Re: nest predation

["Matthew Troutman" <m_troutman@hotmail.com>]

<<Unfortunately, there's another dimension to cowbird behavior.  
Cowbirds are open-ground birds, and you typically find them in grassy 
fields and open thickets.  But the birds they parasitise - certain 
warblers, vireos, maybe thrushes - nest in trees.  As a result, cowbirds 
only parasitise birds that (a) won't throw an alien egg from the nest, 
and (b) happen to nest at the edge of a forest.>>

Exactly.  They parasitize over 200 species of birds, most or all are 
tree nesters.  Cowbirds do have some host birds that throw out alien 
eggs occasionally; I find the occasional cowbird egg on the ground by a 
bird nest high up in the tree.  Sometimes the hosts build another nest 
layer over the alien egg as I pointed out elsewhere.  

Another point that has to be made is about the social structure of these 
birds.  A friend of mine has observed that the sub-adults (presumably 
males) tend to hang out in groups, much like crows.  It seems that by 
podding together that they can attract more females, thus increasing the 
population.  

<<The thing is, there's much more forest edge now than there was two 
centuries ago, and cowbird populations are exploding as those of
parasitised birds - black-capped vireos, Kirtland's and golden-cheeked
warblers, and several others - drop.>>

Exactly.  

<<Some areas have begun cowbird control programs, which raise complaints 
from many groups but do show a measureable affect on the populations of 
putative parasite species.  My wife and I had to stop filling our 
birdfeeders in Austin when we discovered cowbirds taking up about 85% of 
the bird biomass at the feeders - they sound cool, but I don't want to 
help them.>>

The cowbird control program where I frequent ( stomping a cowbird or 
sparrow chick to death ) is still not bringing down the populations of 
these two parasites.  As a matter of fact, you are not allowed to kill 
any birds for any reason unless you have permission.  Its rather stupid, 
the park manager over there has only roller-bladed through the park once 
and she has no idea about anything that is going on.  Whenever a bird is 
killed you are certain to hear from some compromising forester about how 
you cannot start population control, even though it helps all the 
avifuana.  

Matt Troutman

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