Migratory birds possess some of the most remarkable navigational adaptations which allow them to precisely return to breeding territories after migrating thousands of miles. Over 60 years of study have revealed sensory, physiological, and genetic navigational adaptations (1). Many birds achieve pinpoint navigation with specialized cryptochrome proteins in the eye to sense magnetic field lines (2) and magnetite deposits in the bill to sense magnetic intensity (3, 4). Migratory routes are often encoded genetically, allowing juveniles to navigate from an endogenous migration program inherited from their parents (5–7). Yet, the importance of learning, both through experience and social interaction, is likely undervalued in our current understanding of avian navigation.
Bird navigation is not perfect: migratory birds occasionally end up in unexpected places, called vagrants. Vagrancy may be particularly revealing of the mechanisms behind avian navigation (8). External factors may drive vagrancy through interference with navigation senses. For example, natural, disruptive shifts in the Earth’s magnetic field driven by geomagnetic storms produce abnormal magnetic intensities (9) which are correlated with incorrect migration direction in lab experiments (10, 11) and increased vagrancy in free-flying birds (12, 13). Additionally, increased solar activity disrupts avian magnetoreception which may mask the effects of geomagnetic disturbance, allowing birds to migrate with natural cues (13). These results provide in situ demonstrations of the importance of magnetoreception for navigation. Visual landmarks are also likely key for navigation (8, 14), as clouds may drive vagrancy by obscuring landmarks (15). The extent that geomagnetic disturbance, solar activity, and visibly drive vagrancy may reveal the relative role that magnetoreception and visual landmarks play in migratory navigation. However, vagrancy may also arise from winds driving birds off course, called wind drift (16, 17). A successful study of vagrancy must disentangle external factors that interfere with navigational senses from those that displace birds from their route.
Vagrancy may also occur from errors in migratory route inheritance. Vagrancy is more common in juveniles, possibly suggesting a short lifespan of birds with vagrancy mutations or naivety of younger birds (1, 18). Candidate genes regulating migration direction and timing have been identified in several species of songbird, the largest avian order (19–22), but the genetic architecture of migration still poorly known (23). Mutations in genes encoding migratory direction may misorient birds. The propagation of such mutations has been implicated in the rapid formation of new migratory routes in Sylvia atricapilla, Hirundo rustica, and Petrochelidon pyrrhonota (24–26). To properly evaluate the role of genetics in route inheritance in passerines, it is critical to quantify the relative contribution of social learning to route inheritance (8). Social learning is the use of inadvertent cues from other individuals as a source of information (27), as observed in large, day migrating birds, where juveniles join flocks of adults to learn migration routes (28, 29). Conventionally, it is assumed that juvenile passerines do not learn migration routes from adults because most migrate at night, making flocking difficult (6, 18). However, there are cues in passerine migration that may function in social route learning. Many nocturnally migrating passerines produce short vocalizations while migrating (nocturnal flight calls) which likely act as social cues aiding in navigation, orientation, and possibly group decision making (30, 31). Some passerines migrate diurnally in flocks, providing both visual and auditory cues (e.g. Icteridae, Yasukawa and Searcy 2020). The failure of a juvenile to join a migratory flock may cause them to miss out on route learning opportunities, thus leading to vagrancy.
Using vagrants to study migratory navigation presents a challenge because defining vagrancy is difficult (33). The rarity of a species at any given location is often based on observation data limited by low sampling density (e.g., survey or banding data) or inconsistent effort (e.g., community science data). Species abundance is often patchy, making it easy for local range fragments to be overlooked. Any birds dispersing from unknown range fragments may be erroneously identified as vagrants (33). Additionally, vagrants may represent the extremes of a normal curve of migration orientation, rather than true misorientation (34). To provide a clear index of vagrancy, I chose to study migratory passerines lost offshore (Fig. 1). While some migratory routes cross bodies of water (35), flying over the open ocean can be a costly mistake for a passerine. Offshore vessels document these lost passerines, presenting an opportunity to study vagrancy (36).
I studied common migratory passerines in the Western United States lost over the Pacific Ocean. I asked the questions: (1) to what extent do external factors cause vagrancy by interfering with navigational senses or displacing birds from their migration routes? And (2) what species characteristics affect vagrancy propensity? I studied the relative likelihood of offshore vagrancy under different levels of external factors including weather, geomagnetic disturbance, and solar activity. Then, I studied how the vagrancy frequency for each species varied in relation to morphology, migration route distance, and phylogenetics. My analysis revealed that external factors interfering with navigational sensing were the primary predictors of offshore vagrancy, particularly geomagnetic disturbance. The greatest variation in offshore vagrancy occurred between species. Variation between other species was best predicted by migration distance and wing pointedness. Brown-headed Cowbirds were a notable outlier, occurring offshore more often than any other species.