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

Abstract

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.

Demographic responses of eastern bluebirds to climatic variability in northeastern Arkansas

“…Consistent with other studies (Koenig & Walters, 2018;Musselman, 1935;White & Woolfenden, 1973), a post hoc analysis revealed that within each breeding season, hatchability declined from start to end of the season (0.92 ± 0.004 to 0.89 ± 0.013, p < .001). Our best hatchability model contained variability in precipitation in the nonbreeding season (Table 3), with this variable having an inverse relationship with hatchability.…”

High temperatures drive offspring mortality in a cooperatively breeding bird

“…One prominent explanation for the occurrence of cooperative breeding in birds in variable environments is that it represents a 'bethedging' strategy [38], whereby breeding individuals share the costs of reproduction with helpers and are thus able to reduce interannual variation in reproductive success in response to unpredictable rainfall and food availability [9]. This implies that cooperation might buffer breeding attempts from failure during adverse environmental conditions [13,41].…”

Nestling provisioning behavior of Black‐backed Woodpeckers in post‐fire forest

“…In some species, lower‐quality parents have smaller broods (Sydeman et al. 1991) and tend to nest later in the breeding season (Koenig and Walters 2018). Thus, higher provisioning rates earlier in the season could be due to the presence of a greater number of nestlings.…”