The Sneaky Brown-headed Cowbird

Jul 18, 2018

 

There’s a sneaky bird in our midst. A bird so devious, so duplicitous that even this bird guy was recently fooled. To make matters worse, the tragedy I will now describe happened in my own backyard, just a few feet from my door. The innocent victims in this story are a pair of Carolina Wrens. The villain is a bird obscure to most but infamous to some: the Brown-headed Cowbird.

 


The cowbird is our only obligate brood parasite, which is ornithologist-speak for “they trick other birds into raising their chicks”. They are incapable of building a nest of their own, instead relying on a strategy whereby the females lay dozens of eggs in the nests of various species in hopes that some of those chicks will be raised by the host parents. The female sneaks in and adds a single egg to a clutch of say, Yellow Warbler or Song Sparrow eggs. If the host species doesn’t notice, the cowbird egg hatches and the aggressive chick demands more food and grows faster than the host chicks, often pushing them out of the nest altogether.

When I was in my twenties I worked on various bird research projects around the country that involved finding and monitoring songbird nests. In the course of this work, I became all too familiar with the deviant ways of the cowbird. The females are brilliant at quietly lurking in the periphery, watching for nesting behavior among their victims. But the males get in on the action too. I remember watching in riparian woodlands of desert Arizona as male cowbirds stalked nesting Yellow Warblers. They would creepily watch the warblers work on their nests, then alert a nearby female cowbird about the egg-depositing opportunity. Presumably, this was to curry favor with the female so he could mate with her.

My point is, I should have known better. I should have noticed the cowbird egg and removed it. Suburbanization has tipped the scales in favor of cowbirds, giving them access to way more nests than they did in the big, intact forests of the pre-colonial landscape. Their nest parasitism has contributed to the near-extinction of certain rare songbirds, like the Kirtland’s Warbler of the jack pine forests of Michigan.  


I had watched as these charming little wrens brought nesting material to a flower box by our back door. I even took a photo of the eggs, using my phone camera to get a view of the well-concealed nest that I couldn’t with my eyes. The eggs hatched on Father’s Day, and I was pleased. The busy parents brought insects gathered from every possible nook and cranny of the yard, including our grill. The male often took breaks from foraging to trumpet his fatherly pride from the deck railing, a song startlingly loud for such a small creature. A few days later I took another photo of the nest, somehow missing that one of the chicks in the confusing jumble of naked bird parts was monstrously bigger than the others, a hallmark of cowbird parasitism.

About a week before the wren chicks should have fledged, I noticed one of the wrens at the edge of the yard feeding a big fledgling, which I quickly realized was a cowbird. In the past, I had seen adult songbirds feeding cowbird fledglings that didn’t even come from their own nests, so strong is the primal urged to feed a hungry mouth, and I briefly thought that was happening here. Hey, get back to feeding your own chicks, I thought to myself. But alas, a check of the nest revealed only a dead wren chick and an unhatched egg. Looking back at my photos I quickly saw what I had missed.