Clutch size tracks seasonal availability of flying insects, which are a likely ecological driver of the seasonal decline in reproduction and, because of the nonlinear relationship of flying insects with date, constitute a likely factor constraining even earlier nesting. This population exhibits an 11% seasonal decline in clutch size, a 60% seasonal decline in fledging success, and a 77% seasonal decline in fledgling overwinter survival. We contrasted the relative importance of these two hypotheses using a long‐term dataset of the cooperatively‐breeding acorn woodpecker (Melanerpes formicivorus) in central coastal California (USA). Two competing hypotheses have been proposed to explain such declines: the “timing” hypothesis, which states that conditions affecting reproduction decline intrinsically with date, and the “quality” hypothesis, which proposes that high‐quality individuals or individuals in high‐quality situations breed earlier.
This is especially true given that fall nesting is likely to increase as global warming takes place.Ĭlutch size and reproductive success decline seasonally in a wide range of temperate avian taxa. Thus, this phenomenon is at least an irregular part of the breeding biology of a substantial fraction of North American birds and should be considered a possibility in population studies of temperate-zone species. Furthermore, fall nesting is likely to have been underreported in the literature. Among North American terrestrial birds in general, fall breeding has been reported in 16% of all species and is significantly more common among residents and colonially nesting species, in which the frequency exceeds 25%.
Ecologically, fall nesting is closely tied to the acorn crop and thus to breeding success in the following, rather than the prior, spring. Fall nesting is less likely in groups in which either the mate or female breeding adults have undergone a change from the prior year, but groups are otherwise indistinguishable. Fledglings from fall nests constitute 4.3% of the population's total productivity and survive and recruit to the population at levels comparable to spring fledglings. Fall nests occur in about one-third of all years, taking place when the acorn crop is large and summer temperatures are relatively high. These latter nests are mostly initiated in late summer but often do not fledge until at least late September and are thus referred to here as 'fall' nests. One peak occurs in spring during which the majority of breeding takes place, while a second is centered in late August as the new acorn crop matures. Acorn Woodpeckers (Melanerpes formicivorus) at Hastings Reservation in central coastal California exhibit a bimodal peak in annual breeding activity.
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