Friday, July 16, 2010

PostHeaderIcon Bird census: Brown headed cowbird?!

Br00dal parasitism on Alcatraz! This is seemingly benign young sparrow is no sparrow at all.

Though its adopted parents are sparrows, this little guy is actually a black bird, and it's thematic to discover that he or she is being reared on Alcatraz because this is truly the mafioso racketeer of the bird world.

They are known as brood or nest parasites. Around 40 times each breeding season, the female will lay a single egg inside the nest of another bird. If the nest contains eggs or young, the cowbird will eject or destroy some or all of them to ensure that proper effort and attention is paid by the adoptive parents to the young cowbird and not the young of their own species.

The brown headed cowbird is such a gangster that if the nesting bird recognizes and ejects the foreign presence in its nest, the cowbird will often return to demolish the nest and everything inside it.

This is what happens if a sparrow or finch forgets to look after the package left for them by their caring and magnanimous neighbor, the brown headed cowbird. These generous creatures would be positively grief-stricken if anything were to happen to your gorgeous nest.

If the victimized bird simply builds a new nest, the cowbird will return and lay another egg in it. They take notes and they are thorough.

I spotted the seemingly harmless little guy, hopping around with two white crowned sparrows, presumably the adoptive/host parents. It would hop near to one or the other, place its beak near the side of the smaller sparrow's head and shriek for food.

Interestingly, We've never seen a cowbird on Alcatraz. I hope I've got the ID right. At this point, I'm pretty sure.

Recognition as a nuisance species:

American song bird populations are declining and one contributing factor may be the change in land use patterns towards agricultural purposes. The cowbird is now better able to feed and thus better able to breed and terrorize its host species. As such, it has been designated a pest or nuisance species in parts of the United States.

There are even active trapping programs designed to give the songbirds a leg up on the mob.


I found this video from earlier in the year. It's a bird digging for insects or perhaps seeds in the undergrowth of the snowy egret thicket.

Perhaps it's the same individual. Or perhaps there are a number of these little gangsters on Alcatraz.


Laura said...

Neat! I've seen cowbird males singing/displaying on Alcatraz (in March & April), and earlier this summer saw what I suspected was a cowbird nestling being fed by two song sparrows by the egret colony. Interesting to see photographic proof.

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