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.
Downy WOopeckers increase in size with increasing latitude. Downies in Alaska are 12% larger than Downies in Florida. (Source: Ritchison, 1999)
