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  1. Free, publicly-accessible full text available August 1, 2022
  2. 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 ) andmore »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.« less
    Free, publicly-accessible full text available June 29, 2022
  3. Many animals differentially express behaviours across the annual cycle as life stages are coordinated with seasonal environmental conditions. Understanding of the mechanistic basis of such seasonal changes in behaviour has traditionally focused on the role of changes in circulating hormone levels. However, it is increasingly apparent that other endocrine regulation mechanisms such as changes in local hormone synthesis and receptor abundance also play a role. Here I review what is known about seasonal changes in steroid hormone receptor abundance in relation to seasonal behaviour in vertebrates. I find that there is widespread, though not ubiquitous, seasonal variation in the expressionmore »of steroid hormone receptors in the brain, with such variation being best documented in association with courtship, mating and aggression. The most common pattern of seasonal variation is for there to be upregulation of sex steroid receptors with the expression of courtship and mating behaviours, when circulating hormone levels are also high. Less well-documented are cases in which seasonal increases in receptor expression could compensate for low circulating hormone levels or seasonal downregulation that could serve a protective function. I conclude by identifying important directions for future research.« less
  4. Abstract Although the endocrine system likely plays an important role in orchestrating the transition to a migratory state, the specific mechanisms by which this occurs remain poorly understood. Changes in glucocorticoid signaling are one proposed mechanism that may be important in migratory transitions. Although previous work has focused on the role of changes in circulating glucocorticoids, another potential mechanism is changes in the expression of its cognate receptors. Here, we test this hypothesis by comparing mRNA expression of the genes for the mineralocorticoid receptor ( MR ) and glucocorticoid receptor ( GR ) in two brain regions implicated in themore »regulation of migratory behavior (the hippocampus and hypothalamus) in pine siskins ( Spinus pinus ) sampled before or after the transition to a spring nomadic migratory state. Compared to pre-migratory birds, migratory birds had body conditions more indicative of physiological preparations for migration (e.g., larger body mass), and greater levels of nocturnal migratory restlessness. However, we found no differences between pre-migratory and migratory birds in the expression of GR or MR mRNA in either the hippocampus or hypothalamus. Thus, differences in expression of receptors for glucocorticoids do not appear to underly the observed differences in physiology and behavior across a migratory transition. Taken together with previous results showing no change in circulating corticosterone levels during this transition, our findings provide no evidence for a role of glucocorticoid signaling in the spring migratory transition of this species.« less