‘Floating Populations’ of Seabirds: The Bane of Demographic Modelers and Managers

’Floating’ portions of seabird populations (mature but non-breeding individuals) are generally ignored in seabird research and management despite frequent evidence of their existence, especially in cavity-nesting species for whom nest habitat is often limiting. Here we demonstrate, using a few among an appreciable number of cases, that often more adults contribute to regional populations than colony-based censuses reveal, and that these birds are able to breed but do not. Once given the chance through the creation of nesting habitat, either by natural or human-caused processes, these populations reveal themselves by occupying the newly created habitats to become breeders. We include a brief discussion of the degree to which natal philopatry contributes to relatively sudden colony establishment. Not recognizing the existence of floating populations due to exclusively colony-based management, which is often politically necessary (e.g., Wildlife Refuge management), hinders conservation because it ignores the source and role of potential immigrants. Instead, management tends to emphasize supposed natal philopatry. Floaters will exploit mortality-caused vacancies in a breeding population, masking temporal variation in adult mortality, falsely indicating colony-size stability as a measure of the ‘health’ or resilience of a colony/habitat. In addition, the most successful efforts at establishing new colonies or restoring others by ‘social attraction’ are those in which a floating population is present, although unrecognized until it is revealed by the social attraction ‘experiment.’ Success comes when the artificially established breeding aggregation becomes an attractant to ‘floaters.’ Thus, recruitment of these floaters (especially on a predator-free colony or island) accelerates the limited growth provided by the return of hand-raised translocated nestlings. A lack of appreciation for the presence of floaters also limits the validity of assessments of the impact of bird wrecks and the ability of populations to recover from them.