Causes of seasonal decline in reproduction of the cooperatively-breeding acorn woodpecker

Clutch size and reproductive success decline seasonally in a wide range of temperate avian taxa. 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. 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). 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. 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. By parsing lay date data into within-female and between-female components, we found that only the within-female component had a statistically significant direct effect on clutch size, supporting the timing hypothesis. For both fledging success and overwinter survival, however, both within- and between-female effects were highly significant, with between-female effects (differences in female quality) being stronger than within-female effects. These results suggest that timing per se is a key factor affecting the seasonal decline in clutch size, but that both differences in female quality and an intrinsic seasonal decline in conditions drive the even more dramatic seasonal declines in fledging success and overwinter survivorship of fledglings.