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Form-shifting Birds – Ornithology


There is a “rule,” actually a rule of thumb, that says as distance from the equator increases (like going farther north in the northern hemisphere) the body sizes of warm-blooded animals tend to change, the surface area of the body becoming proportionately less than the volume – the surface area shrinks as volume increases. Biologically explained, the body (volume) produces heat and the surface area (skin) radiates heat. According to this idea, a large body is advantageous in cold conditions not only because it has more cells and therefore produces more heat than a smaller body, a larger body also loses less heat per unit volume than a smaller body. So over evolutionary time we would expect thatmammals and birds in cold northern regions would have developed stockier bodies than those in the south. This is called Bergmann’s Rule. There are lots of examples of Bergmann’s Rule among birds such as a recent paper on two bird species from China.

The other “rule” is basically an extension of Bergmann’s Rule and is termed Allen’s Rule. The same principle of heat loss applies here but in this case it applies to appendages. Long skinny appendages such as arms, legs, wings, and ears ought to become shorter and thicker in cold climates.

Perhaps we can make a prediction here. If the earth is warming, and it certainly is, then birds and mammals of northern regions do not need to inhibit heat loss as they once did. Natural selection would not work as intensely to keep bodies and appendages stocky. Can we find examples? Yes, a recent study by researchers in Israel, measuring 8000 specimens of preserved birds, found that over the past 70 years the morphology of birds has been changing. They say “We identified two different types of morphological changes: some species had become lighter — their mass had decreased while their body length remained unchanged; while others had become longer — their body length had increased, while their mass remained unchanged. These together represent more than half of the species examined, but there was practically no overlap between the two groups — almost none of the birds had become both lighter and longer. “

There are more examples. Several parrot species in Australia have increased their beak size between 4% and 10% since 1871. Beak size increases among the parrots corresponds with rises in average summer temperatures. As scientists have known for years, beaks can help in thermoregulation so larger beaks means more heat loss. And in the United States, a recent study of over 70,000 birds of 52 species killed by flying into buildings in Chicago showed that their body sizes were becoming smaller and their wingspans longer, indicating an adaptation to a warmer environment.

Animals 'shape-shifting' as climate warms: study | Europe – Gulf News

What does this all mean? Well, it is more evidence that the climate is warming, but what it means for birds is unknown. Presumably the birds whose body proportions are changing will be more fit for warmer weather, but what other changes in their physiology or behavior might occur? Are there bird species who might not change their body type? And are there negative effects resulting from shape-shifting? New questions to which there are yet no answers.

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