Male dark-eyed juncos in New England are slate gray above with white undersides, but colors change in different locations. Those from the Pacific Northwest have a dark brown head and light brown back, while those from the Dakotas have some white in their wings. Rocky Mountain males have pink sides. Shutterstock

If you take a good look at birds with streaks on their breasts like yellow warblers, song sparrows or pine siskins, you will notice that within each species the pattern of streaking is a little different. It doesn’t take a lot of practice to be able to distinguish different downy or hairy woodpeckers coming to your feeder by the shape of the patches of black, white, and red on their heads. I’ll bet you will be surprised at how many different woodpeckers are visiting your feeder.

If we compare individuals within a species across their range, we often see more striking variability. For instance, our yellow-rumped warblers have white throats while those west of the Rocky Mountains have yellow throats. Male dark-eyed juncos in New England are slate gray above with white undersides. Male dark-eyed juncos from the Pacific Northwest have a dark brown head and light brown back, while those from the Dakotas have some white in their wings and Rocky Mountain males have pink sides. The different forms do disperse occasionally, and the Pacific form can find its way to Maine.

In most cases, we do not know the reasons why certain forms occur in particular areas. Perhaps, pink sides give dark-eyed juncos a survival advantage or perhaps the variation is just random.

However, biologists commonly use comparative techniques to try to understand variation. If a scientist looks at a number of species and all individuals show a particular plumage color or body shape in a habitat, one can begin to draw strong inferences about the value of those variations.

These comparisons have yielded several rules. Bergmann’s Rule says that birds and mammals that live in higher latitudes or higher elevations are generally larger in size than animals from lower altitudes or from temperate and tropical habitats.

Allen’s Rule looks at the same gradient and predicts that the smaller body appendages like ears, tails, limbs, or bills should be smaller in arctic or montane species.

Both rules can be explained in terms of heat loss. For birds in colder habitats, minimizing heat loss is critical. Larger birds have lower surface to volume ratios and therefore lose heat less rapidly than smaller birds with their high surface/volume shapes. Heat can be lost very rapidly across long appendages. Think about the huge ears of a desert jackrabbit that effectively lose heat to the short ears of arctic rabbits.

Gloger’s Rule involves within-species variation of plumage color. In wetter areas, the plumage tends to be darker. The basis for this pattern seems to be that feather-eating bacteria thrive in humid environments and darker feathers are more resistant to those bacteria.

I read with interest a recent article in the Journal of Biogeography by a team of Italian ornithologists. They were interested in regional variation of the barn owl. Although barn owls barely sneak into southern Maine, the species is extremely widespread. They occur on all continents except for Antarctica and on many oceanic islands. Their distribution spans 65 degrees north to 55 degrees south. Barn owls are therefore excellent model organisms for this study.

As one might expect, significant variation occurs across the range. The Italian team used Bergmann’s Rule and Allen’s Rule to guide their study. They were interested to see if they could detect regional changes in morphology in response to climate change. As the climate changes, one can predict that owls will become smaller with longer body appendages.

The investigators took advantage of the fact that until 1950 or so, ornithology was practiced with a shotgun rather than binoculars. Those specimens were placed in museums. The authors measured specimens taken from 1809 until 2018.

Two of the three rules were supported. Barn owls are becoming smaller in areas subject to climate warming and their ventral plumage is becoming darker in areas where annual precipitation is increasing. However, bill length is becoming shorter in warming areas, contrary to Allen’s Rule.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu