Bird Reproduction: Parental Care
In the nidicolous species one of the first duties of parents is to dispose of the egg shells which could otherwise give away the position of the nest; they are either eaten or carried away as soon as they have been vacated by the nestlings. The nidifugous young vacate the empty eggs and the nest itself fairly rapidly so there is little point in moving the empty egg shells, though many do. In the nidicolous species brooding of the nestlings appears to be a continuation of the incubation instinct and the parents share brooding in the same proportion as they shared incubation.
After the eggs have hatched the males sing less often and show less aggressiveness in the defence of the territory, although they become more aggressive towards predators and human beings as the time approaches for the young to leave the nest. Even though they take only a small part in incubation, the males share in the feeding of the young and obviously they then have less time to sing or to fight with territory competitors. In the first few days after hatching the female broods tight, particularly until the young gain temperature control. In open nests, even after brooding proper has finished, the females will shelter the young from the sun and rain, spreading their wings in order to do so.
Parents do not feed nidicolous young immediately after hatching. Normally they bring the first food after an interval of two or three hours: in some species the interval may be much longer. But once they start bringing food the parents work at an increasing rate until by the time the young are ready to leave the nest they are rushing from one food source to the next in a frenzied fashion. The young birds soon become aware of their parents’ approach and they instinctively respond by a begging behaviour which initially involves the thrusting up of their necks with open mouths which show their bright-coloured gapes and tongue spots and which in their turn provide targets at which the parents can aim the food.
When the parents arrive at the nest the young respond with varying degrees of intensity. It is the young bird with the quickest and most effective response that gets the food until the swallowing response is inhibited by a full oesophagus. If the food is in small particles and is not swallowed quickly enough the parents may remove it and place is in another mouth. The rate of feeding increases daily because of the increasing needs of the fast-growing young.
In many instances it is the largest bird which gets the most food but there are ways of ensuring fairly equal sharing out of food amongst young birds. Wheatears nest at the end of a long burrow and consequently there is only one direction from which food can be brought. Once the eyes of the nestling are open and they have learnt where the food comes from the hungriest chick tends to sit at the edge of the nest nearest the entrance. As it is closest to the adult it is fed in each successive visit until it is so full that it has to defecate. It then clambers up on to the rim of the nest, and turns its cloaca outwards. Whilst it is up, and the parent is removing the faecal sac, the next hungriest young bird shuffles into its place. The bird on the rim, his task completed, has to struggle over the others to the back of the nest and sit there until it has digested what it has eaten and begins to feel hungry enough to struggle for the chief and favourite positions. Thus a rotation in the order of feeding is ensured.
In birds of prey the incubation of their two or three eggs which are laid two days apart usually begins with the first egg laid and consequently, if three eggs are laid, the first-hatched gets a lot more food than the last-hatched bird. Once started, this disparity in size is maintained throughout the period that the youngsters are in the nest. Thus, if there is an abundance of food the female eagle may be able to rear all three nestlings, but if there is a shortage then perhaps only the first-hatched will survive, having eaten its nest siblings.
Nest sanitation is important in order to prevent disease, and to lessen the chance of the nest being discovered. In nidicolous young, defecation usually occurs immediately after feeding although it may not occur after every food particle is brought. Generally speaking, the faecal sac, depending on age, is a mass of semi-solid uric acid and darkish intestinal excreta enveloped in thick mucus which is easily portable by parents. These sacs are regularly disposed of by the parents either throughout the nest life or, in some species, only in the first part of the nestling period. When the nestlings of some species are very small the parents will eat the excreta.
There is no necessity for nidifugous species to produce faecal sacs. Seabirds void their excrement over the edge of a cliff ledge and other nidifugous birds are constantly on the move and there is little chance of an accumulation of faeces.