bird’s nest.
Cowbirds babies are tricky too. They generally hatch more quickly than other birds, so for a day or two, there is one hungry cowbird chick in the nest getting 100% of the food the parents bring to the nest. After a couple of days of that, the young cowbird is way bigger than the tiny hatchlings with which it shares its nest, and it continues to out-beg, out-reach, and out-grow them until it’s ready to leave the nest days before the other baby birds. Many of the host babies starve in the nest because the cowbirds get all the food.
After she laid her egg, the mother cowbird would move on to follow the bison. When she was getting ready to lay another egg, she’d find another nest for it in a new area and repeat the process. Cowbirds have been known to lay their eggs in the nests of at least 220 different species of birds. Mother cowbirds can lay as many as 30 eggs in a season – at least twice as much as most other songbirds! But she has to be able to lay a lot to be successful. That’s because some birds recognize cowbird eggs in their clutch and do things like throw out the cowbird egg, or abandon the whole nest and start another one nearby. But other species simply accept the baby cowbird as one of their own. Larger birds that can find plenty of food for a growing cowbird chick will often raise 1 or 2 cowbirds and 2 or 3 of their own young in the same clutch. This cuts into the reproductive success of the host bird, but the effect is more dire on smaller species that have trouble finding enough food for the monster in their nest. When a lot of mother cowbirds converge on the same area to find nests for their eggs, it can really be a burden for some of the host birds. One scientists in Illinois concluded that in most of that state, Wood Thrushes were only raising cowbirds! As a result, Wood Thrush numbers went way down in Illinois.
Cowbirds are also a serious problem for populations of some endangered species, like Black-capped Vireos in Oklahoma or Kirtland’s Warblers in Michigan. Some biologists trap and remove cowbirds from nesting areas for these rare species. It’s not really clear how effective these cowbird control programs are, but they are probably helping those rare species at least a little bit.
Kirtland’s Warbler. Photo by Jim Ownby.
Why are cowbirds such a problem? Us.
We removed bison from the landscape and brought in cattle. Then we put the cattle in fenced pastures. Now the cowbirds had access to animals like bison that stayed put! The cattle didn’t rove around, so the cowbirds stayed put too. So the same mother cowbirds were now hanging around the same area all spring and summer, and attempting to lay eggs in the nests of the same hosts over and over again. To find nests that didn’t already have someone else’s cowbird egg in them, the mothers had to fly farther and farther away from the cattle pastures. Instead of just leaving eggs in the nests of other grassland birds or host birds in scrubby fields, the cowbirds began laying eggs in nests of host birds even in deep forests. Most of those forest birds had no defense against cowbirds, so they’ve really been paying a price in terms of lost reproductive success.
The other thing we’ve done is cut down forest to make more pasture. This has allowed cowbirds to become abundant even in parts of the country like New England that didn’t have roving herds of bison when Europeans landed at Plymouth Rock. We also started growing corn on an enormous scale, much of it to feed our cattle. Cowbirds love corn! Now we’ve made it even easier for cowbirds by supplementing their food.
So the story of cowbirds is complex and exciting. It involves their own unique evolutionary history combined with our modern quest for cheap hamburgers. A lot of people who love birds think cowbirds are evil. I think they’re fantastic, even if in some cases we’ve let them become too abundant. That’s our fault, not theirs. The cowbirds are just doing what they are beautifully adapted to do.
There’s still much research to be done on cowbirds. My favorite question is this: How do the baby cowbirds, raised by parents of so many different species, figure out that they are cowbirds? By late summer, the baby cowbirds join in flocks with other cowbirds. Why do they join those flocks, and not flocks of the species that raised them? This gets at the heart of imprinting and song learning research, as well as group behavior in general. Maybe some bright young ornithologist will figure this out some day . . .
Truly excellent posts on cowbirds and their influence (including key literature references) may be found here.