Kl. Wiebe et Gr. Bortolotti, FOOD-SUPPLY AND HATCHING SPANS OF BIRDS - ENERGY CONSTRAINTS OR FACULTATIVE MANIPULATION, Ecology, 75(3), 1994, pp. 813-823
Clutches of altricial birds generally hatch over a period of hours or
days. Numerous hypotheses have been generated to explain hatching asyn
chrony, but most experiments have focused on the nestling stage to det
ermine some post-hatch benefit such as increased reproductive success.
We concentrated on proximate causes in the prelaying period, which ha
ve largely been ignored. In northern Saskatchewan, we censused prey sp
ecies (small mammals) in early spring and measured the asynchrony of 2
73 clutches of the American Kestrel (Falco sparverius) over 4 yr. Hatc
hing asynchrony was quantified in two ways: time for all the eggs with
in a clutch to hatch and mass hierarchies among nestlings. In good foo
d years, the hatching spans of clutches were more synchronous than in
poor years. Similarly, parents on good territories or females in good
physical condition had more synchronous broods than parents under food
stress. In an experiment, we supplemented food to kestrels during the
prelaying period. Parents that were supplemented laid larger eggs and
hatched those eggs more synchronously, but control pairs were more su
ccessful at raising young to the fledging stage. Synchronous hatching
when food is abundant is consistent with the hypothesis of facultative
manipulation of hatching spans in relation to nestling food supply, b
ut not with the energy constraint hypothesis.