Hypothermic tolerance in an embryonic American kestrel (Falco sparverius)

Embryos of several bird species tolerate acute hypothermia. However, the ex tent to which it can be tolerated by wild species living under natural cond itions is poorly documented. At a single American kestrel (Falco sparverius ) nest 15 days into incubation, we observed a 21-h bout of egg neglect by t he parents during which nest (egg) temperature averaged 12.7 degrees C. Nor mal incubation patterns resumed thereafter, and one of three viable eggs ha tched 32 days after incubation onset, an incubation period 2.5 days longer than the mean in this study. The nestling appeared to develop and fledge no rmally. Although embryonic tolerance of extended hypothermia is known to oc cur in some seabirds, its presence in Falconiformes has not heretofore been recorded. Embryonic hypothermic tolerance may be adaptive in species with extended periods of parental absence during incubation.