Offshore vagrancy in passerines is predicted by season, wind-drift, and species characteristics

Migratory birds accomplish remarkable feats of long-distance navigation. Vagrants, few individuals who migrate to incorrect locations, reveal conditions where orientation and navigation fail. Studies of vagrancy on a continental scale reveal the importance of external factors such as strong winds driving birds off course, clouds obscuring migratory landmarks, and natural disruptions in the Earth's magnetic field interfering with migratory orientation. Species may also possess characteristics that make them more prone to vagrancy. The external drivers of vagrancy on a smaller scale are less understood.

I used eBird, a community science dataset comprising millions of bird observations, to study land passerines observed over the Pacific Ocean, here termed offshore vagrants. These data present the opportunity to study a particular case of vagrancy: small-scale displacement into highly inhospitable areas. I modeled how season, wind, lack of visibility, interference with magnetoreception, and species differences may predict offshore vagrancy. Then, I modeled how species vagrancy likelihood is predicted by morphological and life history traits.

Vagrancy was more common in the fall and positively associated with stronger tail winds in the spring. Species with greater preference for understory foraging habitat were less likely to occur as vagrants. Species vagrancy likelihood was higher in birds with a longer migration distance and rounded wings, but the relationship was weaker in birds with a pointed wings. Brown-headed Cowbirds were the most common offshore species in terms of absolute number of records and proportional to onshore frequency.

Offshore community science records proved revealing of mechanisms for small scale vagrancy in passerines. Offshore vagrancy can be driven by wind drift in the spring, but not in the fall despite higher overall levels of vagrancy. Life history characteristics like foraging habitat preference and migration duration may make some species more vulnerable to the effects of wind drift. Species with longer migrations may have more time to encounter vagrancy causing events, but greater aerodynamic efficiency may counteract this effect.