modified as wings and nearly all can fly. Which of the above traits do you think might be
of importance to flight?
Adaptations for Flight
In comparing birds with other vertebrates, what do you think distinguishes them the most?
Of course, in most birds flight is the most obvious difference (Figure14.2), and birds have
adapted their body plan for this function. Their skeleton is especially lightweight, with
large pneumatic (air-filled) cavities connecting to the respiratory system. Cervical, or neck,
vertebraeareespeciallyflexibleandinbirdswithflightthesternumhasakeel, orlongitudinal
ridge, for the attachment of two large flight muscles: the pectoralis, which encompasses 15%
of the bird’s total mass, and the supracoracoideus, the primary upstroke muscle for flight.
What other traits do you think might be important for flight? Of course, feathers are
lightweight too and a forelimb modified as a wing serves as an aerofoil. This surface is
designed to aid in lifting or controlling by making use of the air currents through which it
moves. A bird’s wing shape and size will determine how a species flies. For example, many
birds have powered, flapping flight at certain times, while at other times they soar, using up
less energy
Figure 14.2: One bird’s flight, as seen in a tern species. ( 32 )About 60 living bird species are flightless, such as penguins, as were many extinct birds.
Flightlessness often arises in birds on isolated islands, probably due to limited resources and
the absence of land predators.