Within-colony nest distribution of a waterbird mixed colony in a small alluvial forest at El Salvador

ABSTRACT Breeding in colonies is a defining characteristic of many waterbird species and several behavioural and habitat traits determine nest-site selection. Although there is relatively extensive literature on the topic, information on the within-colony distribution of nests lacks for mixed colonies, especially for ardeid-cormorant associations and in regions such as Central America. Here, geospatial data on vegetation associated with a mixed colony, established in a small alluvial forest northwest of El Salvador and collected from June to November 2015 was assessed to identify some behavioural and habitat features that could influence nest-site selection. Four ardeid species (Ardea alba, Egretta thula, Bubulcus ibis, and Nycticorax nycticorax) and one cormorant (Phalacrocorax brasilianus) conformed the colony, where the last was the outstanding species. The total area of the colony was 2.14 ha (14% of La Barra forest) with high nest densities (584.58 nests/ha). Spatial analyses indicate that some species prefer to occupy trees/shrubs close to other trees/shrubs with conspecific or similarly sized species. A remarkable height-strata distribution was found, with large-sized species in the canopy, and small-sized species in the lower strata. The resulting estimates of the generalized linear models applied suggest that the within-colony nest-site selection is determined by the combination of higher heights of the tree/shrub used for nesting, nearest distance to a food source, fewer species sharing the same tree/shrub and an increasing effect of the interaction of the latter two factors. Finally, the temporal evolution of La Barra’s colony shows that the increasing number of Neotropical Cormorants (Phalacrocorax brasilianus) could be shaping and threatening the colony’s species structure and possibly the vegetation as well. Conservation measures should be implemented to manage the population of this species.