Cassin's Auklet (Ptychoramphus aleuticus) Population Size, Reproduction, and Habitat Management on a Recently Colonized Island in California, USA

Abstract

Responses to climate change by seabirds in the North Pacific may include range restrictions and require colonizing new habitats. To inform conservation actions supporting climate adaptations, we examined the colonization of Ano Nuevo Island, a nearshore island in central California, by Cassin's Auklets (Ptychoramphus aleuticus). We quantified population growth, reproduction, band returns, mortality, and habitat metrics from 1995-2017 and described habitat management that facilitated colony persistence. Cassin's Auklet breeding population grew to 136 birds by 2014, despite population declines during 2005-2007 and 2016-2017 concurrent with reproductive failures and die-offs that affected Cassin's Auklets regionally. Annual productivity of this small colony was similar to larger populations in the region at 0.72 ± 0.23 SD chicks fledged per pair from 1999-2017 (n = 15 years). Band returns indicated population connectivity with the Farallon Islands, California. Annual rates of burrow damage were 14 ± 8%, with up to 30% damaged a year. Habitat management to prevent erosion damage to nesting burrows included sea lion exclusion, erosion control, and ceramic artificial burrows. Describing conditions that facilitated the colonization and growth of this Cassin's Auklet breeding site can inform actions that support other locations and multiple burrowing seabird species.