A boreal songbird's 20,000~km migration across North America and the Atlantic Ocean
Migration is one of the most fascinating natural history events on the planet, and our understanding of these seasonal movements continues to rapidly increase with the advancement of tracking technology. For the 12 g Blackpoll Warbler (Setophaga striata; hereafter, Blackpoll; Fig. 1), the ability to follow year-round movement is essential given that they are one of the fastest declining songbirds in North America (Rosenberg et al. 2016). Although the study of Blackpoll migration has a long history and their non-stop flight over the Atlantic Ocean is often highlighted as a flagship example of remarkable songbird migrations (Gill 2007), much of this history has been anecdotal or based on indirect evidence (reviewed in DeLuca et al. 2015). Using small geolocator tracking devices, DeLuca et al. (2015) documented Blackpolls from breeding populations in the Maritimes and New England, at the southeastern margins of their range, depart from the northeast Atlantic coast and embark on a nonstop transoceanic flight of up to 3 d and 2,770 km on their way to overwintering grounds in South America. Yet, because their breeding range spans the entirety of North America's boreal forest, questions remain about the migration of Blackpolls from central and western breeding populations. Specifically, how does the phenology, duration, and routes of southward and northward migration vary geographically and do central- and western-breeding Blackpolls undertake similar overwater flights to eastern-breeding birds? Answers to these lingering questions can contribute vital natural history information for this declining species and provide insight into the physiological and ecological constraints that may limit extreme migration strategies.