Spatial demography and behavioural correlates of coloniality in Australasian gannets

Coloniality is wide-spread in many animals but clear associations between spatial patterns of nest-site preference and the behavioural and ecological consequences of nesting location within colonies only rarely have been assessed. Using a suite of integrated geospatial data-capture and analytical methodologies, we quantified nesting demographics, early-season occupation behaviours and reproductive success patterns in a colonial seabird. To this end we mapped the nesting locations of Australasian gannets Morus serrator at Muriwai, New Zealand, developing a novel approach that combines LiDAR ('Light Detection and Ranging') remote sensing, geographic information systems, and geospatial analyses. We visualize colony topography and nest-density patterns with 3D models and surface maps at an unprecedented spatial resolution, and assess the temporal sequence of territorial establishment, the associations between territorial behaviour parameters and the spatial demography of the colony. Consistent with established knowledge on seabird settlement patterns, central high-density nesting areas were shown to be colonized first and were positively correlated with high rates of aggression and mating, and those were, in turn, inversely correlated with fledgling density. Net reproductive success, measured as abundance of fledglings, however, was higher at central nest sites, despite the increased levels of aggression in these high-density areas. Benefits of coloniality are supported by the observed reproductive advantages of central nests, in line with contested theory of why colonial breeding evolved in seabirds. We consider that the protocols we present here are readily modifiable and applicable to a very broad range of colonial species and can contribute to a general understanding of the proximate drivers of breeding-site choice in colonial systems.