Incubation is defined as adding heat to the eggs. Most birds, except ratites, pelicans and relatives, and waterfowl, develop brood patches (featherless areas) in the ventral abdominal wall just before incubation. The brood patch becomes very infused with blood vessels and sensitive to temperature. The incubation period is the time between the laying of the last egg in a clutch and the hatching of that egg – except that birds can sit on eggs without adding heat to them- the brood patch must be applied to the eggs. In about 54% of all bird families, both sexes incubate; in 25% only the female does so, in 6% only the male, in 15% by either, and in one family (brush turkey) none.
Incubating birds roll their eggs around. This serves to distribute the heat evenly and prevent the egg membrane from sticking to the shell. The appearance of cracks in the eggshell is due to pipping by the chick. The chick extends its neck upward and pokes the shell with the upper mandible on which is an egg tooth. (Goslings, a day before hatching, when they have started to breathe, can actually produce an alarm call if their egg is upside down – the female goose will roll them upright.) The back of the head has a special hatching muscle that degenerates a few days after hatching (as does the egg tooth). Both of these structures are also present in reptiles.
Incubation times vary from 9-11 days in songbirds and Hummingbirds to 60-80 days in penguins and albatrosses; in the Royal Albatross it is 77-80 days.
No eggs can survive continuous exposure above 40 degrees or below 34 degrees once incubation begins. The colder the ambient temperature, the more hours a day the egg will be incubated. The parent bird senses through its brood patch the temperature of the egg and applies more or less heat to it. If the eggs do not hatch, the birds may incubate them for 2-3x normal time.
Generally, eggs hatch in the order laid (asynchronous hatching) which results in different size young. In some species (hawks, owls, storks, crows, and hornbills), they begin to incubate as soon as the first egg is laid. Waterfowl, chickens and relatives, and songbirds begin to incubate only after all eggs are laid – synchronous hatching makes incubation and parental care easier as incubation and feeding don’t have to occur simultaneously. This is especially important for precocial birds so the young can follow parents.
After the young hatch, the altricial birds beg for food. Begging may be set off by parents landing at the nest or marks on the parents’ bill at which the chicks peck (gulls). Many young have gape marks – marks in the mouth that induce parents to feed. Parasitic Weaverbirds’ young have gape marks that imitate that of the host. Some altricial birds such as hawks are simply presented with food and the young manage to eat it. Other parents regurgitate partially digested food into the nest for the young (storks). Precocial birds teach their young what food to eat, but much of it is trial and error.
Most altricial birds leave the nest fully feathered about 2 weeks after hatching – may or may not be able to fly but are usually still dependent on their parents for food. Most songbirds feed their young for two weeks after fledging – leaving the nest. Altricial birds are born blind and helpless and naked but with a high metabolic rate and grow rapidly. Precocial birds are hatched feathered and can walk and feed on their own but have a lower metabolic rate and grow relatively slowly.
Varies widely, but let’s look at a typical passerine, the House Wren. The male builds a nest of sticks in a natural cavity and the female provides the nest lining. 3-5 days after mating the first of 5-7 eggs is laid. Incubation begins gradually after the first egg; the brood patch is fully developed by the time the last egg is laid at which time incubation is total. The female sits on the eggs for 12 minutes and then leaves for 8.5 minutes to feed. Sits on eggs overnight. These times vary with ambient temperatures. When the young hatch they are still brooded by the female. As thermoregulation develops in the young, they are brooded less and less. Feeding rates to nestlings vary with clutch size. The larger the clutch the more foraging trips the adults make, but the less food each nestling gets.
As a result, larger broods stay in the nest a day or two longer than smaller broods to reach the same size. (Lack, 1947) – Starlings (Sturnus vulgaris)
|Brood Size||Number of Nests||Successful # Young (per nest)|
Some parents will divide food among the young by feeding from the same spot in the nest and let the young rotate within the nest.
The amount of food consumed by an altricial nesting in a day may exceed its body weight. Here is an example of development of the young House Wren (Kendeigh, 1963).
House Wren Development
|Days of Age||Weight (gms)||Primary Length||Body Temp. C|
Before the young are 9-10 days old they have no sense of fear. If they did, they could be frightened and fall or jump out of the nest. Not thermoregulating yet, they would die. But after 10 days, they can thermoregulate and would be better off scattering from the nest than facing a predator. Then sense of fear is instinctive but recognizing a threat has to be learned.
Flight may occur right after leaving the nest or not for a week or two later. Flying is instinctive, but some learning is required. Many birds, such as hawks and eagles, develop their muscles by flying short distances from the nest. Sparrows and warblers tend to leave the nest at about 10 days of age and hop around on the ground for another weeks before flying. Cavity nesting birds tend to stay in the nest longer and fly fairly well after leaving.
Members of five families of birds do not build nests or raise their young. The Black-headed Duck of South America, for example, lays its eggs in the nests of rails, coots, ibises, gulls, and terns.The Eurasian Cuckoo is the most well known of nest parasites and various races of the species Cuculus canorus parasitize 125 other bird species. At breeding time the female cuckoo finds a host female building a nest or incubating eggs. This stimulates the cuckoo to ovulate. A few days later the cuckoo waits for the host female to leave the nest and then lays an egg in 5-8 seconds, after removing one of the host’s eggs. Lays one egg to a nest but may lay 12-25 eggs, each in a different nest. The host bird may accept it, throw it out, or abandon nest. The cuckoo egg may resemble host’s eggs in both color and size. About 10 hours after hatching, the young cuckoo instinctively pushes out anything that touches its back. This instinct lasts 3-4 days by the end of which the other young and eggs are gone; host feeds young cuckoo. The Brown-headed Cowbird of North America parasitizes 125-150 species but is not as specialized; eggs don’t match those of host and young don’t push eggs or young out.
A successful nest is one in which some young are fledged. The % of young fledging varies widely within and between species. Among songbirds in temperate areas, the proportion of eggs that eventually become flying young varies between 30 and 80%. M. Nice (1957) analyzed data from 7788 open nests of altricial birds and found that the success rate ranged from 38-77% (avg. 49%). Of cavity-nesters, 66%. For asynchronous hatching species the chief cause of mortality is starvation. For synchronous hatchers it’s starvation, predation, and climate. Predation is probably the chief factor in the tropics.
Once the young leave their parents and nesting areas, their chances of surviving to breed the next year are about 60% – the same as an adult. So, for a typical songbird:
% of survival to fledging = 50%
% of survival to juvenile from fledging =50%
% of survival to breeding from fledging = 60%
Chance of surviving from egg to breeding adult =15%.
The age at which a bird first reproduces is important to the growth rate of the population. Virtually all terrestrial species breed in the first year after hatching except swifts (2 years), many parrots (2-3 years), and raptors (3-5 years) and a few males of some passerines (Yellow-headed and Red-winged Blackbirds).
Heavy mortality is associated with migration. Mortality is mainly due to the weather, food shortage, and predation. Weather can be particularly disastrous at times. Small temperate zone passerines sustain a high mortality. In Song Sparrows and titmice, over 70% of the adults die each year. Thus an average individual can expect to live one year. In most songbirds the figure is 40-60% mortality. In shorebirds it is 20-40%. Yellow-eyed Penguin 10%. Royal Albatross 3%.
But mortality is independent of age after maturity in birds. In other words, birds generallly do not die of old age, but succumb to environmental factors.