SREH holes: are northern birds larger than southern birds?
From: James R. Hill, III
Date: 3/5/02
Time: 10:17:49 AM
Comments
As for body size of birds varying by latitude: There is an ecological "rule," known as "Bergmann's Rule." Basically, what it describes is a phenomenon seen world-wide in many geographically widespread warm-blooded animals (birds and mammals) where they are larger the father north you go within their range, and smaller as you go toward the equator within their range. This is an adaptation to prevailing temperatures. It is explained like this: In northern climates, the larger birds (with their body surface smaller in proportion to their body volume, compared to smaller birds) lose less heat, an adaptation to cold temperatures that conserves their body heat and energy. In warmer, southern climates, smaller body size in warm-blooded animals gives a greater body surface in relation to body volume, and presumably aids in cooling the bodies of smaller birds by radiation of heat.
We do not currently know if martins and starlings follow this "rule" because it is still being debated by ornithologists. If they did, both martins and starlings would be smaller in the south than in the north, and, SREH might actually have to be smaller in the south. Some experts say Bergmann's rule only applies to permanent resident populations of birds (or mammals), not migratory populations, which means it would apply to starlings but NOT to martins. I don't know if this is true. Again, if this premise is true, one size SREH would fit all martins, but would not exclude all starlings. Very interesting topics for future research.
For now, though, the bottom line is: be patient with your crescents. They will come.
James R. Hill, III/PMCA
