Saturday, March 28, 2009

What is happening to red-shouldered hawks in California?

Last week I spoke with Pete Bloom by telephone, and he notified me that band recoveries were coming in from the Bird Banding Lab on birds I banded for him in the past couple of years in Southern California. Most of the reports are of fatalities, including road kills. Pete expressed concern that birds may be dying off faster than they are being replaced by natural reproduction, which could make population sinks an unfortunate reality for red-shouldered hawks. Today, I spoke with Pete again and he said that this year's field work seems to be revealing an unusually high percentage of subadult breeders, unoccupied territories, and presence of youngish adult breeders and absence of old breeders. Pete says that a decade ago he used to find a lot of red-shoulders over ten years of age in his study area, but now they seem nonexistent.

These conversations struck me as I cover my own study area, where over the past dozen years I have banded hundreds of red-shouldered hawks. I seem to continually find unbanded adults in areas where I banded one or both members of pairs as recently as one or two years ago. Clearly, there must be an explanation for the regular presence of unbanded birds on territories where conspecifics were banded in recent times.

Several possibilities will need to be considered and analyzed. Roadkills could be a limiting factor for the local population, which may be reflected, at least partially, in band recovery reports from the Bird Banding Lab. Pete thinks West Nile Virus may be playing a role; perhaps even in sublethal effects such as eye weakness that leads to starvation or road collisons.

Those persons who oppose raptor banding because they think that it harms birds are just plain wrong and their position harms the interests of the birds themselves. Banding is an inexpensive way of gathering reliable data and unbanded birds tell us very little about themselves.

Stan Moore
Fairfax Raptor Research
P.O. Box 342
San Geronimo, CA 94963

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