Class Aves: Evolution and Diversity

Class Aves Definition

• Birds are endothermic amniotes that lay hard-shelled eggs.
• They are characterized by having feathers, wings, and a beak.
• Most birds are capable of flight.

Species Diversity

• Approximately 10,000 bird species exist, classified into 30 orders and 142 families.

Evolutionary Relationships

• Birds, along with Crocodylia, are the only living representatives of Archosauria.
• Birds evolved from Theropoda.

Bird Evolution

• Bird evolution took place approximately 150 million years ago.
• Birds share clear similarities with reptiles.
• Birds are considered a specialized subgroup of theropod dinosaurs.

Evidence for the Theropod Hypothesis

• Birds and theropods share carpal bone morphology.
• Birds and theropods both have shoulder sockets.
• Both theropods and birds lay eggs in nests and incubate them.
• Feathers are present in both birds and theropods.
• Similar respiratory systems can be seen in birds and theropods.
• Both birds and theropods have uncinate processes.

Evidence Against the Theropod Hypothesis

• There are questions about how running animals could evolve into flying animals.
• Bird bodies exhibit dorsoventral flattening, while theropods show side-to-side flattening.
• Reduction in body size occurred continuously over 50 million years.
• Evolutionary adaptations were acquired rapidly in bird lineages.
• Avialae, the ancestor of all birds, appeared 163 million years ago and weighed 0.8 kilograms.

Specializations for Flight: Feathers

• Feathers develop in follicles along tracts called pterylae, as opposed to apteria.
• Uniform feather distribution is seen in ratites, penguins, and mousebirds.

Feather Types

• Contour feathers are the outermost feathers, including remiges on the wing and retrices on the tail.
  • Notching reduces drag.
• Semiplume feathers provide insulation.
• Filoplumes provide sensory aid in the operation of feathers and transmit information about the position and movement of contour feathers.
• Bristles, found on the bill, eyes, eyelashes, head, toes, and nostrils, act as screens and tactile sense organs.
• Down feathers provide insulation.

Skeletal Modifications for Flight

• Birds have lighter pneumatic bones.
• Birds have a lighter skull.
• Leg bones are heavier for balance.
• Fusion of bones enhances structural integrity.

Pelvic and Vertebral Adaptations

• The pelvic girdle is elongated.
  • The synsacrum is a fusion of 23 vertebrae, the ilium, and the ischium.
  • The pygostyle supports the retrices.
• Thoracic vertebrae are joined by strong, ossified tendons.
• A bird's vertebral column is rigid except for the neck.

Center of Gravity and Sternum

• A bird's center of gravity is beneath the wings.
• Birds have an enlarged, keeled sternum for muscle attachment.
• Large muscle attachment surfaces on the sternum are needed for flight.

Musculature for Flight

• The pectoralis (chicken breast) attaches to the keel and depresses the wing.
• The supracoracoideus (chicken tender) raises the wing.
  • The supercoracoideus is also attached to the keel, underneath the pectoralis, pulling the wing up from below using a "rope & pulley" arrangement.

Cardiovascular and Respiratory Systems

• Birds have a 4-chambered heart, facilitating double circulation.
• Avian respiratory systems use air storage/bellows in addition to lungs.

Flight Mechanics

• Wings function as airfoils and propellers.
• Changes in wing shape alter thrust, lift, and drag, resulting in changes in direction, velocity, and elevation

Hindlimb Adaptations

• Passeriformes have perching adaptations that lock tendons to prevent falling while sleeping.
• Legs and feet are adapted to facilitate walking, running, hopping, swimming, grasping, and gripping.

Feeding Specializations

• Bird feeding specializations include generalist, insect catching, grain-eating, coniferous-seed eating, nectar feeding, fruit eating, chiseling, dip netting, surface skimming, scything, probing, aerial fishing, filter feeding, pursuit fishing, scavenging and raptorial.

Social Behavior: Songs vs. Calls

• Song complexity differs from calls.
• Songs can be related to breeding in males, or duetting between birds.
• Songs are learned via Song Control Regions (SCR).
  • Hormoal control is a factor.
• Songs identify species and regional dialects.
• Birds exhibit non-vocal and visual displays.

Conservation: Market Hunting

• Market hunting involves exploitation of wild animals for sale or trade, creating income.
• Market hunting focused on species gathered in large numbers during annual cycles.
• Severe depletions and extinctions occurred due to market hunting.
• Parts of birds were components of Millinery trade.
• Birds served as food, specifically, waterfowl, prairie grouse, and pigeons/doves
• Feathers were used in mattresses and pillows.

Conservation History

• Feather boycotts occurred in many cities in response to bird slaughter.
• Massachusetts enacted the first state law prohibiting wild bird feather trade in 1897.
• Audubon Society members were labeled as "extremists."
• Politicians feared the loss of "thousands of jobs" due to conservation efforts.
• The Lacey Act (1900) limited killing and sale of plants and wildlife across state lines.
• The Weeks-McLean Act (1913) empowered setting hunting seasons and prohibited sport/market hunting of migratory birds.

Extinctions

• The Passenger Pigeon population went from 3 Billion to zero between the early 1880s to 1900
• The Passenger Pigeon was extirpated in the wild around 1900 and the last one died in captivity in 1914.

Migratory Bird Treaty Act

• The Convention for Protection of Migratory Birds (Migratory Bird Treaty Act of 1918) was established.
  • It involved the US, Canada, and Great Britain.
  • This made it Unlawful to pursue, hunt, take, capture, kill, or sell "migratory birds."
  • There was full protection of live and dead birds and bird parts.
  • 800 species are currently included on the list.
  • Enacted due to threats from the commercial feather trade.
  • Gave the federal treaty-making power to override state law.
  • Allows for a distinction of game birds – for hunting seasons

Conservation Actions

• Mexico (1936), Japan (1972), and USSR (Russia) (1976) took part in conservation efforts.
• In the late 20th century, the North American Waterfowl Management Plan (NAWMP) was created.
  • Intended to address declines in duck populations.

Current Conservation Status

• According to the State of the Birds – NABCI 2016, the vulnerability of all 1154 native birds in North America was assessed.
• Factors include:
  • Population trend and size.
  • Extent of breeding/nonbreeding range.
  • Severity of threats.
  • The level of concern, with highest concern indicating most vulnerable due to multiple factors.
• Vulnerability of habitats is assessed based on the percentage of species on the watch list.
• 432 species on the Watch List are most at risk of extinction without significant action.
• 57% of oceans (54 species) and 56% of tropical and subtropical forests (478 species) are of conservation concern.
• Oceans face threats from invasive predators on nesting islands, bycatch in fishing vessels, overfishing of forage stocks, pollution, and climate change.
• Tropical forests face threats from hunting, deforestation, and forest degradation, putting 41% of all species at risk of extinction.
• Coasts face threats from habitat destruction, sea-level rise/climate change, recreation and coastal development, and oil spills/pollution.
• Grasslands face threats from remnant habitat loss, with grasslands/shrublands having >50% due to human settlement, and 1/3 of its species on the Watchlist.
• Wetland Losses in the US began in the 1600s with an estimated 220 million acres.
• The current extent of wetlands = ~108 million acres
• Between 1986-1997 = 58,500 acres were lost/yr
• This shows the wetland losses are increasing again.
• Wetland birds generally doing well due to:
  • Concerted efforts for wetland conservation
    • Duck Stamp funding for acquisition, and protection
    • North American Wetlands Conservation Act
  • Careful management of hunting
    • North American Waterfowl Conservation Plan.