Saturday, August 22, 2009

Hawks and summer heat

Here in Philadelphia we have been enduring an August heatwave with several days straight of 90 degree plus temperatures and drenching hunidity. It's been much harder to find the hawks, especially the eyasses.

Several of this blog's readers have asked me whether the heat affects the hawls and how they deal with it. Great questions -- and I asked John Blakeman to weigh in with his bottomless expertise:

"The 90-degree heat is a mere inconvenience to the birds. They don't like it any more than we do. To keep cool they either soar off high in the sky (haggards, mostly), or they sit around quietly and do some panting, evaporating a bit of moisture off their lung and air tube surfaces to cool down.

But they don't have much water to allow significant evaporative cooling, so they do something that most mammals can't do. They just turn down their body thermostat and burn less food energy. Body temperatures of red-tails (and other similar hawks) can be rather variable---not as much as poikilothermous animals ("cold-blooded," such as reptiles). But red-tail body temps only hover around 103 F or so. At night, during sleep, it can dip into the mid 90s, from some experimentation I helped with in undergraduate school.

However, I don't know how hot the body temperature can get when a red-tail does a full-out pursuit of a rabbit or other prey animal on a hot day. Actually, I don't see red-tails doing much of that on hot days. They aren't dumb.

On the other hand, the birds are not intimidated by normal winter temperatures. They are so well insultated, and have the resources to generate massive amounts of body heat if required, that the winter is a welcome time. Unless the temperatures drop below zero F or more, red-tails seem to thrive in cold weather.

I see this when hunting my red-tails. In warm September, the birds are rather lackluster in pursuing fleeing rabbits. But with a bit of autumnal chill in October, their hunting attitudes change altogether. Cool and cold are preferred over warm and hot.

But the birds live from the low Arctic down through much of desert Mexico. They can handle it all."

--John Blakeman


  1. Della,

    As always, many thanks. I think JB has answered my question in this blog.
    There is one more detail. He wrote about mostly haggards soaring high
    up in the sky when it's hot and I twice witnessed two red tails doing just
    that last week. How I set sight of them, however, was by sound. I heard
    that cry way far away but unmistakable, found higher ground and sure
    enough there they were. Is that typical behavior?
    I know they were probably not our hawks given the distance from here to center city, but quite a thrill nonetheless.

  2. Where were you when you saw those red tails soaring?

  3. In the out back of my home in chestnut hill. Peg