Chapter 8: Birds: Flight, Migration, and Behavior

Class Aves contains approximately 10,000 living species — the most species-rich class of terrestrial vertebrates. Birds are warm-blooded, feathered theropod dinosaurs, and their success is largely attributable to the evolution of powered flight, which opened aerial and long-distance ecological niches unavailable to other vertebrates.

Adaptations for Flight

Skeletal Modifications

• Hollow bones (pneumatized): air-filled cavities reduce weight without sacrificing strength
• Fused clavicles (furcula/wishbone): acts as a spring, storing elastic energy during the downstroke
• Keeled sternum: enlarged surface for attachment of powerful flight muscles (pectoralis and supracoracoideus)
• Fused hand bones (carpometacarpus): rigidity for primary feather attachment
• Reduced/fused tail vertebrae (pygostyle): supports tail feathers

Feathers

Feathers are derived from reptilian scales (keratin). Types include:

• Contour feathers: outer body covering, aerodynamic shape
• Down feathers: insulation layer beneath contour feathers
• Filoplumes: sensory feathers around contour feathers
• Flight feathers: primaries (thrust), secondaries (lift), retrices/tail feathers (steering)

Respiratory System

Birds have the most efficient respiratory system of any vertebrate — a unidirectional flow system using air sacs: Air flows through anterior air sacs → lungs (gas exchange) → posterior air sacs → lungs again → out. This means gas exchange occurs during both inhalation and exhalation, unlike the tidal breathing of mammals.

Selected Bird Orders

Order	Common Name	Key Feature	Examples
Passeriformes	Perching birds	Syrinx song organ	Robins, finches, crows
Falconiformes	Falcons	Speed, notched beak	Peregrine falcon
Accipitriformes	Hawks, eagles	Broad wings, soaring	Bald eagle (Haliaeetus leucocephalus)
Psittaciformes	Parrots	Learning, zygodactyl feet	Amazona, cockatoos
Sphenisciformes	Penguins	Flightless, wing-propelled swimming	Aptenodytes forsteri
Struthioniformes	Ostriches	Ratite (no keel), fastest land bird	Struthio camelus

Migration: Navigation Mechanisms

Many bird species undertake extraordinary seasonal migrations. The Arctic Tern (Sterna paradisaea) migrates ~70,000 km per year — pole to pole — the longest migration of any animal.

How Birds Navigate

• Magnetic compass: magnetite crystals in the beak detect Earth’s magnetic field; cryptochrome molecules in the eye may sense magnetic inclination via quantum effects
• Sun compass: birds use the sun’s arc and their internal circadian clock to determine direction
• Star navigation: tested in Indigo Bunting — juveniles learn north from stellar rotation patterns
• Olfactory cues: Homing pigeons (Columba livia) use smell maps to return home
• Topographic landmarks: learned route features used in experienced migrants

Bird Song and Communication

Passerine birds produce complex songs using the syrinx (unique to birds — located at the bronchial junction, unlike the mammalian larynx). Song functions:

• Territory defense: advertise occupancy to rivals
• Mate attraction: females often prefer complex songs (honest signal of genetic quality)
• Species recognition: interspecific breeding barriers

Some species show vocal learning — the ability to acquire songs culturally, like humans acquire language. Documented in songbirds, hummingbirds, and parrots.

Key Checklist

• I can list at least five skeletal or feather adaptations that enable bird flight
• I can describe three different navigational mechanisms birds use during migration
• I can explain the function of the syrinx and why bird song is considered an honest signal in mate choice