See more of the story

Why are some songbirds shrinking? German biologist Carl Bergmann gave us the answer in 1847.

He described a natural phenomenon driven by — what else? — environmental temperature.

Bergmann wrote that colder climates cause animals to be large and compact. Bigger bodies preserve heat more efficiently than smaller bodies.

The corollary is, warm-blooded animals can afford to be smaller in warmer climates, to shrink. Think of it as choosing furnace size here or in Arizona.

An animal's surface area determines its rate of heat loss. Its volume determines the rate of heat production. In a warming climate, animals need less heat. That means they need less volume for heat production, therefore less surface area for heat dissipation, therefore they can be smaller.

Some bird species exposed to a warmer climate are showing signs of this change. The same species in a cooler climate wouldn't be doing so.

A study published two years ago in an ornithological journal, the Auk, focused on house sparrows in Australia and New Zealand, although the change could impact many species in many places.

"Our results suggest that higher temperatures during the breeding season could reduce body size," the authors wrote. That would drive what they called significant changes in the size of the bird.

The change is termed significant by and for scientists. It's not an obvious change to you or me. The birds must be carefully measured for differences to be recognized.

The authors said this finding could prove useful in understanding how future climate change could affect warm-blooded animals.

There is a second rule about animal body shape in warm and cold climates. Allen's Rule was described by Joel Allen in 1877. He was the first curator of birds and mammals at the American Museum of Natural History.

Warm-blooded animals living in cold climates need to conserve as much heat as possible. Allen's rule postulates that animals in cold climates should have evolved to be more compact, rounder. These animals would have shorter limbs and body appendages, like the polar bear with its short legs, stocky body and small ears.

Animals living in warm climates should evolve to maximize the surface area through which they dissipate heat. These animals would have longer legs and appendages, creating more surface area. A giraffe would be a good example.

Both rules apply to tendencies, not absolutes.

An interesting aside: During geologic periods known as the Paleocene-Eocene thermal maximum and the Eocene Thermal Maximum 2, some animals became so small that science uses the word "dwarf."

Thermal maximum means just what it says — hot. Global temperatures had risen as much as 5 to 8° C. This was about 55 million years ago.

A headline in the journal Nature described this succinctly: "Heat could lead to tiny mammals."

Archaeologists took teeth from four fossil mammals found in Wyoming. They determined that animals the size of both rabbit and horse shrank by about 15% during this warming event. Previous research had suggested that another horse species became 30% smaller.

A Wikipedia article says increased carbon dioxide levels during that period, which would have raised global temperatures, could have promoted dwarfing.

At this point in our climate adventure we are worried about an increase of 1.5 to 2° C. No shrinkage on the horizon. Yet.

Lifelong birder Jim Williams can be reached at Join his conversation about birds at