Devil's in the details: A multi-colony perspective on the foraging ecology of western gulls

Abstract

**Abstract:** Plasticity in foraging behavior among individuals within a colony can reduce competition but few studies have compared the generality of foraging patterns among individuals from multiple breeding colonies. Western gulls (Larus occidentalis) are generalist foragers that consume a range of marine and terrestrial foods but multi-colony comparisons are limited. Here, we used tail-mounted GPS loggers to compare the foraging behavior of 156 western gulls breeding at seven colonies ranging from southern California to Oregon, USA. Colonies varied dramatically in population size (100 to 10,000 nests) and distance from the mainland (150 km roundtrip) and for longer durations (12-24 hrs) compared to foraging excursions at sea. More than 50% of all gulls (predominantly from the largest colonies) exhibited a mixture of marine and terrestrial foraging whereas gulls from both smaller colonies in Oregon foraged mainly at sea. Foraging sites on land included landfills, waste management centers, and parks whereas marine habitats were primarily over neritic waters or along coastlines. Colony-specific home range varied with colony size (2,034 km^2 vs. 14,230 km^2, smallest and largest colony, respectively) as did mean individual home ranges (309 ± 311 SD km^2 vs. 986 ± 786 SD km^2, respectively), consistent with greater resource competition. Population-level plasticity in foraging behavior was evident and dependent on habitat type. In general, gulls from the larger more distant colonies were away longer when foraging in terrestrial habitats than gulls foraging at sea, and this may impact territorial defense and attempts at nest predation by conspecifics. **Authors:** Scott Shaffer¹, Rachael Orben², Pete Warzybok³, Russell Bradley⁴, Jaime Jahncke³, Leigh Torres², Robert Suryan⁵, Corey Clatterbuck⁶, Hillary Young⁷ ¹San Jose State University, ²Oregon State University, ³Point Blue Conservation Science, ⁴California State University - Channel Islands, ⁵National Oceanic and Atmospheric Administration, ⁶San Diego State University, ⁷University of California