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Have you heard this myth? A few hundred years ago brown-headed cowbirds (Molothrus ater) roamed the Great Plains of North America following herds of bison and the insects they stirred up. People called these small blackbirds buffalo birds.
Because bison are nomadic, buffalo birds had little time to settle down, build nests and raise chicks. Therefore, buffalo birds started depositing their eggs in the nests of other birds. They let these foster parents incubate, feed and raise their chicks. Later, as European settlers replaced forests with farmlands, buffalo birds moved east. In the East, people called them cowbirds, for the birds followed and even rested on cattle, eating insects associated with the herds.
This description makes a nice story, but it is not true. It is true that cowbirds were once called buffalo birds, and they did follow bison around – but they didn’t start putting eggs in the nests of other birds because bison are nomadic. Cowbirds have lived in North America for about 1 million years and have been putting eggs in the nests of other birds for a very, very long time – even in places without bison or cows. Females most often lay their eggs in the cuplike nests of perching birds. Because they do not waste energy building nests or feeding their young, female cowbirds have the vigor to lay as many as 40 eggs per season.
How does a cowbird sneak her eggs into the nest of another bird? She is patient and observant. She watches a bird fly by with nesting material. After it’s built a nest and laid one egg, the cowbird moves in. Early in the morning (often around 5 a.m.), when the parent leaves the nest to eat, the cowbird flies quickly to the nest and deposits one egg. Her egg is often bigger than the other egg, but some birds do not care. Each day, the cowbird continues in this manner searching for available nests, laying one egg early in the morning in a nest she spots. If she can’t find a nest, she will lay an egg on the ground; when an egg is ready to be laid, it is ready, whether there is an available nest or not.
Cowbird eggs develop inside a female cowbird’s body for longer than eggs in other birds. This means a cowbird egg does not need to be incubated as long and is often the first to hatch in a foster parents’ nest. Because the cowbird has often deposited her egg in the nest of a smaller bird, the large cowbird chick outcompetes the smaller chicks: It gets more food from the parents, grows faster and leaves the nest earlier – in about seven days. Despite its size, often bigger than the parents, the parents still feed the cowbird after it has left the nest.
How do young cowbirds learn cowbird ways and not the ways of their foster parents? It turns out that when cowbirds are about 20 days old – making them teenagers – they sneak away from their host parents’ territory as soon as it gets dark and spend the night gallivanting in fields with other adolescent cowbirds. They return to their host family’s territory just before daybreak to be fed and protected by the foster parents. After about 25 to 40 days, young cowbirds leave their host parents for good and join a cowbird flock.
Not all potential foster parents are amenable to having a cowbird egg deposited in their nest. The American yellow warbler often builds a new nest on top of a cowbird egg. In 1907, Neltje Blanchan sermonized the process:
Scarcely is [the yellow warbler’s nest] finished before the skulking cowbird watches her chance to lay an egg in it that she may not be bothered with the care of her own baby … [The] yellow warblers weave a new bottom to their nest, over the cowbird’s egg … Suppose the wicked cowbird comes back and lays still another egg in the two-storied nest: what then? The little Spartan yellow bird has been known to weave still another layer of covering rather than hatch out an unwelcome, greedy interloper to crowd and starve her own precious babies. Two and even three-storied nests are to be found.
In fact, yellow warbler observers have recorded as many as five nests atop the first nest – a cowbird egg in each of the buried nests.
One bird, the catbird, stands up to cowbirds, rejecting cowbird eggs 95 percent of the time. One study showed that cowbirds check back on their eggs. If they find a catbird has removed the cowbird egg, the cowbird ransacks the nest, forcing the catbird to build a new one. According to this study, the cowbird lays another egg in the new nest 85 percent of the time. (I’d make a political analogy, but you can do this without my help.)
Because of its behavior, people love to hate the cowbird. It is often blamed for things that are not its fault. For example, in 1971, there were only 201 adult male Kirkland’s warblers left. Scientists blamed the cowbird and began cowbird extermination programs. More than 153,000 cowbirds were killed – yet the number of Kirkland’s warblers stayed the same for 20 years. It was not until 1990 that the population began to increase.
What happened? It turns out that Kirkland’s warblers nest only in young jack pine forests six to 24 years after a fire – and in 1980, there was a big wildfire that created suitable habitat for them. So it is likely that human fire suppression efforts caused the population decline, not the parasitic behavior of cowbirds. Today, there were about 4,800 breeding Kirkland’s warblers because biologists have re-created suitable habitat for the birds.
Cowbirds are not evil; they are just doing their best to survive. Many other species prey on bird eggs and young birds – about 1 percent of all birds are parasitic, or about 100 species of parasitic birds out of 10,000 total bird species – but are not reviled. The cowbird’s evolutionary strategy has served it well. By not putting all of its eggs in one basket (literally), it is better able than most other species to withstand changes in the environment. If one species of host birds declines, the cowbird will put its eggs in other nests.
Despite this survival strategy, cowbird populations peaked in the 1970s. They’re declining most likely due to habitat destruction and environmental changes.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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