Intercolony movement of pre‐breeding seabirds over oceanic scales: implications of cryptic age‐classes for conservation and metapopulation dynamics

Abstract

Aim Demographic linkage between subpopulations plays a critical role in population processes. Metapopulation dynamics, however, remains one of the most poorly understood aspects of population biology. This is especially true for small, pelagic seabirds because their discrete subpopulations are located on offshore islands, separated by vast areas of open ocean, making monitoring logistically challenging. Seabird populations often contain large numbers of immature pre‐breeders that may be important for subpopulation connectivity and demography, but are poorly studied. Here we provide evidence for intercolony movement of pre‐breeding Leach's storm‐petrels Oceanodroma leucorhoa among three colonies spanning the North Atlantic Ocean. We discuss their influence on metapopulation dynamics and the extinction risk of a subpopulation under threat from extreme predation. Location North Atlantic Ocean Islands (Scotland, Canada & Iceland). Methods We use a novel application of Bayesian stable isotope mixing models to infer recent movement of pre‐breeding birds between three major breeding populations in the North Atlantic. Carbon and nitrogen stable isotope values from breeding birds (central place foragers) sampled at each colony were used as model sources and pre‐breeding birds as model mixtures. Results Of 134 pre‐breeding Leach's storm‐petrels sampled at three colonies across the North Atlantic, five had isotope mixing model estimates dissimilar (< 25%) to their colony of capture and were instead isotopically similar to another breeding region. Nineteen further pre‐breeders had highly mixed signatures (< 50% for the colony of capture), indicating possible recent movement between colonies. Main conclusions Our findings provide evidence for interpopulation connectivity of pre‐breeding Leach's storm‐petrels among colonies spanning the North Atlantic. These results highlight the significance of cryptic young age‐classes in metapopulation dynamics and the demographic processes. Moreover, they provide us with a better understanding of how one subpopulation remains extant, despite experiencing extreme predation rates.

Apparent survival of adult Leach&#8217;s Storm-petrels (Oceanodroma leucorhoa) breeding on Bon Portage Island, Nova Scotia

“…In the St. Kilda island archipelago, Scotland, the storm-petrel population has declined by 54% in fewer than 10 years, with growing populations of Great Skuas that kill ~21,000 adult storm-petrels per year (Newson et al 2008, Miles 2010. With such a high number of adults being killed, the colony should be declining at a faster rate, but it is possible that it is being supplied with new breeders from larger colonies (e.g., from Iceland, Bicknell et al 2012Bicknell et al , 2013a.…”

“…Once again, additional years of data are needed to properly investigate influences of gulls and other predators on storm-petrel survival. It would be useful for future studies to include quantities of different species suspected to be depredating storm-petrels at this colony, as well as estimations of the number of storm-petrels killed annually by each predator species, and to assess whether this predation is on breeding or prospecting birds (Bonnaud et al 2009, Bicknell et al 2013a). …”

Predicting population responses to environmental change: the importance of considering informed dispersal strategies in spatially structured population models

“…Our assumption that individuals can gather information on a large set of colonies before making a settlement decision is untested in natural systems. However, there is evidence that individuals perform prospecting movements over large distances (Votier et al, 2011;P eron & Gr emillet, 2013;Ponchon et al, 2013;Bicknell et al, 2014), suggesting that dispersal could subsequently occur at such scale. That is why we do not limit dispersal distance.…”

Migratory movements and wintering areas of Leach's Storm-Petrels tracked using geolocators

“…Historically, banding programs provided the only information about migratory connectivity (e.g., Imber , Gaston et al ). More recently, stable isotopes have provided information about connections between breeding and non‐breeding areas (Cherel et al , Hedd and Montevecchi , Philips et al , Bicknell et al ), but resolution of these data limits their utility. Recent advances in technology have, however, made it possible to deploy smaller, longer‐lasting tracking units on birds (Bridge et al ) that provide information on travel routes, staging areas, and non‐breeding locations.…”