The colder it is, the more energy birds need to survive in winter. In northern latitudes, metabolic rates and energy consumption are higher because there is usually less food and daylight for foraging. Smaller birds tend to lose heat faster than larger birds because they have more surface area per unit of weight.
For example, Downy Woodpeckers increase in size with increasing latitude. Downies in Alaska are 12% larger than Downies in Florida. (Source: Ritchison, 1999)
Meiri S. and Dayan T. 2003. On the validity of Bergmann’s rule. Journal of Biogeography 30:331–351
As the pressure of population increasingly regiments us and crowds us closer together, an association with the wild, winged freedom of the birds will fill an ever growing need in our lives. - Edwin Way Teale, introduction to Songbirds in Your Garden, 1953
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