The effects of individual and environmental heterogeneity on long-term population dynamics of Cassin's auklets (Ptychoramphus aleuticus)

Abstract

Reproductive output and survival are expected to be balanced through a tradeoff between current success and future potential, in response to environmental conditions that vary on spatial and temporal scales. Long-term datasets that follow uniquely marked animals through time are excellent tools for describing how heritable or derived traits that influence reproduction and survival can be attributed to individual quality, and how the added reproductive performance of these individuals influence population dynamics. A 37-year record of breeding histories from known-aged Cassin's auklets from Southeast Farallon Island, a colony off the coast of central California, was used to examine these ideas in the context of a behavior unique to long-lived birds called double brooding. The results of generalized linear mixed modeling and multistate mark-recapture models revealed that double brooding, a form of increased immediate breeding effort, was associated with both higher reproductive output and longer lifespans. Older individuals that initiated breeding early in the season were most likely to attempt a second brood, particularly when food availability was high. Multistate mark-recapture analyses showed individuals that double brooded many times throughout their lives incurred no apparent longterm costs to survival or longevity. Oceanographic conditions related to prey abundance in the summer months affected the rates of double brooding, and using three years of movement data were shown to be important drivers of winter habitat selection as well. Findings at the individual level present strong evidence of a positive relationship between double brooding and survival that can only be attributed to some measure of individual quality. At the population level, when competition for breeding sites was relaxed, higher rates of double brooding had a positive effect on future recruitment rates; buffering the population against climate-driven periods of low adult survival.