Who's At Four-Mile-Run Park this Summer?

At Four-Mile-Run Park, you may notice that bird sounds have quieted down with the onset of hot summer weather. By now, many migratory songbirds from the tropics, such as yellow and blackpoll warblers, have headed south after fledging single nests. They’re getting a leg up (wing up!) on their long journeys to Central and South America.

But the year-round residents are still breeding during July. Watch and listen for nests with squeaky broods of sparrow hatchlings, thumb-sized broken American robins’ blue eggshells, or fat, newly fledged grackle nestlings poking around on the ground.

If you see an extra-large egg or hatchling in a nest, you’re witnessing parasitism in action. Spot evidence of a parasite: 

Brown-headed cowbird egg in black-capped vireo nest in Oklahoma. 

Photo Credit: Christine Fallon/USFWS.

Four-Mile-Run Park is one of many places where warblers and larger birds risk getting parasitized by the brown-headed cowbird (Molothrus ater). After they’ve mated with glossy black-and-brown males, the grayish female cowbirds and drop eggs into other bird’s nests. Often, a laying female cowbird punctures and tosses out one or more of the “host” nest’s eggs before adding her own. The ~220 species of potential host birds (nationally) for the brown-headed cowbird offer her ample options for the upwards of 40 eggs she'll produce in one season. 

Brown-headed cowbird nestling in Kirtland’s warbler nest. 

Photo Credit: Cassandra Waldrop/USFWS.

Female cowbirds have evolved to be strategic, spreading out their investment by laying just one or two eggs in each nest. If the targeted host bird is savvy enough to recognize the cowbird egg, she’ll eject it, such as an American Robin did HERE. But when the parasitized female fails to detect the errant egg, she feeds and raises the oversized cowbird nestling as if it were her own, often at the expense of her offspring. 

What nest is this cowbird female at 4-Mile-Run scoping out?

Photo Credit: Matt Strachan.

Why doesn’t a cowbird just build her own nest? You’d think her large baby could be better cared for by a parent of its own species. As their name denotes, cowbirds in their native range of midwestern prairies follow cows around, benefiting from insects stirred up by the herd. Historically they followed wild, roaming bison and, thus, evolved a means to continue to reproduce along the way by coopting other birds to raise their young.

So, the brown-headed cowbirds spotted this month at Four-Mile-Run are likely up to no good, well good for them but not for other bird species. Ameliorating the impacts of cowbirds on the reproduction of native birds has long been a conservation concern, prompting the formation of a North American Cowbird Advisory Council to make recommendations for managing cowbird populations. 

Historically, cowbird impacts would have been muted by their movements with bison, but cow herds are fenced in and not migratory. These days, brown-headed cowbirds threaten other bird species as far north as Alaska and south into central Mexico. 

Brown-headed cowbird males spotted at Four-Mile-Run Park.
Photo Credit: Matt Strachan.