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Toothless Beak
The first birds had a full complement
of reptilian teeth, but over time, these were reduced and eventually
lost, as in modern birds. Modern birds have a beak which is covered
with a horny sheath, making it hard and rigid. There are a variety
of beaks seen between modern birds which evolve primarily in response
to diet. Below are just a few examples. The flamingo beak is designed
forstrainig algae and invertebrates from mud and water. The beak
of the avocet is designed for probing mudflats for crustaceans,
insects and aquatic seeds. Shrikes are the only songbirds which
consistently feed on small vertebrates, and they kill their prey
by biting it repeatedly with their hooked bill. They also eat
insects.
Reversed, Fully Descended Hallux
The hallux is basically the big toe. A reversed, fully descended hallux is an essential component of the perching foot. This is a feathue unique to birds, which in some cases has been secondarily modified. We see a large variety of adaptive foot morphologies in modern birds. Below are just a few examples. The feet of the kingfisher and chicken maintain a fully reversed hallux enabling them to perch or roost. The foot of the duck is adapted for swimming. here the anterior toes are webbed and the hallux is reduced. The most highly modified feet occur in flightless birds. The ostrich provides an extreme example of a foot designed for running, in which the hallux and one anterior toe are lost.
Fused Digits of the Hand
The earliest birds had a clawed hand (e.g. for climbing trees), but gradual specialisation of the wing as flight apparatus has resulted in the loss of a functional hand. The "handless" state is seen in modern birds.
The Pygostyle
The earliest birds had a long, bony reptilian tail made of numerous vertebrae, but in most modern birds the distal-most 4 to 7 caudal vertebrae are fused and reduced to a single, bony stump called the pygostyle. From this, the tail feathers arise. A true pygostyle is absent from most moder, flightless birds.
For diagram of the avian skeleton click here
The Keeled Sternum
This is an adaption to powered flight. The sternum is keeled to accommodate the extensive flight musculature (up to 40% of body weight). It is not present in earliest birds which would have probably been weak flyers, but it is present in all modern flying birds. The sternum is reduced t a flattened plate in modern flightless birds.
For diagram of the avian skeleton click here
The Avian Wing
The various wing morphologies seen in modern birds are indicative of their lifestyle. For example, long, narrow wings are designed for speed (e.g. the swift), whereas short, broad wings aredesigned for manoeuverability (e.g. songbirds). There are also certain features of the wing which enhance flight, such as the propatagium (a piece of skin which enhances the wing surface area) and the alula (a structure on the leading edge of the wing enabling low speed flight and precision landing).
Physiology
All modern birds are endothermic (warm-blooded) meaning they can regulate their own body temperature independently of their external environment. When the shift from cold- to warm-bloodedness occured is a hotly debated issue. It is hard to say whether this happened at some point in the theropod lineage or within the avian lineage.
Another universal feature of modern birds is a 4-chambered heart, for efficient pumping of blood around the body, in order to meet the physiological demands of flight.
Author: Robert Davis
Last updated: 21/11/05
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