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Re: THrush Forebodings

On Tue, 25 Apr 2000, Shantanu Phukan wrote:
> So, my question is this. Is this ribbon of woods
> (and others like it throughout Chapel Hill/Carrboro) thick enough to allow
> Wood Thrushes to breed successfully, or do we have, instead, doomed
> breeders? Has anyone seen their local Wood Thrushes successfully fledge
> the young?

According to research by Dr. Scott Robinson in Illinois, strips of 
habitat like this will never have successfully breeding thrushes. These 
are, as you correctly surmised, population sinks. A few Wood Thrushes 
take residence on forest fragments on the Duke Campus every summer, 
probably with similar results. The birds attempting to breed there have 
emigrated from source populations, large blocks of natural woodland 
elsewhere. Dr. Robinson's research found far lower breeding success than 
Alex Netherton cites; he found over 90% nest failure. However, I was told 
by one Michigan-based biologist that Robinson studied only first nesting  
attempts, and that second attempts had lower rates of cowbird parasitism, 
possibly leading to Alex's more cheerful statistics.

The individual thrushes may not necessarily be "doomed breeders" 
permanently. While they will probably fail to fledge young in the ribbon 
of woods you describe, birds generally don't attempt to re-nest in the 
same territory where they failed the previous year; instead, they only 
return and nest in the same territory if they bred successfully in that 
territory. These birds may, as Alex suggests, be young adults, who as 
they age will be better able to compete for higher-quality habitats. As 
long as we maintain large blocks of undisturbed native forest somewhere 
in the neighborhood, those large blocks should continue functioning as 
sources and populating the surrounding areas with Wood Thrush and other 
forest bird species. I am hopeful that the thrushes I hear from my house 
are breeding successfully; I live right next to the Duke Forest, and have 
yet to see a cowbird in my yard in nearly a year living in this location.

I keep stressing natural, undisturbed, or native forests because Wood 
Thrushes, like the Painted Buntings that John Fussell recently discussed, 
will not persist in the Loblolly Pine plantations which foresters have 
been spreading across this state. Thrushes depend on a healthy 
undergrowth of shrubs and herbs for successful nesting, and Loblolly 
plantations have only a carpet of pine needles underneath for the first 
few decades. In my native Massachusetts, overpopulation of deer has wiped 
out the undergrowth in fragments of otherwise intact forest, resulting in 
similar absences of Wood Thrush and other species.

Good birding,


Joshua S. Rose
Duke University
Department of Zoology