Panayiota Siskos ’23
Vertebrates have different seasonal reproductive times depending on the hypothalamic–pituitary–gonadal axis, which is associated with breeding latitude and a sensitivity to changes in length of daylight. Migratory animals in particular work to optimize reproductive timing since breeding conditions are needed later at migrationary regions than overwintering regions (regions where animals stay throughout the winter). There are multiple migrationary bird species with populations that breed in separate locations (allopatric) but overwinter together (sympatric), known as heteropatry. However, there is no prior information on how environmental cues regulate phenology differences in such birds. In this study, scientists employ the varying junco’s migrationary tendencies to understand if populations wintering together but breeding differently have different physiological responses to local conditions in wintering areas before migration, thus shedding light on how population-level differences in phenology can regulate differential responsiveness to conditions in a common environment.
Researchers hypothesized that hypothalamic–pituitary–gonadal activity and migratory physiology reflect differences in migration patterns and predicted that testosterone elevation response to gonadotropin-releasing hormone would be larger in short-distance migrants than longer-distance migrants, a change correlating to the time it takes the birds to travel to their breeding location. It was also predicted that slate-colored juncos, the long-distance migrant, would have a higher migratory preparedness index than the sympatric pink-sided junco, their shorter-distance counterpart. Seasonally sympatric pink-sided juncos and slate-colored juncos were captured before spring migration and injected with gonadotropin-releasing hormone to measure testosterone. Stable isotope analysis used on feathers and claws helped approximate breeding latitude and to ensure that the groups overwintered at their capture site. Fat score, pectoral muscle condition, and size-corrected body mass were measured in terms of migratory preparedness.
In pink-sided juncos, δ2Hf values were higher and predicted latitudes were lower than that of slate-colored juncos. Both of their δ2Hf values suggested non-overlapping breeding altitudes, making the two populations allopatric. Between the two groups, baseline testosterone and migratory preparedness score had no major differences. However, pink-sided juncos had higher GrNH-induced testosterone levels, which indicated earlier reproduction and local adaptation of reproductive phenology.
Future directions for this study include utilizing more populations, having geolocator data for departure and arrival dates, and finding which parts of the hypothalamic–pituitary–gonadal axis regulate variation in daylight. The ability to understand changes in phenology is not only crucial in understanding species divergence via reproductive isolation, but also for predicting how species breeding at varying latitudes react to environmental changes.
 S. Wanamaker, et al., Local adaptation from afar: migratory bird populations diverge in the initiation of reproductive timing while wintering in sympatry. BIOLOGY LETTERS 16, 1-6 (2020). doi: /10.1098/rsbl.2020.0493.