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.