Interacting effects of nest shelter and breeder quality on behaviour and breeding performance of herring gulls

Physical attributes of a nest site can be important in determining the outcome of a breeding event. However, high-quality individuals may be more able to obtain particular sites, and thus habitat quality and individual quality are often confounded in correlative studies. We examined the potential sheltering effect of nest vegetation on the nest microclimate and on the behaviour and breeding performance of herring gulls, Larus argentatus. In addition to comparing birds at unmanipulated vegetated and unvegetated sites, we experimentally manipulated vegetation around nests to tease apart breeder quality and habitat effects. Birds nesting at unmanipulated vegetated sites had earlier laying dates, heavier eggs and more fledglings than those at comparable unvegetated sites. Vegetation provided shelter from weather conditions for the incubating birds and chicks; vegetated nest sites had on average milder air temperatures and lower wind speeds than unvegetated control nest sites. During incubation, breeders at naturally vegetated nests assumed alert postures less, and spent more time sleeping, than those at naturally unvegetated sites, indicating less disturbance from neighbours. In nests where vegetation was experimentally removed, both nest microclimate and breeder behaviour became similar to those at the naturally unvegetated nests; however, despite these changes, the absence of vegetation did not alter the higher breeding performance of birds at these sites. Therefore, nest microclimate alone is not responsible for the improved performance at vegetated sites; such sites appear to be occupied by better-quality individuals.