Colony-nesting gulls restrict activity levels of a native top carnivore during the breeding season

Although nesting in colonies can offer substantial reproductive benefits for many seabird species, increased visibility to predators remains a significant disadvantage for most colony-breeders. To counteract this, some seabird species have evolved aggressive nest defense strategies to protect vulnerable eggs and chicks. Here, we used an experimental approach to test whether colony inhabitance by breeding gulls Larus spp. in western Norway impacts visitation rates of a native, mammalian predator, the Eurasian otter Lutra lutra during the breeding season. Camera traps were placed inside of and on the periphery of seabird colonies prior to the breeding season and left to run for one continuous year. Sighting frequency of otters on these cameras was compared to a control region free of gull nesting. We found that otter activity was significantly reduced in the colonies when gulls were incubating and rearing chicks, compared to time periods when gulls were building nests and absent from the colonies. Rhythmic activity patterns did not seem to be significantly impacted by the presence of gulls. This study provides clear evidence that certain colony-nesting species can have a direct, negative impact on visitation rates of a native carnivore. Seasonal carnivore activity patterns are likely to be highly dependent on differing nesting strategies and level of nest defense by seabirds.