ECOLOGICAL CORRELATES OF REPRODUCTION FOR A BET-HEDGING SEABIRD, THE KITTLITZ’S MURRELET BRACHYRAMPHUS BREVIROSTRIS

For longer-lived species, individuals hedge their bets by skipping breeding when conditions are poor to preserve the opportunity for future breeding attempts. Thus, understanding the ecological drivers behind the ‘choice’ to reproduce or not is a key step in identifying and diagnosing causes of population change. We investigated both marine and terrestrial factors that might influence whether Kittlitz’s Murrelets Brachyramphus brevirostris—a long-lived seabird of conservation concern that is often associated with glacially-influenced habitats—choose to breed in a given year. We used a suite of proxies to explain variation in reproductive metrics of radio-tagged Kittlitz’s Murrelets, exploring three hypotheses related to prey availability, nest depredation risk, and energetic costs of nesting in Icy Bay, Alaska, 2007–2012. In two of the years, we compared results with the locally co-occurring, closely-related Marbled Murrelet B. marmoraus. Across the six-year period of our study, the posterior mean breeding propensity of Kittlitz’s Murrelets was 0.21 (95% credible interval [CrI] = 0.15–0.27), nesting success was 0.41 (CrI = 0.25–0.57), and fecundity was 0.04 (CrI = 0.02–0.06). Although none of our three hypotheses were strongly supported by the data, our results collectively suggest that successful nesters expended greater effort than failed nesters and non-nesters to acquire sufficient prey—they stayed longer in the study area, had larger marine core use areas, spent slightly less time diving, and had shorter commuting distances and longer incubation shifts. Also, in years when fecundity was highest, overlap of individual marine core use areas was greatest, suggesting predictable foraging hotspots for murrelets in those years. Importantly, when comparing Brachyramphus species, we found that Marbled Murrelets outperformed Kittlitz’s Murrelets reproductively by three to four times in overlapping years, suggesting different drivers of reproduction. From our collective results, we posit that the limitation to reproduction of Kittlitz’s Murrelets may not be prey abundance or quality, but instead efficient access to prey. While our associations with indices of prey availability, depredation risk, and energetic costs to nesting did not explain much of the variation in fitness metrics, our study demonstrates the value of using a comparative approach and multiple metrics to diagnose factors limiting a population, especially when data are sparse. For bet-hedging species with complex life histories, we recommend using a combination of integrated modeling and a monitoring framework designed to accumulate evidence across studies to estimate population dynamics and improve inferences about ecological drivers.