Birds

          Birds are vertebrates - they have a vertebral column and a skull. They are ‘warm blooded’; that means they maintain their body temperature at a constant level and above that of their surroundings. Their chief distinguishing characteristics are that they have feathers and their forelimbs are modified to form wings. 

          The feathers consist of a shaft with rows of fine filaments (barbs) on each side. The barbs themselves have finer filaments (barbules) branching from them. In the down feathers, the barbs and barbules are loose and fluffy. They trap an insulating layer of air close to the body and so reduce heat losses.

           The barbs and barbules of the flight and covert (or contour) feathers are organised in a regular way forming a smooth, water resistant cover. The barbules overlap so that the hooks on one set of barbules engage with grooves on the other set rather like ‘Velcro’. If the feathers get damaged in flight, they can be restored to shape by preening.

Adaptations to flight

The vertebrae of the back fused together and fused to pelvis; gives rigidity to skeleton during flight.

Deep ‘keel’ to sternum allows attachment of powerful flight muscles.

Hollow bones reduce weight.

Note. Wings are not an adaptation to flight; they are essential for flight

         The flight feathers overlap in a way that lets air pass between them during the upstroke, so reducing air resistance. During the downstroke the gap is closed so offering maximum air resistance and giving lift. 

Gliding flight. The wings are outstretched. Lift is provided either by upcurrents of air (e.g. thermals)

or by slowly losing height.
REPRODUCTION

          Birds reproduce by laying eggs which are fertilised internally before laying. The eggs have a hard shell and a food store in the form of yolk and albumen. The number of eggs laid depends on the species, and varies from 1 to about 20. They are laid in a nest which may be a carefully constructed structure in a bush or tree or hole, or simply in vegetation on the ground. It may be little more than a scrape in the shingle or a ledge on a cliff depending on the species.

          The eggs are kept warm by incubation; that is, the bird ‘sits’ on the eggs so that they acquire the parent’s body temperature. Incubation continues for a few weeks, during which time, the embryo develops and finally the young bird pecks its way out of the shell. 

          The parent birds ‘brood’ the chicks in the nest by covering them with their bodies, thus protecting them from heat loss and rain until their feathers develop. Most birds also feed the chicks until they are old enough to fly, and continue to feed them for a period even though they are out of the nest. In ground-nesting birds and some waterfowl, the chicks leave the nest soon after hatching and forage for food, though always in the company of a parent.