Nest density and competing risks: A long-term investigation of Black Brant (Branta bernicla nigricans) nest survival

Many species of seabirds and waterfowl are predominantly colonial breeders, suggesting that colonial breeding is beneficial in some way. For Arctic nesting geese, colonial breeding may be an adaptive approach to minimize the risk of nest failure through predator swamping. Yet evidence for predator swamping is inconclusive. Species in low-lying coastal zones may have to manage tradeoffs between historical (e.g., predation) and increased frequency of other threats (e.g., coastal flooding associated with storm events). As the climate continues to change, coastal obligates will be subject to rising sea levels and an increase in extreme weather events, which could exacerbate the tradeoff between flooding and predation risk. We studied the effect of nest density on nest success in the colonial nesting Black Brant (Branta bernicla nigricans) on the Tutakoke River Colony in western Alaska and explored how density effects change during years of heavy Arctic fox (Alopex lagopus) predation and coastal flooding associated with storm events. We classified the colony as containing 2 strata that differed in elevation and vegetation, and were separated by the Tutakoke River. The stratum north of the Tutakoke River was 0.25 m lower in elevation and subject to periodic tidal flooding, while the southern stratum never flooded. Nest density positively affected brant nest success in years with high levels of fox predation. Nest success was greatest at the highest density during Normal years (low levels of fox predation and no flooding; northern stratum nest success 0.81 +/- 0.01, southern stratum nest success 0.81 +/- 0.01) and lowest in areas of low density during Fox years (northern stratum nest success 0.38 +/- 0.01, southern stratum nest success 0.31 +/- 0.01). Nest success was routinely higher during Normal years at all densities compared to Flood and Fox years. Our results support the hypothesis that colonial nesting, at least in tundra habitats, may have developed as a defense mechanism to swamp mammalian predators.