Annual regulation of adrenocortical function in migrant and resident subspecies of white-crowned sparrow

Flexibility underlies differences in mitochondrial respiratory performance between migratory and non-migratory White-crowned Sparrows (Zonotrichia leucophrys)

Migration is one of the most energy-demanding behaviors observed in birds. Mitochondria are the primary source of energy used to support these long-distance movements, yet how mitochondria meet the energetic demands of migration is scarcely studied. We quantified changes in mitochondrial respiratory performance in the White-crowned Sparrow (Zonotrichia leucophrys), which has a migratory and non-migratory subspecies. We hypothesized that the long-distance migratory Gambel's subspecies (Z. l. gambelii) would show higher mitochondrial respiratory performance compared to the non-migratory Nuttall's subspecies (Z. l. nuttalli). We sampled Gambel's individuals during spring pre-migration, active fall migration, and a period with no migration or breeding (winter). We sampled Nuttall's individuals during periods coinciding with fall migration and the winter period of Gambel's annual cycle. Overall, Gambel's individuals had higher citrate synthase, a proxy for mitochondrial volume, than Nuttall's individuals. This was most pronounced prior to and during migration. We found that both OXPHOS capacity (state 3) and basal respiration (state 4) of mitochondria exhibit high seasonal flexibility within Gambel's individuals, with values highest during active migration. These values in Nuttall's individuals were most similar to Gambel's individuals in winter. Our observations indicate that seasonal changes in mitochondrial respiration play a vital role in migration energetics.

Comparison of the Phenotypic Flexibility of Muscle and Body Condition of Migrant and Resident White-Crowned Sparrows

AbstractSeasonally breeding birds express variations of traits (phenotypic flexibility) throughout their life history stages that represent adaptations to environmental conditions. Changes of body condition during migration have been well studied, whereas alterations of skeletal and cardiac muscles, body mass, and fat scores have yet to be characterized throughout the spring or fall migratory stages. Additionally, we examined flexible patterns of muscle, body mass, and fat score in migrant white-crowned sparrows (Zonotrichia leucophrys gambelii) in comparison with those in a resident subspecies (Zonotrichia leucophrys nuttalli) during the stages they share to evaluate the influence of different life histories. Migrants showed hypertrophy of the pectoralis muscle fiber area on the wintering grounds in late prealternate molt, yet increased pectoralis muscle mass was not detected until birds readied for spring departure. While pectoralis profile and fat scores enlarged at predeparture in spring and fall, pectoralis, cardiac, and body masses were greater only in spring stages, suggesting seasonal differences for migratory preparation. Gastrocnemius mass showed little change throughout all stages, whereas gastrocnemius fiber area declined steadily but rebounded in fall on the wintering grounds, where migrants become more sedentary. In general, residents are heavier birds with larger leg structures, while migrants sport longer wings and greater heart mass. Phenotypic flexibility was most prominent among residents with peaks of pectoralis, gastrocnemius, and body masses during the winter stage, when local weather is most severe. Thus, the subspecies express specific patterns of phenotypic flexibility with peaks coinciding with the stages of heightened energy demands: the winter stage for residents and the spring stages for migrants.

Perspectives on environmental heterogeneity and seasonal modulation of stress response in neotropical birds

Corticosterone (CORT), the main glucocorticoid in birds, regulates physiological and behavioral traits linked to predictable and unpredictable environmental fluctuations (i.e., stressors). Baseline and stress-induced CORT concentrations are known to fluctuate seasonally, linked to life history stages (LHS) such as breeding, molt, and wintering stage. These variations have been relatively well described in North American birds, but poorly addressed in neotropical species. To fill this gap, we explored how baseline and stress-induced CORT variation by LHS was affected by seasonality and environmental heterogeneity (i.e., frequency of unpredictable events such as droughts, flashfloods, etc) within the Neotropics using two approaches. First, we reviewed all currently available data about CORT concentrations for neotropical bird species. Second, we performed an in-depth analysis comparing the CORT responses of the two most common species of the Zonotrichia genus from North and South America (Z. leucophrys and Z. capensis, respectively) and their subspecies to seasonality and environmental heterogeneity. These species have been analyzed with the same methodology, allowing for an in-depth comparison of CORT variations. Despite scant data on neotropical bird species, we observed overlap between molt and breeding, and lower fluctuations of CORT among LHS. These patterns would be considered atypical compared to those described for North temperate species. Further, we found no significant associations between environmental heterogeneity and the stress-responses. In Zonotrichia we observed a positive association between baseline and stress-induced concentrations of CORT and latitude. We also observed differences by LHS. Both baseline and stress-induced CORT concentrations were higher during breeding and lower during molt. In addition, for both species, the overall pattern of seasonal modulation of stress response was heavily influenced by the migration strategy, with long-distance migrants showing significantly higher stress-induced CORT levels. Our results highlight the need for more data collection in the Neotropics. Comparative data would shed further light on the sensitivity of the adrenocortical response to stress under different scenarios of environmental seasonality and unpredictability.

Gene expression of sex steroid metabolizing enzymes and receptors in the skeletal muscle of migrant and resident subspecies of white-crowned sparrow (Zonotrichia leucophrys)

Circulating sex steroid concentrations vary dramatically across the year in seasonally breeding animals. The ability of circulating sex steroids to effect muscle function can be modulated by changes in intracellular expression of steroid metabolizing enzymes (e.g., 5α-reductase type 2 and aromatase) and receptors. Together, these combined changes in plasma hormones, metabolizing enzymes and receptors allow for seasonally appropriate changes in skeletal muscle function. We tested the hypothesis that gene expression of sex steroid metabolizing enzymes and receptors would vary seasonally in skeletal muscle and these changes would differ between a migrant and resident life history strategy. We quantified annual changes in plasma testosterone and gene expression in pectoralis and gastrocnemius skeletal muscles using quantitative polymerase chain reaction (qPCR) in free-living migrant (Zonotrichia leucophrys gambelii) and resident (Z. l. nuttalli) subspecies of white-crowned sparrow during breeding, pre-basic molt, and wintering life history stages. Pectoralis muscle profile was largest in migrants during breeding, while residents maintained large muscle profiles year-round. Circulating testosterone peaked during breeding in both subspecies. Pectoralis muscle androgen receptor mRNA expression was lower in females of both subspecies during breeding. Estrogen receptor-α expression was higher in the pectoralis muscle, but not gastrocnemius, of residents throughout the annual cycle when compared to migrants. Pectoralis aromatase expression was higher in resident males compared to migrant males. No differences were observed for 5α-reductase 2. Between these two subspecies, patterns of plasma testosterone and androgen receptors appear to be conserved, however estrogen receptor gene expression appears to have diverged.

Seasonal differences in hypothalamic thyroid-stimulating hormone β, gonadotropin-releasing hormone-I and deiodinase expression between migrant and resident subspecies of white-crowned sparrow (Zonotrichia leucophrys)

Across taxa, the seasonal transition between non-breeding and breeding states is controlled by localised thyroid hormone signalling in the deep brain via reciprocal switching of deiodinase enzyme expression from type 3 (DIO3) to type 2 (DIO2). This reciprocal switch is considered to be mediated by increasing thyroid-stimulating hormone β (TSHβ) release from the pars tuberalis, which occurs in response to a change in photoperiod. Although well characterised in a handful of model organisms in controlled laboratory settings, this pathway remains largely unexplored in free-living animals under natural environmental conditions. In this comparative gene expression study, we investigated hypothalamic thyroid hormone signalling in two seasonally breeding subspecies of white-crowned sparrow (Zonotrichia leucophrys), across the entirety of their annual cycles. The migratory Gambel's (Z. l. gambelii) and resident Nuttall's (Z. l. nuttalii) subspecies differ with respect to timing of reproduction, as well as life history stage and migratory strategies. Although DIO3 mRNA expression was elevated and DIO2 mRNA expression was reduced in the wintering period in both subspecies, DIO2 peaked in both subspecies prior to the onset of reproduction. However, there was differential timing between subspecies in peak DIO2 expression. Intriguingly, seasonal modulation of TSHβ mRNA was only observed in migrants, where expression was elevated at the start of breeding, consistent with observations from other highly photoperiodic species. There was no correlation between TSHβ, DIO2 and gonadotropin-releasing hormone-I mRNA or reproductive metrics in residents. Based on these observed differences, we discuss potential implications for our understanding of how changes in medial basal hypothalamic gene expression mediates initiation of seasonal reproduction.