Thursday, April 17, 2008

Here, but NOT Welcome!

Oh, the hated brown-headed cowbirds are back. They return in the spring just before the songbirds that they parasitize so thoroughly. Here's a male (right) and female hoping to fatten up on cracked corn in our side yard.

I won't recount the entire natural history of the brown-headed cowbird, except to say that they evolved following roving herds of bison, eating the insects these large mammals kicked up. Being constantly mobile, cowbirds could not afford to build a nest, incubate eggs, and raise young. So they adapted to let someone else do all the work. Female cowbirds lay their eggs in the nests of other birds. They are amazingly good at finding hidden bird nests and waiting until the coast is clear to slip in and make an unwanted egg deposit.

Two weeks later the "foster" parents are raising a youngster that is not their own (and probably arguing over whose side of the family the nestling's ugliness comes from).

If it weren't illegal and bad for the soul, I'd consider doing something to control this species. Cowbird control measures have resulted in increased reproduction by the endangered Kirtland's warbler in Michigan.


Every year here on our farm we see fledgling cowbirds and it makes me heartsick to think of the wood thrushes, blue-winged warblers, and indigo buntings that were fooled into raising them.

One species that never has to raise cowbirds to fledging is the American goldfinch. Their nests get cowbird eggs to be sure, but the nestling cowbirds cannot survive on the goldfinches' all-seed diet.

Instead of "offing" cowbirds, we'll quit feeding so much corn and mixed seed (which the cowbirds love) and we'll curse these parasites whenever we see them.

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8 Comments:

At 10:19 PM, Blogger Mary said...

Thanks for this info, Bill. Cowbirds have been visiting here for a few weeks and I've noticed they enjoy a millet/corn/nut mix I've offered. As of today, the supply ran out and I don't intend to replace it! Spring brings a lot of undesirables to the feeders... now it's time to make adjustments.

 
At 10:43 PM, Blogger Seabrooke said...

But Bill.... if the cowbirds don't parasitize those nests, then the cowbirds go extinct. It's like being mad that a hawk is hunting your feeders. The hawk's gotta eat, just like the sparrow's gotta eat, and it's not really fair to favour one over the other. The cowbird can't help who it is. I agree that they need to be controlled in extreme situations such as with the Kirtlands, but I don't mind them parasitizing the Yellow Warblers and other common birds that aren't going to suffer by raising somebody else's kid one year. I would miss their "swee-tiddley" in the spring if they weren't here.

 
At 11:43 PM, Blogger BT3 said...

Thanks Mary!

Seabrooke:
Let me rephrase: I'd rather the cowbirds did their damage elsewhere. Or that they'd leave just the warblers and thrushes alone.

If you've ever seen a winter feeding flock of cowbirds in the South, you'll agree that cowbirds have a LONG way to go to get anywhere near "threatened" status. Meanwhile, wood thrushes and cerulean warblers are crashing....I know it's not cool to play favorites. I just worry about the warblers breeding here in the fragmented forests of SE Ohio.iepfxbox

 
At 12:57 AM, Anonymous Birding Tours said...

Nice pictures. I'm a bird lover myself.

 
At 9:46 AM, Blogger RuthieJ said...

So, if I "happen" to find a cowbird egg in a chipping sparrow or other songbird nest, is it also illegal to destroy the cowbird egg?

 
At 10:46 AM, Blogger Julie Zickefoose said...

Nope, Ruthie, like other birds that have the misfortune to be born with black feathers, cowbirds are not protected under the Migratory Bird Treaty Act. Nor are red-winged blackbirds, common crows, or common grackles. Which is why townships spray massive roosts of these "blackbird" species with the detergent Tergitol to wet them down and kill them in winter; why mass poisonings of "pest" blackbirds with granular pesticides like Dursban raise hardly a collective eyebrow. And ironically enough, the common grackle is in significant decline, along with the wood thrush and cerulean warbler. Is it time to take a look at our prejudice against black birds?

I agree with Seabrooke that the brown-headed cowbird has a place in nature, but unfortunately the pressure from exploding populations of cowbirds helps to tip the balance against sensitive, quickly declining species like wood thrush and many woodland warblers. We've thoroughly messed things up by fragmenting contiguous woodland, giving cowbirds ingress into areas they never haunted before, and the opportunity to parasitize species that are not evolutionarily prepared to eject their eggs. Cowbirds were never this common in pre-colonial times; they were likely a fairly rare, quite specialized bird that followed the bison out West. But now, they are everywhere, having burgeoned with our clearing and development of the land. I wouldn't be surprised if mass cowbird control, using catch traps of the type seen in Kirtland's warbler habitat in Michigan becomes routine maintenance for forest preserves wishing to slow the stunning decline of migrant songbirds.

 
At 1:21 AM, Blogger Seabrooke said...

Yes, like everything else to do with nature, it seems, humans are causing some serious imbalance when it comes to cowbirds and their hosts. You can really get a sense of how new they are to the landscape by looking at the rates of anti-parasitism strategies in host species here and in their native range. I don't think the solution is to eradicate cowbirds, though, as much as it is to slow fragmentation and the other forces of which cowbirds are just a symptom. Removing the cowbirds is a band-aid solution, which will perhaps protect, but is not going to heal the wound. So I usually stand up for the scapegoat underdog. :)

The first time I heard about it I was rather disturbed to learn of the spraying of blackbird roosts, particularly given their declining trends, and of the Rusty especially. It's somewhat ironic that we get all up in arms about farmers killing off huge numbers of Swainson's Hawks and Dickcissels in South America, but don't blink twice about doing the same to our blackbirds.

 
At 4:00 PM, Anonymous Eric said...

As a cowbird apologist, I would highly recommend that all birders read Stephen Rothstein's excellent article on the Brown-headed Cowbird in the August 2004 issue of Birding (available at http://www.americanbirding.org/birding/v36n4p374.pdf ).

Rothstein refutes the accepted "dogma" that cowbird parasitism has been responsible for widespread population declines in songbirds. He effectively argues that cowbird parasitism has simply not been shown to be a limiting factor in populations of such species as Kirtland's Warbler, Black-capped Vireo, and other poster children for anti-cowbird practices.

Rothstein exhorts us that "unnecessary cowbird-control programs have diverted our attention and resources from a much greater threat facing many host species -- habitat loss."

Here in Texas, thousands of cowbirds are trapped and killed annually as mitigation for take of Golden-cheeked Warber and Black-capped Vireo habiat, when it is that very loss and degradation of habitat that has caused these species' decline, not the native cowbird.

 

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