Food Supply and Hatching Spans of Birds: Energy Constraints or Facultative Manipulation

Clutches of altricial birds generally hatch over a period of hours or days. Numerous hypotheses have been generated to explain hatching asynchrony, but most experiments have focused on the nestling stage to determine some post—hatch benefit such as increased reproductive success. We concentrated on proximate causes in the prelaying period, which have largely been ignored. In northern Saskatchewan, we censused prey species (small mammals) in early spring and measured the asynchrony of 273 clutches of the American Kestrel (Falco sparverius) over 4 yr. Hatching asynchrony was quantified in two ways: time for all the eggs within a clutch to hatch and mass hierarchies among nestlings. In good food years, the hatchling 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 successful 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, but not with the energy constraint hypothesis.