Nest defense intensity does not vary with time of day in American Herring Gulls (Larus smithsonianus)

Nest defense in birds is vital for the protection of their young but can prove energetically costly. Birds often show plasticity in nest defense depending on factors such as threat level, mate presence, nest stage, and body condition. Such factors can vary over temporal extents ranging from a diel cycle to a breeding season or an individual's lifetime. Quantifying diurnal variation in nest defense intensity can be particularly useful for understanding time-energy budgets in breeding birds, but it remains understudied. Here we investigated how nest defense by American Herring Gulls (Larus smithsonianus) in response to a predatory threat varies with time of day. We simulated predatory threat at 26 Herring Gull nests during four different times of day: Early Morning (0530 - 0600 h), Late Morning (1000 - 1030 h), Afternoon (1400 - 1430 h), and Evening (1900 - 1930 h). Based on previous findings of diurnal activity patterns in American Herring Gulls and closely related species, we predicted that nest defense intensity - measured as aggressiveness of response and latency to calm - would be greater during the early morning and late evening than at other times. Contrary to our prediction, we did not detect evidence of difference in nest defense intensity among the four sampled time periods. We found a marginal effect of the presence of the non-incubating mates on the incubating gulls' likelihood of moving off the nest, but their response scores and latency to calm were not significantly different than when the non-incubating mate was absent. Previous work has shown that nest defense intensity in American Herring Gulls is low overall during the incubation stage which could explain our results.