Todd-Zaragoza et al: Egg size of Scripps’s Murrelets at Santa Barbara Island 1 Marine Ornithology 51: 1–9 (2023) Contributed Papers 1 LAYING SEQUENCE AND OCEANOGRAPHIC FACTORS AFFECT EGG SIZE IN SCRIPPS’S MURRELETS SYNTHLIBORAMPHUS SCRIPPSI AT SANTA BARBARA ISLAND MARCELA I. TODD ZARAGOZA 1 , AMELIA J. DuVALL 2 , JIM A. HOWARD 3 , DAVID M. MAZURKIEWICZ 4 & SARAH J. CONVERSE 5 1 School of Environmental and Forest Sciences, University of Washington, Seattle, Washington 98195, USA (marcelat99@outlook.com) 2 Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, USA 3 California Institute of Environmental Studies, Davis, California 95617, USA 4 Channel Islands National Park, Ventura, California 93001, USA 5 U.S. Geological Survey, Washington Cooperative Fish and Wildlife Research Unit, School of Environmental and Forest Sciences & School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, USA Received 30 December 2021, accepted 01 September 2022 ABSTRACT TODD ZARAGOZA, M.I., DuVALL, A.J., HOWARD, J.A., MAZURKIEWICZ, D.M. & CONVERSE, S.J. 2023. Laying sequence and oceanographic factors affect egg size in Scripps’s Murrelets Synthliboramphus scrippsi at Santa Barbara Island. Marine Ornithology 51: 1–9. Egg size is an important avian life history parameter, with larger eggs indicating greater investment of resources in the chick. Prey availability can affect such investment. We investigated the effects of oceanographic conditions and laying sequence on Scripps’s Murrelet Synthliboramphus scrippsi egg size at Santa Barbara Island, California during 2009–2017. We evaluated oceanographic covariates characterizing marine productivity for their effect on egg size, including large-scale oceanographic indices such as the Pacific Decadal Oscillation (PDO) index, Oceanic Niño Index (ONI), and North Pacific Gyre Oscillation (NPGO) index. We also evaluated a larval anchovy catch-per-unit-effort (ANCHL) index and the Biologically Effective Upwelling Transport Index (BEUTI) as region-wide indices, and sea surface temperature (SST) as a local index. We evaluated oceanographic conditions over the entire year and during the breeding season only. We also considered the contribution of lagged effects to oceanographic conditions. Our results generally ran counter to our hypothesis that increased ocean productivity should increase egg size. Based on Akaike’s Information Criterion, the four top-ranked models provided support for an association between larger eggs and conditions indicative of lower oceanographic productivity, including lower values of BEUTI and NPGO, and higher values of ONI, PDO, and SST. The only result that supported our hypothesis was a positive relationship between ANCHL and egg size, although the 95% confidence interval for the effect included 0. The strongest relationship detected was between laying sequence and egg size, as second eggs were considerably larger than first eggs. Our results indicate substantial complexity in the relationship between ocean productivity and seabird demography. A better understanding of how ocean productivity affects seabird breeding outcomes through multiple mechanisms will help improve predictions of how seabirds will respond to changing ocean conditions. Key words: Alcidae, egg size, laying sequence, index, ocean productivity, monitoring INTRODUCTION Fluctuating marine conditions that affect prey availability have direct impacts on seabirds. For example, Cassin’s Auklets Ptychoramphus aleuticus at Triangle Island (British Columbia, Canada) and the Farallon Islands (California, USA) exhibit reduced offspring survival and fledgling mass due to limited prey availability in years with warm sea surface temperature (SST; Ainley et al. 1990, 1995; Hipfner 2008). Changes in marine conditions have also been linked to declines in growth rate and fledging success of Tufted Puffins Fratercula cirrhata off the coast of British Columbia (Gjerdrum 2003) and changes in clutch size of Scripps’s Murrelets Synthliboramphus scrippsi off the coast of California (Roth et al. 2005). In general, egg size varies with the amount of energy invested in egg production, and the energy available to invest can vary with environmental conditions (Williams 2005). In Norway, Atlantic Puffin Fratercula arctica populations at two separate colonies (investigated during 1980–2011) showed parallel declines in population size and egg volume (Barrett et al. 2012). Investigators found relationships between egg volume and various prey population indices (both positive and negative effects), the North Atlantic Oscillation (positive effects), and SST (negative effects), and they concluded that low food availability led to lower egg size. In contrast, Hipfner (2012) found that egg size increased with SST for Glaucous-winged Gulls Larus glaucescens at Triangle Island, but the effect was small and limited to one and three-egg clutches. The Scripps’s Murrelet (hereafter “murrelet”) is a small seabird in the Alcidae family that breeds on islands in the southern California Current, from southern California, USA, to central and Baja California, Mexico. Little is known about the relationship between marine or other oceanographic conditions and murrelet breeding success. Research has indicated that higher ocean productivity leads to earlier clutch initiation and larger clutch sizes (Roth et al. 2005). Also, Thomsen & Green (2017) found a negative correlation between murrelet nest success and drought severity at Santa Barbara Island, USA, due to drought-induced predation on eggs by deer mice Peromyscus maniculatus introduced to the island by Indigenous peoples. However, there has not been any analysis of TODD ZARAGOZA, M.I., DuVALL, A.J., HOWARD, J.A., MAZURKIEWICZ, D.M. & CONVERSE, S.J. Laying sequence and oceanographic factors affect egg size in Scripps’s Murrelets Synthliboramphus scrippsi at Santa Barbara Island