Ocean warming events are becoming more frequent, with potential consequences for birds that use marine resources. We analyzed the annual population growth of Heermann's Gulls (Larus heermanni) through matrix-based demographic models under normal and high sea surface temperature (SST) conditions. Under stable, normal conditions, predicted population growth was ∼4%, with survival of fledglings and juveniles contributing to population growth. Under high SST conditions, predicted population growth dropped to −15% and adult survival became the key factor determining species fitness. Simulations showed a declining growth rate as the frequency of warm SST anomalies increased. Heermann's Gulls can resist oceanographic anomalies of up to 1 event every 5 yr without seriously compromising their population growth rate or their individual fitness, but populations may decline rapidly if the frequency of warm-phase anomalies continues to increase, as it has done during the last decade.
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Vol. 120 • No. 2