Unravelling the range expansion of a migratory waterbird: environmental factors underlying the selection of new breeding colonies by the Eurasian spoonbill

Abstract Context While wetland degradation has caused declines in many species, the Eurasian spoonbill ( Platalea leucorodia ), a colonial waterbird, is increasing along the East Atlantic Flyway, currently (re)colonising its former breeding range over the past two decades. Objectives Evaluate the environmental parameters determining the species distribution in Portugal, and assess whether range expansion can be driven by environmental changes improving conditions locally and/or by population increase, as space limitation in previous colonies due to high productivity may force dispersal and recruiters to new colonies. Methods National census of breeding waterbirds was used to develop Ecological Niche Models. We compared rates of change in environmental conditions from 2002 to 2021 between wetlands that were colonised by spoonbills and those that were not. Results Distribution of spoonbill colonies was best explained by distance to previous occupied sites. Rates of change in environmental conditions were similar between wetlands that were or not colonised by spoonbills. Thus, range expansion seems to be mostly driven by population growth and occupation of previously empty suitable areas. National protected wetlands, in proximity to active colonies and surrounded by different wetland habitat types, including semi-natural habitats, seem to be the best suited for future colonisation. Conclusions This work reenforces the need to identify species’ habitat requirements to support the conservation and management of suitable areas even if those are yet to host the target species, as this allows anticipating range expansion as well as other distribution shifts, and can contribute towards providing resilience to environmental changes.