Storms, budworms and vagrant birds in a changing world.

The seasonal migration of billions of birds is one of the most spectacular events in the natural world's annual cycle. Migration lets birds exploit spatiotemporally dynamic resources and involves complex trade-offs for survivorship. The migratory movements of individual birds are routinely influenced by atmospheric and ecological connections across long distances. Suboptimal conditions during active migration often lead to bird displacement, sometimes en masse. Birds that are displaced outside of their regular geographic ranges are known as vagrants, arguably the most spectacular instances of which involve transoceanic and transhemispheric movements of small songbirds. Vagrants have been catalogued by amateur ornithologists for decades, but there is now a renewed scientific interest in the causes and consequences of vagrancy - in particular as a prelude to range shifts that are prone to become more likely under global change. Against the backdrop of an unprecedented arrival of vagrant North American songbirds in Western Europe in September 2023, we discuss the environmental drivers that influenced this transoceanic movement and the latest scientific literature exploring the phenomenon.