[CT Birds] snowy owls (long)

Chris Elphick elphick at sbcglobal.net
Sat Dec 7 08:53:25 EST 2013


Hi Dave,

I agree about the hunting.  We know they hunt during the day and we know they hunt at night.  How much they do one vs the other probably depends as much on when food is available and easy to capture as anything.  The key point I was trying to make though is that no one (as far as I can tell) has systematically measured the behaviour.  This would be a great undergraduate research project for all those young birders looking for something to do over the winter break!


Also, you wrote: "My take on what they presented is that in a region that offers a greater area of familiar and supportive habitat than the northeast, 55% of the available specimens had less than a moderate fat pack".  


Some counter points are: 


(a) I'm not sure how important familiarity is.  My understanding is that snowy owls are pretty nomadic under normal conditions, so that (perhaps) means that data from their normal winter range may be more representative of what happens in an irruption year than might be so for other species. 


(b) I'm not sure that habitat in the NE is any less supportive.  Snowy owls don't normally winter this far south, not because the habitat is necessarily less suitable than where they do winter, but because they can normally find adequate habitat farther north.  There's normally no reason for them even to try to winter as far south as they are this year.  Southerly habitat may be less suitable, but we have no reason to assume that it. 


(c) You don't say this but your wording implies that it is (at least potentially) a problem that 55% of specimens had less than moderate fat.  That idea seems logical, but the situation is more complex.  There is lots of evidence that how much fat a bird has is not a simple indicator of how well it is faring.  This is because birds have to balance two opposing goals - carry enough fat to not starve (implies good to be fat) vs. carry as little fat as possible so as to minimize the energetic cost of flight (implies good to be skinny).  This means that the ideal amount of fat will depend on how predictable the food supply is.  If it is very unpredictable then carrying more fat is important, as it provides a reserve if food cannot be found.  If the food supply is really good though, then the best solution is to carry virtually no fat because that keeps flight costs down.  So, lots of emaciated/starving birds would be a clear sign of a problem, but
 lots of birds with little fat (as implied in the Kerlinger & Lein paper) could simply mean that there is a decent steady food supply.  

The bottom line to all of this though is that it is easy to speculate, but much, much harder to gather the data that is really needed to inform.

Chris

 
Chris Elphick
Storrs, CT
elphick at sbcglobal.net



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