No abstract

The transition to a migratory state involves coordinated changes in physiology and behaviour. In species with regular, predictable (obligate) migrations, increasing day length triggers the expression of a spring migratory state and androgens play an important role in stimulating its development. By contrast, we know little about the environmental cues and endocrine mechanisms that regulate migration in species with less predictable (facultative) migrations. Here, we tested whether photoperiod stimulates a migratory state in a facultative nomadic migrant, the pine siskin (Spinus pinus). We exposed wintering birds to either a naturally increasing or short-day photoperiod and measured physiological and behavioural changes indicative of a migratory state. We also examined changes in circulating hormones that may play a role in the migratory transition. Natural-day, but not short-day, birds displayed physiological preparations for migration, including increases in fat deposition, and showed increased levels of migratory restlessness. We found no evidence for a role of corticosterone in the migratory transition, but testosterone may be important. This study is the first experimental test of the role of photoperiod in regulating facultative migration and demonstrates that the predictive cue used by many obligate migrants to time spring migration is also important in a facultative migrant.

No abstract

Physiological preparations for migration generally reflect migratory strategy. Migrant birds fuel long-distance flight primarily with lipids, but carrying excess fuel is costly; thus, the amount of fat deposited prior to departure often reflects the anticipated flight duration or distance between refueling bouts. Seasonal pre-migratory deposition of fat is well documented in regular seasonal migrants, but is less described for more facultative species. We analyze fat deposits of free-living birds across several taxa of facultative migrants in the songbird subfamily Carduelinae, including house finches (Haemorhous mexicanus), American goldfinches (Spinus tristis), pine siskins (Spinus pinus) and four different North American ecotypes of red crossbills (Loxia curvirostra), to evaluate seasonal fat deposition during facultative migratory periods. Our data suggest that the extent of seasonal fat deposits corresponds with migratory tendency in these facultative taxa. Specifically, nomadic red crossbills with a seasonally predictable annual movement demonstrated relatively large seasonal fat deposits coincident with the migratory periods. In contrast, pine siskins, thought to be more variable in timing and initiation of nomadic movements, had smaller peaks in fat deposits during the migratory season, and the partial migrant American goldfinch and the resident house finch showed no peaks coincident with migratory periods. Within the red crossbills, those ecotypes that are closely associated with pine habitats showed larger peaks in fat deposits coincident with autumn migratory periods and had higher wing loading, whereas those ecotypes associated with spruces, Douglas-fir and hemlocks showed larger peaks coincident with spring migratory periods and lower wing loading. We conclude that population averages of fat deposits do reflect facultative migration strategies in these species, as well as the winter thermogenic challenges at the study locations. A difference in seasonal fattening and wing loading among red crossbill ecotypes is consistent with the possibility that they differ in their migratory biology, and we discuss these differences in light of crossbill reproductive schedules and phenologies of different conifer species.