This story was originally published in June 2023.
In a way, it’s hard not to admire the female cowbird. This is a bird that has evolved to not only never need a nest, but also to have other birds raise its young.
In the bird world, this is known as brood parasitism. Instead of building a nest, the female cowbird puts all of her energy into producing eggs that she lays in the nests of other bird species.
This is happening now in Maine, according to a bird expert with Maine Audubon. It is not uncommon at this time of year to spot the much larger egg of a cowbird tucked in with the tinier eggs laid by the oftentimes smaller bird who built the nest.
This can be a hard thing for bird lovers to see. Once the cowbird lays her eggs, she abandons them to be raised by the other bird, often at the expense of the host bird’s own chicks.
That kind of natural behavior can be difficult for people to accept, according to Doug Hitchcox, staff naturalist with Maine Audubon.
“What they are doing is what they should be doing,” Hitchcox said. “From a naturalist’s perspective, it is part of nature.”
The brown-headed cowbird is the variety found in Maine. They often select nests of birds smaller in size that build cup-shaped nests like the Eastern phoebe. Because of that, it’s easy to spot the much larger cowbird eggs next to the smaller phoebe eggs.
The incubation period is often shorter than that of the host bird’s eggs so the cowbird chick will hatch first.
Even though she did not lay the cowbird egg, the owner of the nest will treat it and raise it as her own chick.
“The cowbirds will outcompete the other chicks and all the mother bird sees is a baby begging to be fed,” Hitchcox said. “They will eat more and grow faster [and] there are reports they will push the other chicks out of the nest either on purpose or simply by taking up all the space.”
Cowbirds are native to North America. Historically, their range was the prairies where bison also roamed.
“They followed the bison herds so they never stayed in one place long enough to raise chicks,” Hitchox said. “So they evolved to lay eggs in other nests.”
As European settlers killed off the bison through overhunting and changed the landscape, the cowbirds expanded their range. Now they are found throughout the U.S., north into Canada and south into Central America and bringing their brood parasitism with them.
Most bird species that end up with cowbird eggs in their nests aren’t majorly impacted. However, cowbirds have been blamed for the decline in populations of the endangered Kirtland’s warbler around the Great Lakes and the endangered black-capped vireo in the southern midwest.
In those cases, conservation plans include removing cowbird eggs from those nests.
“That is a one-time intervention policy that should only happen if it’s an endangered species that needs all the help it can get,” Hitchox said. “Otherwise it should be left and [let] nature take its course.”
Most often though, removing cowbird eggs from nests isn’t allowed. Doing so violates the federal Migratory Bird Treaty Act which protects this and other native species.
It can also endanger the host bird’s eggs, according to Hitchcox. Because even though the cowbird abandons her eggs for another bird to raise, she will stick around for a bit to make sure it all goes according to plan.
“There is a 2007 natural history study that found cowbird females can engage in retaliation,” he said. “So in some cases by removing the [cowbird] egg, you are doing more harm than good,” Hitchox said.
Once the cowbird chicks have grown enough to leave the nest, they will fly off. Female cowbirds need no instructions on how to lay their eggs in other birds’ nests. They are born programmed to engage in brood parasitic behavior as adults.
“It’s called an innate behavior,” Hitchcox said. “They get no care from their actual parents and are born knowing what to do — it’s pretty cool like you being born with a college degree.”
