[Tweeters] winter survival

Dennis Paulson dennispaulson at comcast.net
Mon Dec 3 16:27:19 PST 2007

I think it's interesting that the small passerines in my yard eat up
feeders full of seeds and suet cakes sufficiently fast that I can see
a change over the course of a day, and we have to put out seeds every
few days and suet every few weeks.

Yet there are at least two accipiters, including both species, in the
neighborhood all winter. I see them with some frequency. They have to
eat also, and I've often wondered why there wouldn't be a perceptible
decline of the small feeder birds (appropriately named in this case)
in the immediate neighborhood over the course of the winter. But such
a decline has never been evident to me.

It's difficult to find out how much food these hawks need during the
winter to stay alive. From the BNA account, a captive male Cooper's
Hawk ate 63 grams/day over the winter, and that's about a robin-sized
bird each day (robins weigh around 75 grams). So if one male Cooper's
Hawk fed well, there should be 150 or so fewer robin-equivalents in
the neighborhood by the end of the winter. But apparently the
"neighborhood" is quite large. In fact, one male Cooper's had a
winter home range of around 800 acres, or 3200 times the size of my
yard. Doing the math, that means my yard ought to lose only 150/3200
robin/winter, which is a pretty small slice of robin, so no wonder I
don't notice it.

However, home ranges overlap, and that means that very likely
multiple individuals of these hawks are using my yard, so the loss
might be a bit higher through the winter, but probably still
undetectable. Apparently the hawks do get enough to eat, and they
don't have a substantial effect on local bird populations.

My final thought is that surely we are supporting larger populations
of passerines in the cities than might be here, by supplying the
former with tons and tons of food (seed, suet) that is produced
elsewhere, and this should have an effect right up the food web to
the bird-eating hawks.
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
dennispaulson at comcast.net

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