Hello, tweets.

There is an interesting paper in a recent Condor.

Timothy Roth II, William Vetter, and Steven Lima. Spatial ecology of
wintering Accipiter hawks: home range, habitat use, and the influence
of bird feeders. Condor 110: 260-268, 2008.

They radiotracked Sharp-shinned and Cooper's Hawks in Indiana over a
winter.

The average size of Sharp-shinned home ranges through the winter was
12.8 sq km for adults, 33.0 sq km for immatures. Immatures apparently
need a considerably larger area to find enough to eat (they are not
fully competent), or, alternatively, they are just predisposed to
move around more.

The average size of Cooper's home ranges in urban landscapes was 3.9
sq km for adults, 10.1 sq km for immatures. Again, immatures wandered
more than adults. But notice how Cooper's home ranges are much
smaller than Sharp-shinned.

In rural landscapes, Cooper's home ranges averaged 14.2 sq km for
adults, 14.9 sq km for immatures. I'm not sure why the figure is the
same for both age groups, and now I can't recall if the authors had a
reasonable explanation for this.

They also censused "prey" species (birds of chickadee to flicker
size) and found they were 8x more abundant in urban study sites. This
probably explains why the Cooper's home ranges were much smaller in
urban areas.

Of great interest, bird feeders did not prove to be significant to
either species. They were no more likely to concentrate their efforts
in an area with a lot of feeders than in an area with no feeders.
That should make a lot of us feel better.

I wrote some time ago wondering why accipiters didn't seem to
decrease the abundance of their prey species in my yard over a
winter, and now my speculation that each bird roams over a quite
large area in the winter is confirmed. Ranging over several to many
square kilometers, their predation may be negligible at any one spot.
But I see that I haven't explained several of the differences cited
in the paper, and I plan to check it again when I can.

-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net



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