Size and color vs. cold and hot

Do birds of the same species look alike throughout their ranges?
 
You might think that a species, by definition, would look the same wherever it was, but in fact most non-migratory birds differ within their ranges. 

Hairy Woodpeckers are bigger
in the northern part of their
range than they are in
the southern part.

Birds of a wide-range species that live in the cold end of the range tend to be bigger than birds that live in the warm end. A Hairy Woodpecker from Central America is noticeably smaller than a Hairy Woodpecker from Canada. 

Larger birds tend to be able to conserve heat better than smaller ones. Over many generations smaller birds born in cold regions have been more likely to die from mid-winter cold, leaving the larger siblings to carry on the species.
 
The process of size variation doesn’t take all that long, either. The House Finches, introduced into North America in the mid-1800s, have already developed smaller versions in the southern United States than are found in northern states and Canada.
 
The tendency of a species in cold climates to be larger than its fellow members in warm climates applies not only to birds, but all warm-blooded animals. In fact, it is a scientific principle is called Bergmann’s Rule, named for 19th Century German biologist Carl Bergmann, who developed it.
 
A similar principle is called Allen’s Rule, which says that cold-region birds have shorter wings, beaks and legs than those in the warm regions – all techniques, again to conserve heat.
 
Gloger’s Rule is not about size, but color. In 1833, Wilhelm Gloger, a German naturalist, postulated that, due to the intensity of the sun, warm-blooded members of the same species living in warmer climates tend to be darker than those in colder climates. It works for humans and other animals including birds. Birds of a single species that ranges from cold to hot will have deeper feather colors in the south than in the north.

Ranges big and small

The dwelling ranges of birds can vary from tiny to global.

The 50 small Galapagos Islands about 600 miles off the coast of Ecuador are home to two-dozen species of birds found nowhere else on Earth.

In the United States, the Everglades Kite is found only in certain parts of southwestern Florida. The summer range of the Kirtland’s Warbler is limited to young Jack Pine forests of Michigan, found in an area only about 60 by 100 miles at the top of the Lower Peninsula. This warbler winters in the Bahamas.

The Osprey can be found
on every continent.

Both birds are classified as endangered species, which is not surprising, given the limited ranges in which they live.

Some species span the globe.

Many like the Herring Gull are circumpolar, a term that means a bird can be found around the world on one side of the equator or the other. A few species, such as the Osprey and Short-eared Owl, can be found on every continent.

Among migrating species, the summer and winter ranges can be astonishingly far apart. The American Golden-Plover spends its summer on the north shores of Alaska and Canada and its winters in Argentina and Paraguay.

What is the 'range' of a bird?

What does “range” mean when talking about a species of bird?

The range of a bird species is the territory in which one would expect to find it living at one time of the year or another. Ranges vary from small areas of less than the size of Rhode Island to nearly the entire world.

Several factors determine a species’ range, chief among them the climate and the kind of soil. Climate and soil affect the kind of vegetation that grows, and vegetation is the source of food, nesting sites and materials, and protection for most birds. Few birds can survive in the desert, where vegetation is spare, but some have adapted to it. Other birds require the sea, or at least lakes or rivers, for their survival.

Various factors also limit range. Oceans are the most obvious; few birds are capable of crossing an ocean under normal conditions. Temperature limits range; few birds can survive an Arctic or Antarctic winter, for instance. High mountains define some range boundaries -- many species are found either east or west of the Rockies.

The Red Knot can be seen in 
Connecticut, but Connecticut is not 
part of the Red Knot's range.

Many birds migrate and their territory includes a “summer” or “breeding” range and a winter range. Often the summer and winter ranges overlap so that a species might live year round in some states or provinces while in other regions, the birds appear only in summer or winter. In the case of such species as the Tundra Swan and the Snow Goose, summer and winter ranges are widely separated.

Although during migration these birds fly over a lot of land and may stop for a bite to eat on the way to and from ranges, the territory in between the winter and summer grounds is not considered part of its range.  For example, the Red Knot has a winter range at the southern tip of South America and a summer range in the northern Arctic. While it can be seen on Connecticut shores in March, Connecticut is just a brief stopover on its long way north and not part of its range.