Lost at sea: interference with navigational senses and failed social route learning may drive offshore vagrancy in passerines

Abstract

Background Migratory birds possess remarkable navigational adaptations. Vagrants, few individuals who migrate to incorrect locations, offer a unique opportunity to study how navigation is accomplished. Vagrancy may occur due to external factors forcing birds off course, such as strong wings, or navigational errors. Natural disruptions in the Earth’s magnetic field may cause navigational errors interfering with bird magnetoreception. Failures of other navigation tools like visual landmark recognition and social route learning may also contribute to vagrancy, but these factors are difficult to quantify.Methods I used eBird, a community science dataset comprising millions of bird observations, to study the relative likelihood of offshore vagrancy under different external factors including weather, geomagnetic disturbance, and solar activity using mixed effects logistic regression. Then, I studied how variation in species vagrancy propensity is be driven by morphology and migration distance using multiple linear regression, or inheritances of vagrancy alleles with phylogenetic analysis.Results High geomagnetic disturbance and low visibility increased offshore vagrancy, and winds did not appear to blow birds off course. Most variation in offshore vagrancy stemmed species specific differences, best explained by an interaction between wing pointedness and migration. A longer migration distance was strongly correlated with increased vagrancy in birds with rounded wings, but the relationship was absent in birds with a pointed wing shape. Brown-headed Cowbirds were notably more prone to vagrancy than any other passerine.Conclusions External factors primarily cause vagrancy by interfering with magnetic and visual senses, rather than physically forcing birds off course. Species with longer migrations have more time to encounter these vagrancy causing events, but a more pointed wing shape may allow birds to reorient more efficiently. While migration routes are primarily genetic in passerines, I found evidence of an overlooked role of social learning in route inheritance. I contend that as a brood parasite, solitary juvenile Brown-headed Cowbirds experience more difficulty in joining migratory flocks, thus missing out on social route learning opportunities. These results clarify the relative role of different navigational adaptations in migratory birds and demonstrate the utility of studying vagrants to understand bird migration.