Bird Reproduction: Incubation Period of Eggs
There are no sharp divisions between one stage and the next. Even though many of a bird’s actions have been building up to egg-laying, nest-building may continue after the first egg has been laid as may also some of the courtship displays, particularly copulation and courtship feeding. Normally small birds will lay one egg each day, often in the early morning, which is the time that it takes the eggs to form round the ripe ovum. Large birds may lay theirs every other day as the larger eggs take longer to form. Each species tends to lay a clutch size which has been adapted by natural selection to correspond with the largest number of young for which the parents can, on average, provide enough food. Dr David Lack has shown that clutches above the normal size are at a disadvantage because the young are weakened by under-nourishment and as a result fewer survive than from clutches of the normal size.
Some birds, normally those large birds with long incubation periods, characteristically lay one clutch a year. Others may lay two, three or infrequently more clutches, and probably all but a few species of birds will replace the first if it is destroyed shortly after it is laid.
The number of days needed to incubate the eggs may be as few as eleven in some of the warblers, but up to fifty-three days, in the case of a Fulmar incubating a single egg. Normally the species which have open nests incubate their eggs for the shortest period: Wrens and Long-tailed Tits, for instance, which have domed and soundly constructed nests, may have incubation periods lasting fourteen to eighteen days. Most birds incubate, or cover the eggs, for about 60% to 80% of the time it takes them to hatch but some species may remain on them for longer periods. For instance, the female Goldfinch, fed on the nest by the male, may incubate the eggs for as much as 96% of the time the eggs are in the nest. Birds like Manx Shearwaters and Fulmars, whose eggs need to be incubated for seven to eight weeks, may stay as long as five days on the nest before they go off for a similar period to feed and let their mates take over. Full incubation usually begins, in passerines, just before the last egg is laid, which helps to ensure that the eggs hatch at about the same time.
Normally, the female incubates the eggs and keeps them warm by shuffling them in order to touch the incubation patches on her belly. Most species of birds, prior to incubation, develop these feather-free patches of thickened skin on the ventral surface of the body, where a rich supply of blood vessels facilitates the transfer of body heat to the eggs. These patches generally develop about four to six days before the eggs are laid. They persist throughout the incubation period and into the early part of the brooding period when they begin to return to normal.
When males do share in incubating, they are generally amongst the less colourful, and in certain passerine species they lack incubation patches. They sit on the nest quite regularly, just keeping the eggs warm whilst their mates are away. In the exceptional cases of the Dotterel and the Red-necked Phalarope the males do all the incubating. The Gannet has no brood patch and places both webs of its feet over the egg, and the warmth of the web is responsible for incubation.
The male’s duties in this early part of the nesting period are to defend the territory and the nest. At this point, the males, some of which have been silent since pair formation, start singing again. At times they give the appearance of considerable idleness. Now song has not only the effect of keeping intruders out but also, at least in the early stages of the incubation period, of giving the female confidence in her new situation.
- Reproductive System of Birds (birdwatchtips.info)