A study published in Avian Conservation and Ecology has quantified the scale of gull predation in a colony of Leach's Storm Petrels in Newfoundland, Canada, finding that up to 143,000 were killed by large gulls in one year alone.
The study found that the number of Leach's Storm Petrel on Gull Island in Witless Bay had fallen by an average of 6% per year since 2001, leaving approximately 180,000 breeding pairs there in 2012. Witless Bay is one of the most important breeding areas for globally Vulnerable species, making it both an Important Bird Area and an Important Biodiversity Area.
The populations of American Herring Gull on Gull Island and nearby Great Island have also decreased in recent years, with Gull Island experiencing a drop from 3,852 pairs in 1979 to 1,608 in 2011. However, while their numbers have been falling the gulls have taken to nesting in and around the coniferous forests where the storm petrels breed. After a moratorium on groundfish fisheries in 1992 reduced the availability of discards, and a delayed capelin spawning, it was anticipated that the storm petrels would suffer increased predation by large gulls but gauging the scale required careful work.
Most of the Leach's Storm Petrels killed by gulls were probably prospecting birds that had not yet undertaken their first breeding attempt (Joe Pender).
The difficulty in measuring the impact of predation on a storm petrel breeding colony is that these tubenoses have high rates of pre-breeding dispersal, with large numbers of young birds raised elsewhere visiting colonies to prospect for a future breeding site. They seldom nest before the age of five and rarely return to the colony at which they hatched.
After mapping the area supporting breeding storm petrels, burrow density and occupancy in the zone containing suitable habitat was surveyed, enabling a fresh estimate of around 179,742 Leach's Storm Petrels on Gull Island, meaning that the population there has declined by an average of 6% per year between 2001 and 2012.
Three strip transects, each two metres wide, were surveyed between May and August for remains of Leach's Storm Petrels. These results allowed researchers to estimate the scale of gull predation on the island and how it varied between years.
Results from the transects were extrapolated to produce an estimate that between 118,000 and 143,000 Leach's Storm Petrels were predated by gulls, mostly American Herring, in 2012, which compares to a renewed estimate of 74,707 killed on Great Island in 1997. It is thought that most of the birds predated will have been from the huge number of prospecting younger birds. Great Black-backed Gull, the other large gull breeding in the area, probably accounted for no more than 1,000 of the predated storm petrels.
As Leach's Storm Petrels are long-lived seabirds producing very few young, adult survival is an important driver of population trends, and the species has low adult survival rates in eastern Canada. Predation by gulls could be an important factor, as could conditions in wintering areas and on the migration routes.
With most of the predation events taking place within the forest, where storm-petrel burrows are located, the recent encroachment of American Herring Gulls seems to be benefitting the species, with a steady supply of nesting and prospecting Leach's sharing the same habitat.
The researchers commented that, while it is possible that American Herring Gulls may be contributing to the decline in Leach's Storm Petrel numbers in Witless Bay, similar declines have been documented at the world's largest colony on Baccalieu Island, which has very few gulls.
Bond, A L, Wilhelm S I, Pirie-Hay, D W, Robertson, G J, Pollet, I L, and Arany, J. 2023. Quantifying gull predation in a declining Leach's Storm-petrel (Hydrobates leucorhous) colony." Avian Conservation and Ecology 18.1: 5. DOI: https://doi.org/10.5751/ACE-02388-180105