Volume 7, Issue 1

Migration routes vary greatly among small passerine species and populations. It is now possible to determine the routes over great distances and long periods of time with emerging monitoring networks. We tracked individual Swainson’s Thrush ( Catharus ustulatus ), Bicknell’s Thrush ( Catharus bicknelli ) and Gray-cheeked Thrush ( Catharus minimus ) in northeastern Quebec and compared their migration routes and paces across an array of radio-telelemetry stations in North America. Swainson’s Thrush migrated further inland than the other two species. Individuals from all three species slowed their migration pace in the southeastern United States, and Swainson’s Thrush was more likely to stopover than Bicknell’s Thrush. Although individuals were tagged in a small area within or close to their breeding range, the results document the variability of migration routes between species with similar ecological characteristics and provide detailed material to be used for migration studies with broader taxonomic or ecological scope.

Migratory movements of facultative migrants are poorly understood due to their irregular and often unpredictable occurrence. However, tracking such movements is important for understanding population dynamics, informing annual cycle conservation plans, and identifying possible cues of facultative migration. We used pinpoint GPS tags to track autumn and winter movements of migratory red-headed woodpeckers ( Melanerpes erythrocephalus ) to better understand migration frequency, timing, and routes for birds breeding in managed oak savanna systems in the North American Great Lakes region. Proportions of individuals migrating differed between the two sites, with 72% of the Ohio population migrating, while no individuals in the Minnesota population migrated. Of the Ohio birds that migrated, their movements were highly variable in distance and direction but generally occurred south of the breeding site. Wintering sites ranged from 111 – 218 km from the breeding site. Cover types occupied during migration and wintering were almost exclusively small patches of closed-canopy hardwood forest within agricultural matrices. We documented one-time movements in migratory and non-migratory individuals during the year that have not previously been described in facultative migrants. We found no evidence of a harness or marker effect on proportions of individuals migrating, migration return rates, or annual survival regardless of migration.

Cerulean Warblers ( Setophaga cerulea ) are among the fastest declining Nearctic-Neotropical migrant wood-warblers (Parulidae) in North America. Despite ongoing conservation efforts, little is known about their non-breeding distribution. In June 2016-2018, we deployed geolocators ( n = 30) on adult male Cerulean Warblers in Indiana, USA, to track annual movements of individuals. Recovered geolocators ( n = 4) showed that Cerulean Warblers occurred broadly throughout northern South America. Autumn migration lasted 44-71 days ( n = 4), whereas spring migration lasted 37-41 days ( n = 3). The average migration distance was 5268 km. During autumn migration, Cerulean Warblers made 1-4 stopovers (i.e., ≥2 days; n = 4) and 1-2 stopovers during spring migration ( n = 3). When crossing the Gulf of Mexico during autumn migration, two birds stopped over after crossing, but not beforehand. Two others navigated through the Caribbean rather than crossing the Gulf of Mexico. During spring migration, one individual stopped after crossing, one individual stopped before crossing, and one individual stopped before and after crossing the Gulf of Mexico. No birds migrated through the Caribbean Islands during spring migration. These results represent novel information describing annual movements of individual Cerulean Warblers and will inform conservation efforts for this declining species.

Probability-of-origin maps deduced from stable isotope data are important for inferring broad-scale patterns of animal migration, but few resources and tools for interpreting and validating these maps exist. For example, quantitative tools for comparing multiple probability-of-origin maps do not exist, and many existing approaches for geographic assignment of individuals have not been validated or compared with respect to precision and accuracy. To address these challenges, we created and analyzed probability-of-origin maps using stable hydrogen isotope values from known-origin individuals of three species of migratory bat. We used a metric of spatial overlap to group individuals by areas of origin without a priori knowledge of such regions. The metric of spatial similarity allowed for quantitative comparison of geographic origins and grouping of individuals with similar origins. We then compared four approaches for inferring origins (cumulative-sum, odds-ratio, quantile-only, and quantile-simulation) across a range of thresholds and probable minimum distance traveled. The accuracy of geographic origins and minimum distance traveled varied across species at most threshold values for most approaches. The cumulative-sum and quantile-simulation approaches had generally higher precision at a given level of accuracy than the odds-ratio and quantile-only approaches, and many threshold values were associated with a relatively high degree (> 300 km) of variation in minimum distance traveled. Overall, these results reinforce the importance of validating assignment techniques with known-origin individuals when possible. We present the tools discussed as part of an R package, ‘isocat’ (“Isotope Origin Clustering and Assignment Tools”).

We used stable hydrogen isotope markers to identify geographic provenance of Blackpoll Warblers (Setophaga striata) and Red-eyed Vireos (Vireo olivaceus) passing through a coastal stopover area in the Gulf of Maine on fall migration, and supplemented δ2 H signatures from feathers with wing length data as an additional proxy for migration origin, since geographic size variation has been documented for this species. We captured migrant blackpolls with putative origins from as close as the Adirondacks, New Brunswick, and northwestern Maine, and potentially as far as Alaska. We captured vireos with putative origins from as close as Maine and Nova Scotia, and potentially as far as British Columbia and Alberta. We found a significant relationship between blackpoll wing length and capture date indicating that birds with longer wings– and thus likely from more western and northern breeding regions – passed through the capture site earlier in the season than more local breeders. In contrast, vireos from more distant breeding latitudes passed through the capture site later in the season. These results demonstrate that mid-coast Maine serves as a catchment area for both Blackpoll Warblers, and Red-eyed Vireos, and provides some evidence that connectivity between breeding areas and stopover sites may be strong for blackpolls; however, additional study linking breeding and stopover sites across the migratory range of both species is necessary to understand more fully how strong connectivity is between breeding and stopover areas for these species.

Migrating animals are known to play an important role in nutrient transfer over short distances; however, this phenomenon has not been well studied for long-distance migrants. In this preliminary study, we focused on nitrogen (N) transfer by 44 bird species that migrate from Eurasia to two regions in sub-Saharan Africa that fall into the lowest 10% quantile of global N-deposition (mean annual deposition ≤ 10.44 mg/m 2 /year). We estimated the number of birds that die during the non-breeding season in these areas and then used N content and species-specific mass values to calculate annual N-deposition rates. For these two areas of low N-deposition, we found that bird mortality contributed 0.2 – 1.1% of total nitrogen deposition, which is a relatively small proportion. Therefore, we conclude that nitrogen transfer by long-distance bird migrants using the East Atlantic Flyway and the West Asian-East African Flyway currently has limited impact on the sub-Saharan nitrogen cycle. However, it is worth noting that this impact may have been more important in the past due to larger bird populations and lower background N-deposition (i.e., less anthropogenic impact).

Determining the processes that shape the relative timing of energetically-costly events in the annual cycle of migrating birds is important to our understanding of avian phenology and ecology. We paired satellite tracking and hydrogen stable isotope analysis (δ 2 H) to examine the relative timing of two such events – migration and feather molting – in tundra swans from four breeding areas in Alaska, USA. Our results show a trend of increasing intra-individual variability in breast feather δ 2 H values with increasing migration distance, suggesting the overlap of breast feather molting and migration. However, when individual samples were pooled by breeding area, the δ 2 H values of breast and head feathers showed no trend with migration distance, presumably resulting from high levels of inter-individual variability in δ 2 H values within each breeding area. We explore potential reasons for this variability, propose potential mechanisms influencing feather δ 2 H values of tundra swans, and recommend further research into methods for exploring the temporal configuration of events in the annual cycle of migrating birds.