Field Notes—

The Cowbirds’ Dark Rites Of Spring

By Curtiss Clark

A joyful society of birds has gathered in our neighborhood for their rites of spring, and the trees around the feeders are full of travelers, some just arrived, some just about to depart.

The cool weather has kept the winter guest juncos around even as April slips away, but it hasn’t deterred the redwing blackbirds from getting started on their accommodations for the summer. The male redwings arrived from the south more than a month ago, and the females are just now beginning to show up, complaining, I’m sure, about how little has been done by their slacker mates to find a nest site and secure a territory.

For all their full-throated singing at this time of year, inspired by the imperatives of the mating game, all of them have a lot of work ahead: building nests, incubating eggs, feeding ravenous nestlings, and fending off threats from hawks, and raccoons, and even snakes. All of them, that is, except the cowbirds.

While all the other species are out building their nests, the cowbirds are sitting around watching — very carefully. They don’t build nests, they don’t sit on eggs, they don’t even feed or raise their own young. They avoid all this work by being consummate opportunists… and thugs.

Cowbirds can be found all over the continent now, but before Europeans settled North America, their range was limited to the open grasslands of the Great Plains. They would follow the vast herds of itinerant bison and feed on the swarms of insects flushed from the grasses by the animals. Because of this migratory existence, they had no time for nesting and raising their young. They made do by laying their eggs in the nests of other bird species and just kept moving with the herd.

As settlers overspread the continent, they slaughtered the bison and replaced them on the plains with their own livestock. Forestland across North America was cleared for farming, and in the past century great swaths of woodland have been converted into suburban landscapes, opening up supportive habitats for cowbirds in areas where bison never dreamed of roaming.

Though the need to migrate in tandem with bison herds has disappeared, cowbirds still migrate north and south with the seasons, abandoning the northern extreme of their range in the winter for the southeastern US, southern California, and Arizona. But they are back, now, watching other birds build their nests, waiting for the first egg to appear so they know the nest won’t be abandoned. Then they lay one of their own there beside it and move on to find another nest for their next egg.

Female cowbirds will lay up to 40 eggs to compensate for the relatively low three percent survival rate for their foster-care young. Researchers have counted 220 bird species that are parasitized by cowbirds in this way.

In Europe, cuckoos do the same thing. But to ensure a greater rate of acceptance for their eggs, cuckoos will lay their parasite eggs only in nests that have eggs of a similar color, so the intruders’ eggs aren’t so obvious to the hosts. Cuckoo eggs come in various colors, but each individual female lays eggs of one color only. Consequently, each female must find the nest of a host species with similarly colored eggs.

Cowbirds don’t rely on such subterfuge. It sounds like too much work. Intimidation is a lot easier.

Dr Jeffrey Hoover, an avian ecologist at the Illinois Natural History Survey in Champaign, and fellow researcher Scott Robinson of the University of Florida, have found that when they removed cowbird eggs from the nests of warblers, the cowbirds would return and destroy the host nest and any remaining eggs in it. It turns out raising a larger, hungrier, cowbird nestling is the price the warblers have to pay for protection from the cowbird mobsters.

In an article published by the Proceedings of the National Academy of Sciences, Dr Hoover reported that the warblers that accepted the cowbird eggs produced, on average, three of their own offspring. The average success rate for warbler nests where the cowbird egg is rejected is just one offspring. The evolutionary track of the warblers now has an incentive for foster care.

It should be noted that female cowbirds run this protection racket. The males aren’t involved. It’s a matriarchal mafia.

So the society of birds in my neighborhood may not be as joyful as it sounds. There is a dark side to their rites of spring. The cowbirds are back in town, and nobody dares look the ladies in the eye.