A new study has presented the likely source of the increasing number of Lesser Black-backed Gulls seen in North America.
Lesser Black-backed Gull has an extensive range in the Old World, breeding from Greenland east to western Asia. Although it has traditionally been considered a vagrant to North America, the incidence of sightings is on the up. Since the first North America record in New Jersey, USA, in 1934, the species has now been seen in every US state and Canadian province.
First recorded in New Jersey in 1934, Lesser Black-backed Gull has now been seen in every US state and Canadian province (Mark Woodhead).
The increase in records of Lesser Black-backed Gull in North America had been particularly marked since the turn of the century, which is at odds with observed declines witnessed over the same period in populations of the species in the UK and Iceland, which both peaked in the 1990s. The researchers point out that declining populations are less likely to be the source of vagrant birds, thus these populations are unlikely to be the source of Lesser Black-backed Gulls reaching North America in recent years.
In contrast, there was a clear correlation between the growth of the Greenland breeding population and the upturn in records across North America, strongly suggesting that the expansion of Lesser Black-backed Gull into Greenland has been instrumental in fuelling the rise in regularity of the species in the New World.
The team cites Iceland's importance in the evolution of Lesser Black-backed Gull vagrancy and migration. It is likely that the then-increasing population in Iceland was integral to early records from North America, before the Greenland population became established in the late 1980s. However, the stark decline in Iceland since the mid-2000s clearly contrasts with the continued rise in North America.
Further evidence which suggests Greenland as the key source population is ringing data. Despite thousands of Lesser Black-backed Gulls now having been seen in North America, only two banded birds have ever been found there: a Dutch-ringed adult in New York in October 1997 and a first-winter from south-west Iceland seen in Puerto Rico in November 2002. Despite widespread ringing projects elsewhere in Europe, including the UK, none has ever been noted in North America; meanwhile, Greenland populations have not been subjected to ringing schemes.
The researchers conclude that data on breeding populations can be used to determine plausible source populations of vagrants. In the case of Lesser Black-backed Gull, Iceland may have contributed to vagrancy in early years of colonisation, but Greenland populations have consistently increased alongside numbers of vagrants, suggesting Greenland as the source population of vagrants in North America.
They go on to predict that Lesser Black-backed Gull will soon occur regularly enough in North America to be considered an established wintering species, and may even form a small breeding population on the continent. In fact, it appears that this is already the case, given the number of birds now wintering widely across eastern North America. The team recommends that further studies of North American Lesser Black-backed Gulls should incorporate GPS tracking to build a better picture of the birds' annual life cycles.
Reference
Zawadzki, L C, Hallgrimsson, G T, Veit, R R, Rasmussen, L M, Boertmann, D, Gillies, N, & Guilford. T. 2021. Predicting source populations of vagrants using breeding population data: a case study of the Lesser Black-backed Gull (Larus fuscus). Frontiers in Ecology and Evolution, DOI: https://doi.org/10.3389/fevo.2021.637452
