Local adaptation from afar: migratory bird populations diverge in the initiation of reproductive timing while wintering in sympatry

Abstract

The initiation of reproduction in many seasonally breeding animals is controlled by photoperiod and tends to be clinal: populations at higher latitudes breed later than those at lower latitudes, often reflecting a higher photoperiodic threshold. Migratory animals presumably time reproduction to match conditions at their breeding grounds, at least in part, by cues perceived on their wintering grounds. We asked how closely related dark-eyed junco ( Junco hyemalis ) populations that overwinter in sympatry but breed in allopatry respond to their shared winter environment by comparing early spring indices of readiness to migrate (fat and muscle condition) and breed (baseline and elevated testosterone). We measured stable hydrogen isotopes from feathers grown the preceding year and claws grown during winter to estimate breeding and wintering latitudes, respectively. We predicted that if reproductive initiation is adapted to the emergence of resources at their respective breeding destinations, then birds migrating to higher latitudes (slate-coloured junco; J. h. hyemalis ) should delay breeding as compared with those migrating to lower latitudes (pink-sided junco; J. h. mearnsi ) despite a common overwinter environment. We found higher testosterone in pink-sided juncos, consistent with earlier reproductive initiation, suggesting local adaptation in reproductive phenology is achieved through differential responses to predictive environmental cues.

Determinants of spring migration departure dates in a New World sparrow: weather variables reign supreme

“…Median latitudinal and distance estimates were calculated from geographic assignment probability maps by extracting the geographic cell coordinates with the highest posterior probability (top 10%) using the raster package (Hijmans, 2019). Migration distance was defined as the distance from the release site to the centroid of the estimated breeding polygon (Becker et al, 2022; Wanamaker et al, 2020).…”

“…Migration distance was defined as the distance from the release site to the centroid of the estimated breeding polygon (Becker et al, 2022; Wanamaker et al, 2020).…”

“…Prior to analyses of migration timing, we used a linear mixed effects model with a random effect of site to derive an index of body condition through the residuals of a regression of mass (measured prior to release) on tarsus length (Schulte-Hostedde et al, 2005;Wanamaker et al, 2020). We modeled days until spring migratory departure under an event-time analysis framework, using additive Cox proportional hazard mixed models (CPHMMs, (Therneau and Grambsch, 2000) and Grambsch, 2000).…”

Novel Rickettsia spp. in two common overwintering North American songbirds

“…Wintering juncos in the Appalachians include migrant and resident subspecies[ 14 ], and we earlier determined this rickettsiae-positive junco to be migratory ( J. h. hyemalis )[ 9 ]. Using feather hydrogen isotopes and geographic assignment models (Figure S1)[ 15 ], we estimated the most likely breeding site of the robin to be the Great Lakes (459 km median migration distance). In contrast, estimated breeding sites of the junco ranged from the western USA to Manitoba in Canada (2,377 km median migration distance; Figure 1 B).…”

Novel Rickettsia spp. in two common overwintering North American passerines

“…Most robins wintering in Indiana remain year-round or migrate elsewhere to breed, while wintering juncos in the Appalachians include migrant and resident subspecies; we earlier determined this positive bird as migratory ( J. h. hyemalis ) (7). Using feather hydrogen isotopes and geographic assignment (Figure S1) (10), we estimated the most likely breeding site of the robin to be the Great Lakes (e.g., Wisconsin; 459 km median migration distance), whereas that of the junco ranged from the western USA to Manitoba in Canada (2,377 km median migration distance; Fig. 1B).…”