For an egg to develop normally, it must be exposed for a considerable length of time to temperatures a few degrees below the normal 104 degrees F (40 degrees C) avian body temperature. Indeed, the ideal incubation temperature for many birds' eggs is about human body temperature, 98.6 degrees F. Almost all birds create the required temperature by sitting on the eggs and incubating them, often transferring heat via a temporarily bare area of abdominal skin called the "brood patch."
On the other hand, the embryo inside the egg is also very sensitive to high temperatures, so that in some situations eggs must be protected from the sun. Ducks with open nests, for example, will pull downy feathers (originally plucked to form their brood patches) over the nest to cover the eggs when they leave it, providing shade if the weather is hot and helping to retard heat loss when it is cold.
Brood patch and nest care exchange
Video sent by glo77
Embryos are less sensitive to cold than to heat, particularly before incubation has started. Mallard eggs have been known to crack by freezing and still hatch successfully. Eggs cool when incubation is interrupted, but this is not usually harmful, and few birds incubate continuously. Instead egg temperature is regulated in response to changes in the temperature of the environment by varying the length of time that a parent bird sits on them or the tightness of the "sit." For instance, female House Wrens (which incubate without help from the males) sat on the eggs for periods averaging 14 minutes when the temperature was 59 degrees F (15 degrees C), but an average of only 7.5 minutes when it rose to 86 degrees F (30 degrees C).
Many birds apparently sense the egg temperature with receptors in the brood patches, which helps them to regulate their attentiveness (time spent incubating) more accurately. Since the embryo itself increasingly generates heat as it develops, periods of attentiveness should generally decline as incubation progresses. Attentiveness is also influenced by the insulating properties of a particular nest.
Eggs are also turned periodically -- from about every eight minutes by American Redstarts to once an hour by Mallards. The turning presumably helps to warm the eggs more evenly, and to prevent embryonic membranes from sticking to the shell.
Eagles and boobies exhibit "obligate siblicide," in which the larger chick invariably kills its smaller sibling. For example, of more than 200 records of two-egg clutches followed in the Black Eagle of southern Africa, only one record exists of both chicks surviving to fledging. Obligate siblicide also occurs among pelicans, owls, and cranes. In obligate siblicide, which occurs even when food supplies are abundant, the second egg serves as insurance against loss of the first egg from infertility, predation, or damage, rather than as a means of rearing two chicks.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.
My added comment. Just this one sentence on obligate siblicide once again points to the
Season of Miracles for 2006, and this particular pair of Birds and their 3 successful fledges!!!!
