Breeding in a den of thieves: pros and cons of nesting close to egg predators

Breeding success of many Arctic-breedingbird populations varies with lemming cycles dueto prey switching behavior of generalist predators. Several bird species breed on islands to escape fromgeneralist predators like Arctic fox Vulpes lagopus, but little is known about how these species interact.We studied brent geese Branta bernicla bernicla that share islands with gulls (Larus spec.) in Taimyr, Siberia(Russia). On one hand, gulls are egg predators, which occasionally steal an egg when incubatinggeese leave the nest for foraging bouts. On the other hand, gulls import marine resources to the islands,enriching the soil with their guano. We considered three hypotheses regarding clutch size of brent geeseafter partial nest predation. According to the “predator proximity hypothesis”, clutch size is expected tobe smallest close to gulls, because of enhanced predator exposure. Conversely, clutch size is expected tobe largest close to gulls, because of the supposedly better feeding conditions close to gulls, which mightreduce nest recess times of geese and hence egg predation risk (“guano hypothesis”). Furthermore, gullsmay defend their nesting territory, and thus nearby goose nests might benefit from this protection againstother gulls (“nest association hypothesis”). We mapped goose and gull nests toward the end of the gooseincubation period. In accordance with the latter two hypotheses, goose clutch size decreased with distanceto the nearest gull nest in all but the lemming peak year. In the lemming peak year, clutch size wasconsistently high, indicating that partial nest predation was nearly absent. By mapping food quantity andquality, we found that nitrogen availability was indeed higher closer to gull nests, reflecting guanofication.Unlike predicted by the nest association hypothesis, a predation pressure experiment revealed thategg predationrate decreased with distance to the focal gull nests. We therefore propose that higher foo