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Posted by Bill Pranty on 11:15:48 08/23/09
In Reply to: the spread of the cowbird posted by Ann Wiley
Good afternoon Ann,
The impacts of cowbirds on other native landbirds is a very complicated one.
Simplifying the issue greatly, yes, the expansion of cowbirds in the New World is due to deforestation and animal husbandry. And yes, the cowbird is often the only nes-tling that fledges.
Determining the true effects of cowbirds is very difficult, since our concern should be on populations of birds, rather than on individuals.
Shiny Cowbirds seem to be the least of our concerns, since they have never become numerous in the 25 years since they colonized Florida, despite an abundant and widespread potential host (Red-winged Blackbird).
Bronzed Cowbirds are currently limited to the southeast coast region, but may become much more numerous -- I can't think of a reason that they would remain restricted to where they are UNLESS it is because they are concentrating on Spot-breasted Orioles. (Two of the three observations of Bronzed Cowbird fledglings were fed by orioles, with the third observation of a Red-winged Blackbird). If Bronzed Cowbirds are targeting Spot-breasted Orioles -- and _Icterus_ orioles are one of their primary hosts throughout their native range -- and if they become numerous, then they potentially could extirpate the oriole from Florida. (Shiny Cowbirds also target _Icterus_ orioles in their native range).
But such a scenario seems a long way off currently -- and it may never happen. The cowbirds may concentrate on blackbirds instead.
Brown-headed Cowbirds are probably the biggest threat to Florida's landbirds, but they remain fairly rare in the southern peninsula and the Keys. If they increase in abundance, then impacts to Gray Kingbirds, Black-whiskered Vireos, Cuban Golden Warblers, and Florida Prairie Warblers may become a conservation issue in the coming decades.
Best regards,
Bill Pranty
Bayonet Point, Florida
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