Spatial structure in migration routes maintained despite regional convergence among eastern populations of Swainson's Thrushes

Abstract

Background: Migratory connectivity links the different populations across the full cycle and across the species range and may lead to differences in survival among populations. Studies on spatial and temporal migratory connectivity along migration routes are rare, especially for small migratory animals.

Methods: We used an automated radio-telemetry array to assess migratory connectivity en route and between early and later stages of the fall migration of the eastern populations of Swainson's Thrush, and to assess the variation of migration pace between consecutive detection from the different receiving stations along the migratory journey. We tracked 241 individuals from across eastern Canada to determine if populations were mixing around the Gulf of Mexico. We also tested the influence of tagging longitude, latitude and age on migration pace.

Results: Migration routes varied and converged towards the northeast coast of the Gulf of Mexico, but in this region, populations maintained finer-scale spatial structure. Migration pace increased as birds progressed south, independent of age and tagging site.

Conclusions: We showed that for songbirds, migratory connectivity can be maintained at fine spatial scales despite the regional convergence of populations, highlighting the importance of detailed spatial tracking for identification of population specific migration routes. Overall, our study provides a portrait of migratory movements of eastern Swainson's Thrush and a framework for understanding spatial structure in migration routes for other species.

Keywords: Migration pace; Migratory connectivity; Motus; Neotropical migrants; Radio-telemetry.

Fig. 1

Number of birds detected in the Motus network from different tagging locations (solid black circles). Receiving stations were aggregated in cells of 1 × 1 degree. Every individual was counted only once per receiving station. Empty cells are receiving stations with no detections and colored cells represent the number of individuals detected per cell

Fig. 2

Variation of the spatial (continuous line) and temporal (hatched line) migratory connectivity (Mantel Rm statistic) en route between tagging sites along a latitudinal gradient of cells of 1 × 1 degree (values close to 0 = weak migratory connectivity, values close to 1 = strong migratory connectivity). Populations converged near the Florida peninsula but maintained a finer scale spatial structure (continuous line). We calculated the median latitude of the 16 intervals of 5°N tested. Observations between 37 and 33 °N did not include enough receiving stations for a robust statistical interpretation. Populations maintained a temporal segregation (hatched line) in the early stages of migration, but no differences associated to the origin was found south of 38°N, despite a slight increase, but weak connectivity, in the last detections south of 30°N. Vertical bars represent 95% confidence limits based on 100 bootstrap samples. The read horizontal line represents the y-intercept = 0

Fig. 3

Time (a: hours, b: days) between successive detections in relation to distance between stations (km)) and migration pace (km/h) of 553 segments from 236 Swainson’s Thrushes for segments within the same day (a) and > 1 day (b). Migration pace is the result of the distance and the time elapsed between two receiving stations. The migration pace does not indicate the ground speed of the birds as the distance between receiving stations is not representative of the distance traveled by the bird

Fig. 4

Fitted splines for a generalized additive model of the relationship between distance between receiving stations and latitude (smooth terms) on migration pace (log). Distance between receiving stations (left) suggest a lack of variation of the migration pace for longer segments. Birds have a slower migration pace in northern latitude, closer to their breeding origin (right). Migration paces were slower and more variable in northern latitudes suggesting more stopover closer to the breeding grounds