Feb 15 2013
Much as we’re unhappy with the results, the introduction of house sparrows from Europe began a grand experiment in avian adaptation.
House sparrows were introduced to both the U.S. and New Zealand in the 1850s where they immediately became isolated from their native populations. More than 150 years later they differ based on where they live.
In addition to changes in plumage the birds are different sizes. In locations where winters are harsh, the birds are large. Where the climate is moderate, they are smaller. This effect is called Bergmann’s rule and is true of birds around the world.
In 1992 William A. Buttermer studied house sparrows at a winter roost in Ann Arbor, Michigan where he found that the largest males survived the best.
Not only were the large birds able to thermoregulate better than the small ones but they had two other advantages. The larger birds claimed the most favored roosts and they were able to fast longer.
During winter storms birds must roost and wait for the weather to improve, so they are forced to fast. The larger birds survived fasting better than small ones.
It’s better to be bigger in winter.