Immature Northern Gannets (Morus bassanus) increase colony attendance following highly pathogenic avian influenza

Abstract

The emergence of Highly Pathogenic Avian Influenza (HPAI) H5N1 in wild bird populations in 2020 changed the landscape of this disease for seabird populations, including Northern Gannets Morus bassanus. In 2023, we photographed the three Northern Gannet colonies in Newfoundland and Labrador, Canada (Funk Island, Baccalieu Island and Cape St Mary's), following an HPAI outbreak in 2022 and documented an overall 43% decline in apparently occupied sites (AOS) from the last population survey in 2018. During the photo analyses, we assigned immature birds present in the core breeding area to one of four age categories according to their plumage characteristics, and estimated that 9% (inter-colony variance ranging from 7 to 14%) of all AOS in 2023 hosted at least one immature bird, an increase compared with rates of 2% or less before the outbreak. Further, 16% of all immature birds present in the core breeding area showed evidence of breeding and were probably 2- and 3-year-old birds. Our results support the social competition theory whereby a higher proportion of immature and/or younger immature birds occupying an AOS within the core breeding area is observed following significant reductions in numbers of established breeders, suggesting the presence of a pool of immature birds capable of recruiting into the Newfoundland Northern Gannet breeding population and help its recovery.