Incubation Time
photograph of European Starling by Rohan Kamath	How much time different species actually spend sitting on the eggs during the incubation period is even more variable than who does the sitting. Individual bouts of incubation by many small passerines such as wrens may last less than ten minutes; an albatross, in contrast, may sit on its eggs continuously for weeks at a stretch. Where only one parent incubates, it usually spends about two-thirds to three-quarters of the daytime hours on the eggs, and the remainder feeding. Flycatchers and others that hunt flying insects spend only slightly over half their time on the nests. Small birds have high metabolic rates, and stoking their fast-burning fires exhausts fat reserves rapidly. They could not survive even a small part of a normal albatross incubating session.
The constancy of incubation often is genetically controlled and adapted to the habitat of the species. It may have a profound effect on the ability of different species to colonize new areas. For example, two starling species, the European Starling and the Asian Crested Myna, were both introduced into North America in the late 1800s. The species have very similar breeding habits, but the former has spread over virtually the entire continent; the latter has remained restricted to the vicinity of Vancouver where it was introduced. One hypothesis to explain the different successes of these two close relatives is that the myna's incubation constancy is genetically attuned to its subtropical homeland. It sits on its eggs for only about half of the day; the starling incubates for almost three-quarters of the hours of daylight. Although both lay clutches of 5 eggs, the starling successfully rears an average of 3.5 young per clutch; the myna manages to fledge an average of 2. This relatively low reproductive rate may account for the myna's limited success in Vancouver, compared with the explosive spread of the European Starling. Since, in general, birds do not begin incubating until the clutch is complete, "incubation time" is defined as the period from the laying of the last egg of the clutch until that egg hatches (or, if individual eggs can't be identified, from the last egg laid to the first egg hatched). It is one more aspect of incubation that varies a great deal. Incubation time is roughly correlated with the weight of the egg. The eggs of small songbirds generally hatch in about 11 days; those of the Royal Albatross in about 80 days. More information on incubation, based on careful, long-term observations of nests, is needed for most species of North American birds. Data to be gathered include who incubates (and if both parents do, how the load is shared), the proportion of time the eggs are covered in different kinds of weather, egg turning frequency, and elapsed time from laying to hatching.
SEE: Incubation: Heating Eggs; Hatching Asynchrony and Brood Reduction; Who Incubates? Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.