Why Some Birds Take so Long to Hatch

In general, incubation periods of bird eggs are positively correlated with egg weights. The eggs of some birds take much longer to hatch than expected. Highly variable and long incubation periods are explained using fork-tailed storm petrels as an example. Their eggs take two to three times as long to hatch as eggs of similar size. Egg neglect causes the extreme variability in incubation periods within a species. Low incubation temperatures prolong incubation, but long incubation periods may also be a result of selective forces that favor hatching well-developed young that can be attended infrequently. Past terminology has obscured the importance of the relationship between incubation periods and the maturity of chicks at hatching. Selection has produced a continuum between cheap eggs and expensive care for the relatively helpless (altricial) young or expensive eggs and inexpensive care for the relatively independent (precocial) young. The distant foraging habits of the Procellariiformes have favored independent hatchlings.