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| Brown-Headed Cowbird |
(published 8-5-13)
One of the most interesting birds around is also among the most disliked. I speak of the villainous brown-headed cowbird.
Understandably, birders are critical of the cowbird, a brood parasite that deposits its eggs in other birds’ nests. This makes child-rearing difficult or worse for the host species.
Cowbirds are on my mind because of an excellent new book
about the Kirtland’s warbler by Bill Rapai, a Michigan author who spoke to the
DuPage Birding Club in May. Any book or article about our rarest warbler
invariably involves the cowbird, a well-known Kirtland’s nemesis.
Kirtland’s warbler numbers have steadily increased in recent
decades. Most believe this is due at least in part to human management of the
cowbird population on the warbler’s north-central Michigan breeding grounds.
Over the years thousands of cowbirds have been trapped and executed under
special permit.
Cowbird control is a fascinating aspect of the Kirtland’s
warbler recovery story. Around here,
however, warblers and other songbirds receive no such assistance. Their nests
stand a good chance of receiving an unwelcome delivery from a female cowbird. The
foreign egg usually hatches first and the chick grows quickly. It is often larger
and more aggressive than the host bird’s true offspring. That spells trouble.
In July I witnessed a male cardinal feeding sunflower seeds
to a cowbird chick in my backyard. The cowbird was nearly adult-sized and it
surprised me that the cardinal didn’t recognize the fledgling as an imposter. Soon
enough, however, genetics kick in and young cowbirds move on to be with their
own kind.
Keriann Dubina, a naturalist at Fullersburg Woods Forest
Preserve in Oak Brook, has studied cowbirds extensively. Last winter I attended
her presentation, “The Truths and Myths About Brown-Headed Cowbirds.”
Dubina said that cowbird eggs have been found in the nests
of 274 kinds of birds, even a hummingbird nest! Unfortunately, only about 30
species seem to possess the ability to recognize a cowbird egg and do something
about it.
However, one of Dubina’s key points was that bird species
suffering the most from cowbirds (such as Kirtland’s warbler) are those that also
have habitat issues. It is a myth, she said, that cowbird control improves a
host species’ reproductive success and population. What matters most is
increasing habitat, a notion well supported in Rapai’s book. Bottom line: even without cowbirds some
threatened or endangered bird species would still face serious challenges.
So we can’t blame the cowbird for everything, even though we
might like to. The bird’s reputation took another big hit in 2012 when The
Nature Conservancy published an article called “The Mafia Birds,” by Madeline
Bodin.
The story reported a shocking discovery by Jeff Hoover, a
researcher with the Illinois Natural History Survey. Working in the Cache River watershed in the
far southern tip of the state, Hoover found that cowbirds don’t just lay their
eggs and move on. Instead, wrote Bodin,
“They stick around and use tactics of intimidation and retaliation—like avian
mobsters—to make sure that warblers and other bird species raise their young.”
True enough, when Hoover and his team removed cowbird eggs
from prothonotary warbler nests, the cowbirds returned and destroyed the
remaining warbler eggs.
Such behavior makes the cowbird hard to like but no less fascinating.
“Here’s a bird that doesn’t have the ability to build its
own nest, evolved with mammal megafauna and once the megafauna were wiped out,
adapted to changing habitat and figured out a way to survive,” said Ron
Skleney, a naturalist at Willowbrook Wildlife Center in Glen Ellyn.
Indeed, the cowbird’s close association with the American
bison earned it the nickname “buffalo bird.” Following the great herds was a
feeding strategy; where there were bison there were lots of insects. But chasing
buffaloes left no time for nesting.
Skleney further points out that the cowbird’s evolutionary
success is due in large measure to humans. The species thrives in edge habitat,
created by the fragmentation of our native forests and woodlands.
Keep these insights in mind the next time you watch a brown-headed
cowbird grazing on your front lawn. The species has quite a complex natural
history. Remember, too, that it’s native to North America, unlike those pesky
house sparrows and starlings. For me that’s a positive.Fair or not fair, birders must also admit that on a life list, the conniving cowbird counts just as much as a Kirtland’s warbler.
Copyright 2013 by Jeff Reiter. All rights reserved.