Meeting Abstract
Migration is an evolved behavior that allows animals to take advantage of resources that are variable in time and/or space, and different migratory strategies depend on the predictability of resource variation. When food varies seasonally, obligate migrants can anticipate and prepare for migration, but it is unknown whether facultative migrants, whose movements are characteristically unpredictable in timing and destination, prepare for migration or rather escape when resources are low. Here we conducted a captive experiment to test two hypotheses about the behavioral and hormonal responses of a facultative migrant (Pine siskin; Spinus pinus) to declining food availability. Prepare Hypothesis: Siskins prepare for departure by increasing fuel stores, and elevations of baseline corticosterone (CORT) support increased locomotor activity. Escape Hypothesis: Siskins don’t prepare for departure, body condition declines as food availability declines, and stress-related levels of CORT induce escape behavior. Throughout a 15-day experiment, we measured body composition using a Quantitative Magnetic Resonance machine, continuous locomotor activity using force perches, and baseline CORT levels among birds given ad libitum food or a slow decline, fast decline, or randomly changing amount of food. We found support for the Escape Hypothesis. Siskins’ body condition declined as food declined, baseline CORT was elevated in birds with reduced lean mass, and birds showed marked increases in activity only when food availability was low. This work shows that facultative movements are physiologically distinct from seasonal, obligate migration, with food availability likely serving as a proximate cue, and birds showing little to no preparation for flight.
