Survival vs. reproduction: dimorphic life history strategy in Heermann’s gulls during warm ocean temperature anomalies

Abstract

Seabirds have thrived over time in marine environments with highly variable productivity. Life history strategies favored by natural selection consist of late sexual maturation, few offspring per breeding event, high adult survival, long lifespan, and deferred breeding when food availability is low. Heermann’s gull Larus heermanni, with 95% of its world population breeding on Isla Rasa, Gulf of California, follows this common pattern, including poor breeding performance during periods of low food availability often associated with high sea surface temperatures. Our interest was to characterize individual variations in this general pattern and assess its consequences for lifetime number of fledglings produced. We compared life history parameters from a set of known-age females banded during 1984-1993 and recaptured during 1989-2013 and found that most (85%) followed the general pattern (hereafter ‘conservative’) of laying ≤2 eggs and fledging no chicks during anomalously warm years. However, 15% of the females (hereafter ‘bold’) laid eggs during all anomalously warm years and fledged chicks in most of those years. Throughout their lifespan, conservative females lived significantly longer and produced significantly more eggs, but the total number of fledglings was not significantly different between strategies. Both strategies coexist over time, persisting within the population under conditions that were common until the late 20th century. If conditions continue to vary, with the occurrence of heatwaves and increases in the frequency of anomalously warm years, bold females may have a selective advantage over conservative females, which might help mitigate the impacts of ocean warming on future populations.