Almost every structure in a bird's body is changed in some way
that enhances flight, and these changes are evident even in flightless
birds. The bones are hollow with a lattice-work internal support,
and so are strong but light. Other adaptations reducing the weight
of birds include the absence of some organs, for example, females
have only one ovary, and all birds have extensive airsacs throughout
the body.
Birds are toothless, an adaptation that trims the weight of the
head. Food is not chewed in the mouth but ground in the gizzard,
a digestive organ near the stomach. The gravel that birds swallow
to use to grind their food probably is as heavy as a full set of
teeth (or more), but this weight is positioned nearer the centre
of gravity than at the end of the neck. The bird's beak, made of
light-weight keratin, has proven to be easily modified during evolution,
taking on a great variety of shapes suitable for different diets.
Flight and Sight
Flying requires great expenditure of energy from an active metabolism.
Birds are homeothermic endotherms; they use their own metabolic
heat from inside to maintain a warm, constant body temperature,
aided by insulation provided by feathers and a layer of fat. However,
like some mammals, a few birds go into a state of torpor to conserve
energy at night.
An efficient circulatory system with a four-chambered heart that
segregates oxygenated blood from oxygen-poor blood supports the
high metabolic rate of the bird's cells. The efficient lungs have
tiny tubes leading to air sacs that help dissipate heat and contribute
to the trimming of density.
For safe flight, senses, especially vision, must be acute. Birds
have excellent eyes, perhaps the best of all vertebrates. The visual
areas of the brain are well developed, as are the motor areas; flight
also requires fine, rapid coordination.
Reproduction
With brains proportionately larger than those of reptiles and
amphibians, birds generally display more complex behaviour. Avian
behaviour is particularly intricate during breeding season when
birds engage in elaborate ritual courtship, and build intricate
nests.
Since the shell of an egg and the surrounding membranes are formed
just before the egg is laid, fertilization must be internal, prior
to the formation of these structures. The act of copulation is somewhat
awkward because the male of most bird species has no penis. He must
climb atop the female's back and then twist her tail so the mates'
vents, the openings to their cloacas, can meet and transfer sperm.
After eggs are laid, the avian embryo must be kept warm through
brooding by the mother, father, or both, depending on their species.
What Makes a Bird: Feathers and Wings
The bird's most obvious adaptation for flight is its wings, Their
shape conforms to the same principles of aerodynamics as the wings
of an airplane, but the avian wings must provide propulsion as well
as lift.
The combination of extreme lightness and strength make feathers
one of the most remarkable vertebrate adaptations. Feathers are
made of keratin, the same protein that forms our hair and fingernails
and the scales of reptiles. In fact, feathers evolved from reptilian
scales. Only birds and a few theropods, dinosaurs considered closely
related to birds, have feathers. Indeed, the presence of feathers
is enough to classify a modern animal as a bird, but some birds
have scale-like feathers (penguins) and others have hairy feathers
(kiwi).
Endangered Species
The great diversity of habitats found in the southern Okanagan
and Similkameen valleys of British Columbia is reflected in the
rich fauna of breeding birds. Because of low or declining numbers
and restricted distribution, many of these species have been placed
on British Columbia's Red List, a compilation of species considered
at great risk.
The Sage Grouse (Centrocercus
urophasianus) is an example of a species of the Family
Phasianidae (Partridges, Grouse and allies); it lived in the Okanagan
Valley in historical times but is now extirpated.
The threatened Yellow-breasted
Chat (Icteria virens) and the endangered Sagebrush
Brewer's Sparrow (Spizella breweri breweri) belong
to the Family Emberizidae (Towhees, Sparrows, Juncos, and Buntings).
The Sagebrush Brewer's Sparrow is not endangered as a species, but
the subspecies S. brewerii brewerii is at risk here in
BC.
The White-headed Woodpecker
(Picoides albolarvatus) is a threatened member of
the Order Piciformes (Woodpeckers).
The Burrowing Owl (Athene
cunicularia) is an endangered member of the Family Strigidae
(Typical Owls).
All of these species of birds are limited in their distribution
by availability of suitable habitat. The grasslands, waterside vegetation
and pine forests in which they live in the Thompson-Okanagan valleys
are being decimated expanding towns, intensive agriculture, and
a multitude of other industrial activities.
this section sponsored by: 
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