Nest-Failure and the Evolution of Hatching Asynchrony in the Least Flycatcher

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Abstract
(1) We used nest survival rates from a population of least flycatchers Empidonax minimus in Manitoba [Canada] to test the nest-failure hypothesis for the evolution of hatching asynchrony in birds. (2) Hatching within flycatcher clutches was asynchronous; mean hatching spread increased from 26.4 h in clutches of three to 45.8 in clutches of five. Hatching sequence did not affect growth rate or early post-fledging survival. Brood reduction from starvation occurred in only one nest (2.2%). (3) In contrast to partial brood loss, only 38% of nests successfully fledged at least one young; predation of the nest contents accounted for 83% of nest failures. Daily survival probabilities of nests were lowest during laying and highest just before fledging. (4) Observed hatching spreads were significantly less than predicted by the nest-failure hypothesis. In all clutch sizes, fledging productivity would have been maximized by initiating incubation on the first-laid egg. (5) Our results did not support the nest-failure hypothesis as fomulated by Clark and Wilson (1981) and Hussell (1985). We suggest several factor that might place constraints upon optimizing hatching asynchrony in order to minimize predation.