It was immediately clear that something was amiss. Above my head, a male yellow-rumped warbler was tending a youngster nearly twice his size. Wherever the warbler flew, the fledgling followed, demanding to be fed. This poor dad had been duped into raising a brown-headed cowbird.
Brown-headed cowbirds breed throughout North America, save for Alaska. Another species, the shiny cowbird, is a South American breeder, though a small population has become established in the Florida Everglades. Bronzed cowbirds are also tropical, with a range that extends into the southernmost portions of Texas, New Mexico and Arizona. All cowbirds lay their eggs in the nests of other birds. It’s called brood parasitism.
Dozens of species worldwide have adopted this reproduction strategy, but the brown-headed cowbird is the North American champion. However, their actual success rates are low. Many songbirds now know when they’ve been suckered, and they have evolved countermeasures. As a result, the female cowbird might lay up to 40 eggs per year. If only 5 percent survive to adulthood, the cowbird population is sustained. She can lay that many eggs because she does not have to withstand the rigors of raising a family. All of her energy goes into egg production.
Also, because neither parent plays a role in raising the chick, pair bonds among male and female cowbirds are rather weak. She may mate with many different males on the way to laying that many eggs.
Evolution is a race to see which species adapts more quickly. Cowbird chicks are genetically engineered to grow bigger and faster than their nest mates. If they don’t outright kill their foster siblings, they demand most of the food, accomplishing the same objective. Over many generations, some host species have figured this out. When they spot an egg that is not like the others, larger birds will push it out of the nest. Brown thrashers are adept at this defense, and experiments have shown that gray catbirds successfully eject a cowbird egg 95 percent of the time. Smaller birds may simply give up and build another layer of nest over the first clutch of eggs and start again. Yellow warblers aren’t very clever about hiding their nests, but they are very clever about recognizing a cowbird egg and adding a layer.
Unfortunately, many species don’t recognize foreign eggs. The Kirtland’s warbler is a threatened species confined mostly to the jack pines of northern Michigan. In order to keep this species from going extinct, state biologists have had to resort to trapping and killing cowbirds in the area.
Brown-headed cowbirds have been documented invading the nests of over 200 species, including ridiculously small hummingbird nests. Some foster parents are better providers than others. Finches feed their young a largely seed-oriented diet, which is contrary to the big protein load that cowbirds need in order to outgrow their siblings. House finch nests are easy to see, and are often victimized. But cowbird chicks usually can’t survive the vegetarian diet.
War is hell. A study conducted 35 years ago suggested that cowbirds remember where they laid their eggs, and if the hosts reject the egg, the cowbird retaliates by dismantling the host nest. When the host rebuilds, the cowbird watches where and tries again.
Two questions immediately spring to mind. First, how did cowbirds come to think this reproduction strategy was a good idea? Cowbirds were originally birds of the short-grass prairies, where they followed buffalo herds. Only later, when Americans converted forests to pastures, did cowbirds spread east. A bird that needs to move with its food source may find it convenient to let another stationary species raise its young. Furthermore, there are many species that sneakily lay their eggs in nests of their own kind. It’s not a big evolutionary jump to figure out how to lay eggs in the nests of another species.
Second, how do baby cowbirds raised by foster parents later figure out how to be cowbirds? How will this cowbird baby I am watching discover it is not a yellow-rumped warbler? Studies show that cowbirds do not learn the songs of their foster parents. Instead, they are genetically hard-wired to recognize and associate with the vocalizations of other cowbirds, just as soon as they are old enough to be on their own. This process must happen quickly, though. If the baby does not find its own tribe soon after fledging, it may never catch on. That could mean years of psychoanalytical therapy.
Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at firstname.lastname@example.org.
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