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1. Food and social cues modulate reproductive development but not migratory behavior in a nomadic songbird, the Pine Siskin

   https://doi.org/10.1093/ornithology/ukae006  

   Vernasco, Ben_J; Cornelius, Jamie_M; Watts, Heather_E (January 2024, Ornithology)

   Abstract Many animals rely on photoperiodic and non-photoperiodic environmental cues to gather information and appropriately time life-history stages across the annual cycle, such as reproduction, molt, and migration. Here, we experimentally demonstrate that the reproductive physiology, but not migratory behavior, of captive Pine Siskins (Spinus pinus) responds to both food and social cues during the spring migratory-breeding period. Pine Siskins are a nomadic finch with a highly flexible breeding schedule and, in the spring, free-living Pine Siskins can wander large geographic areas and opportunistically breed. To understand the importance of non-photoperiodic cues to the migratory-breeding transition, we maintained individually housed birds on either a standard or enriched diet in the presence of group-housed heterospecifics or conspecifics experiencing either the standard or enriched diet type. We measured body condition and reproductive development of all Pine Siskins and, among individually housed Pine Siskins, quantified nocturnal migratory restlessness. In group-housed birds, the enriched diet caused increases in body condition and, among females, promoted reproductive development. Among individually housed birds, female reproductive development differed between treatment groups, whereas male reproductive development did not. Specifically, individually housed females showed greater reproductive development when presented with conspecifics compared to heterospecifics. The highest rate of female reproductive development, however, was observed among individually housed females provided the enriched diet and maintained with group-housed conspecifics on an enriched diet. Changes in nocturnal migratory restlessness did not vary by treatment group or sex. By manipulating both the physical and social environment, this study demonstrates how multiple environmental cues can affect the timing of transitions between life-history stages with differential responses between sexes and between migratory and reproductive systems. 

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2. Social environment influences termination of nomadic migration

   https://doi.org/10.1098/rsbl.2022.0006  

   Robart, Ashley R.; Zuñiga, Hilary X.; Navarro, Guillermo; Watts, Heather E. (March 2022, Biology Letters)

   The final stage of migration, when animals terminate migratory movements and transition to a more sedentary state, remains the least understood phase of migration. Whereas migrants that return to the same locations each year may use mechanisms associated with locating a specific destination, migrants with low site fidelity, such as nomadic migrants, may rely on local environmental cues to determine when to cease migratory movements. Using an experiment with captive birds, we tested whether the presence of a conspecific influences the termination of migration, indicated by changes in behaviour and physiology, in a nomadic migrant (the pine siskin, Spinus pinus ). We paired migratory birds with a non-migratory individual or left migratory and non-migratory individuals unpaired. Migratory paired birds had a significant decline in nocturnal activity immediately after pairing and activity levels remained lower two weeks later, with significant declines in energetic reserves and flight muscle size also observed. By contrast, migratory unpaired birds maintained high levels of activity and energetic reserves. These results provide evidence for a role of the social environment in migratory termination decisions. Social cues may be particularly useful in nomadic migrants, such as pine siskins, to facilitate settling in high quality, but unfamiliar, habitats. 

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3. Food deprivation is associated with telomere elongation during hibernation in a primate

   https://doi.org/10.1098/rsbl.2024.0531  

   Blanco, Marina B; Smith, Dana L; Greene, Lydia K; Lin, Jue; Klopfer, Peter H (February 2025, Biology Letters)

   Telomeres, the protective ends of chromosomes, progressively shorten due to incomplete mitotic replication and oxidative stress. In some organisms, transient telomere elongation may occur, for example, when individuals have an energy surplus to counter stress-induced life events or when elongating telomeres is a last chance to increase fitness. Mammalian hibernators are good models to test telomere dynamics, as they cycle between prolonged bouts of metabolic depression (torpor) punctuated by short surges to euthermia (arousals). We studied captive fat-tailed dwarf lemurs (Cheirogaleus medius), strepsirrhine primate hibernators, that were food-deprived (n= 8) or fed daily (n= 7) during hibernation (4.5 months). We compared telomere lengths, assayed via qPCR from oral swabs, at five strategic time points that span a full year. Food-deprived subjects underwent multi-day torpor/arousal cycles, lost considerable body mass and elongated telomeres during hibernation but shortened them upon emergence. In contrast, food-provisioned subjects ate daily, lost body mass more slowly, underwent shallower and shorter torpor bouts and experienced little change in telomere lengths during the same periods. Our results highlight a complex relationship between telomere dynamics, energy balance and torpor expression. Further investigation is warranted to elucidate the regulation of protective mechanisms in these primate hibernators. 

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4. Seasonal Patterns of Fat Deposits in Relation to Migratory Strategy in Facultative Migrants

   https://doi.org/10.3389/fevo.2021.691808  

   Cornelius, Jamie M.; Hahn, Thomas P.; Robart, Ashley R.; Vernasco, Ben J.; Zahor, Dorothy L.; Glynn, Kenneth J.; Navis, Corrie J.; Watts, Heather E. (June 2021, Frontiers in Ecology and Evolution)

   null (Ed.)

   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. 

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5. Migratory state is not associated with differences in neural glucocorticoid or mineralocorticoid receptor expression in pine siskins

   https://doi.org/10.1515/ami-2019-0001  

   Watts, Heather E.; Rittenhouse, Jeffrey L.; Sewall, Kendra B.; Bowers, J. Michael (January 2019, Animal Migration)

   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 the 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. 

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