Although Charles Darwin lived and worked in the 19th century, modern evolutionary biologists are far from exhausting all avenues of inquiry regarding birds and evolution. For example, in the 1990s, researchers such as Russ Greenberg, ornithologist from the Smithsonian Institution in the United States, began to explore a new question concerning the relationship between climate and the evolution of beak size. This question was inspired by Allen's Rule, which states that warm-blooded animals living in cold climates will have shorter limbs and appendages than those that live in warmer climates. The biological mechanism behind this rule is thermoregulation—more body surface area helps animals to shed heat better whereas less surface area helps them to conserve it. Since a bird's beak plays a large role in thermoregulation—it has lots of blood vessels and is not covered in feathers—researchers wondered whether hotter climates beget larger beaks and colder climates beget smaller ones. Indeed, studies revealed that climate has influenced beak size, but not which type of climate had more of an overall impact.
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Chillier winters, smaller beaks
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