Comparison of Diets in the California Least Tern (Sternula antillarum browni) at Two Sites in Central California, U.S.A.

Abstract. We evaluated diet and diet assessment methods for the California Least Tern (Sternula antillarum browni) at two nesting sites in California over 12 years (2001–2012). California Least Tern diets at Alameda Point (37° 47′ 14″ N, 122° 19′ 12″ W), an estuarine site, and Purisima Point (34° 46′ 39″ N, 120° 37′ 35″ W), an open coast site, were compared using dropped fish and hard parts (otoliths and scales) from regurgitated pellets and fecal samples. SIMPER analyses, Kendall's tau, Kruskal-Wallis, Dunn's test, rank-sum tests, and Welch's t-test determined any differences in assessment methods, chick and adult diets, and sizes of prey items. Diet composition differed between sites for both dropped fish (25% similar) and fecal samples (26%). Assessment methods showed similar results at Alameda Point (79%) for dropped fish and fecal samples but not at Purisima Point when comparing dropped fish to fecal samples (12%) and fecal samples to regurgitated pellets (19%). There was no difference in diet composition at either site or between any method using adult-only samples. All differences in diet composition appeared during the rearing/fledging stage. Fish species dropped at both sites were deeper-bodied (17 mm) than those consumed (11.9 mm), with terns at Purisima Point dropping deep-bodied species typically not consumed and those at Alameda Point dropping larger individuals of usually-consumed species. When comparing adult and chick diets at Alameda Point using fecal samples, composition was similar, and chicks ate larger prey. Our results suggest that more than one assessment method is necessary to provide a complete dietary picture.