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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,
Josh
Joshua S. Rose
Duke University
Department of Zoology
jsr6@acpub.duke.edu
http://www.duke.edu/~jsr6/