Abstract

Autumn 2023 saw the biggest single arrival of Nearctic landbirds ever recorded in Britain and Ireland, with around 99 individuals of 27 species. This paper examines the likely causes of this unprecedented event, including the well-timed and ideally attributed transatlantic storm system that likely carried many birds to Europe, as well as indirect causes, such as solar activity and atmospheric pollution. IntroductionPulses of intense autumn bird migration characterise September and October in northeastern North America (Farnsworth et al. 2016) and, although most of this migration occurs over land, several hundred million birds migrate over the western North Atlantic (Dokter et al. 2018). These transoceanic migrants are vulnerable to displacement by storms, and the annual autumn arrival of small numbers of Nearctic landbirds in Europe is, for many, one of the most anticipated events of the ornithological calendar. These vagrants are typically entrained in circulation around warm and moist air masses with southwesterly winds (i.e. low-pressure systems) ahead of continental cold air masses with northwesterly winds (i.e. high-pressure systems) passing across the eastern coastline of North America and into the western North Atlantic. Once entrained within such a weather system, the birds are carried eastward towards Europe in rapidly moving depressions or along frontal waves (Elkins 1979, 1999; Lees & Gilroy 2009). Between 1967 and 2006, the peak arrival time for transatlantic vagrants in Britain and Ireland was consistently the second week of October (Elkins 2008). This timing has shaped birder behaviour for decades; for example, it has determined the peak of the ‘Scilly Season’ and the maximum numbers of observers on that archipelago. The composition of vagrants has been quite predictable over the decades, too, with a few species dominating – particularly Red-eyed Vireo Vireo olivaceus, which has over 170 accepted records in Britain up to 2022; but there has also been a long statistical ‘tail’ of rare (and hence highly sought-after) species recorded on only a few occasions. The variation in the relative frequency at which different species occur as vagrants in Europe is well predicted by life-history traits, with the species most frequently occurring as vagrants typically being larger-bodied birds that are longer-distance and later-departing migrants, especially species with non-breeding ranges in South America (McLaren et al. 2006).