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The vast majority of what is known about migration comes from studies of obligate migration—regular and predictable “to-and-fro” seasonal movements. Much less is known about facultative migration, which occurs less predictably in space and time. Here, we use pine siskins (Spinus pinus), nomadic and irruptive finches, as a captive model to advance our understanding of facultative migration. Building on approaches used to study obligate migration, we investigate whether pine siskins exhibit migratory restlessness in captivity, similar to obligate migrants, and whether pine siskins express a seasonal pattern of migratory behavior. Pine siskins exhibited nocturnal migratory restlessness that was remarkably similar to that in obligate migrants; it was characterized by jumping and wing whirring behavior that was preceded by a period of quiescence. This behavior was expressed seasonally, peaking in the spring, when free-living pine siskins frequently migrate. As in many obligate migrants, migratory restlessness in pine siskins coincided with elevated body condition and showed minimal overlap with body molt. Although pine siskins are both nocturnal and diurnal migrants, patterns of diurnal activity did not mirror patterns of nocturnal activity and were more difficult to interpret. Free-living pine siskins frequently migrate in the fall, but in captivity showed no clear pattern of fall migratory restlessness. We suggest that this could reflect a relatively predictable and programmed spring migration and a more facultative fall migration. Significance statement Migrations are undertaken by a diverse array of animals and are important to the functioning of many ecosystems. The goal of this study was to shed light on an important, but understudied form of migration: facultative migration. Facultative migrations occur less predictably than the better-studied “to-and-fro” seasonal migrations of obligate migrants. Using a songbird, the pine siskin, as a model, we sought to determine if pine siskins express a migratory state in captivity and whether pine siskins exhibit a seasonal pattern of migratory behavior. We found that pine siskins showed behavioral and physiological changes associated with a migratory state, which were remarkably similar to those in obligate migrants. Pine siskins showed seasonal changes in migratory behavior in the study that suggest that this species has a predictable and programmed spring migratory period and a much more flexible fall migration.
Watts, H. E., Robart, A. R., Chopra, J. K., Asinas, C. E., Hahn, T. P., Ramenofsky, M. R. 2017. Seasonal expression of migratory behavior in a facultative migrant, the pine siskin. Behavioral Ecology and Sociobiology 71(1), 1-12