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Center for

Environment & Society

Form and Function

How does a bird’s body facilitate it’s behavior?


  • Any living animal with feathers is a bird and all birds have feathers.
  • There are many different shapes of feathers: wing feathers, body feathers, and tail feathers.
  • Many are specialized like the facial feathers on owls for better hearing or the ornate tail feathers on tropical birds such as the Blue-crowned Motmot, which are used to attract a mate. Water birds have water proofed feathers to keep them dry while swimming.
  • Feathers are made of keratin.
  • They can be replaced (molt). Vane and then Rachis (shaft) off of it comes barbs and then barbules. Rictal bristles are modified feathers without barbs.


  • Bergmann’s Rule: Within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, and species of smaller size are found in warmer regions. Bergmann’s rule is most often applied to mammals and birds.
  • Allen’s Rule: The rule says that the body shapes and proportions of endotherms vary by climatic temperature by either minimizing exposed surface area to minimize heat loss in cold climates or maximizing exposed surface area to maximize heat loss in hot climates. The rule predicts that endotherms from hot climates usually have ears, tails, limbs, snouts, etc. that are long and thin while equivalent animals from cold climates usually have shorter and thicker versions of those body parts.


  • Plumage: what the bird’s feathers make it look like
  • Some birds look the same all year round: Northern Cardinals, Blue Jays, Bald Eagles.
  • Other birds molt into breeding plumage that can be strikingly different from their non-breeding plumage: Scarlet Tanager, American Redstart, Indigo Bunting, Black-bellied Plover.
  • In some species both sexes look more or less the same: American Robin, Carolina Wren, Bald Eagle, Canada Goose.
  • In other species the females are more drab and plain: Eastern Towhee, Northern Cardinal, Black-throated Blue Warbler, Common Yellowthroat, Eastern Bluebird.
  • Why might females look different?
  • Why are some species so showy, while some are so cryptic?







  • There are many varying adaptations of bills depending on the species, what they eat, and where they live.
  • Compare the bills of Great Blue Heron, Ruby-throated Hummingbird, Osprey, Northern Cardinal, Common Yellowthroat, and the Ibis. What do they have in common and how are they different?

Feet and Legs

  • Wading birds like herons have long featherless legs.
  • Ducks and other water birds have webbed feet for swimming.
  • Raptors have featherless legs with big talons for catching prey.
  • Snowy Owls have feathered legs to help them stay warm.