Body mass and triglycerides predict departure of free-living nomadic pine siskins

Facultative migrations are observed across vertebrate taxa, include irruptive and nomadic movements, and occur in response to ephemeral and unpredictably variable resources. While the physiology underlying seasonal, obligate migrations is thoroughly studied, much less is known about the physiological mechanisms of facultative movements. We test two hypotheses in a free-living, nomadic bird, the pine siskin (Spinus pinus). The Prepare Hypothesis predicts that, like obligate migrants, siskins increase fuel stores to prepare for migratory movements and elevations of baseline corticosterone (CORT) support departure. The Escape Hypothesis predicts that siskins do not prepare for departure, body condition declines as food availability declines, and stress-related levels of CORT induce escape from resource-poor areas. Under the controlled lab conditions of previous studies, food restriction induces declines in body condition and increases in CORT and locomotor activity, supporting the Escape Hypothesis. This study evaluates the ecological relevance of these captive findings by testing the Prepare and Escape Hypotheses in the field for the first time. During two fall field seasons, we radio-tagged siskins and tracked their local movements using handheld and automated telemetry. We assess how body condition and CORT relate to feeding behaviour [estimated via plasma triglycerides (TRIG)], space use and departure. We do not find support for either the Prepare or Escape Hypothesis, but rather observe an intermediate pattern. Birds with higher TRIG, and therefore greater food intake, are more likely to depart. Birds in poor condition stay longer near the field site; however, above a threshold body mass, body condition does not predict departure. These findings suggest moderate energy stores are necessary for departure, but movement decisions depend on other factors among birds with sufficient fuel. Siskin movements are physiologically distinct from both obligate and fugitive movements, and we discuss how food availability and body condition interact to drive different types of movement. Read the free Plain Language Summary for this article on the Journal blog.