In migratory animals, the degree to which individuals return to the same wintering sites across multiple years can affect fitness and population dynamics, and thus has important implications for conservation. Despite this, long-term evaluations of wintering-site fidelity are rare for migratory birds: many populations are intensively studied on their breeding grounds but tracking the migratory movements of small birds once they leave the breeding grounds is challenging. To evaluate patterns of overwintering location and fidelity, we collected winter-grown claw tissue from 301 Song Sparrows (Melospiza melodia; 449 samples) captured in spring at their breeding grounds over 6 consecutive yr and assessed stable hydrogen isotope (δ2Hc) values to determine within-individual repeatability and between-year variation in wintering latitudes. We also retrieved useable data from 8 geolocators over 2 consecutive winters. Geolocator-derived wintering positions correlated with origins based on δ2Hc values. Consistent with previous findings, male δ2Hc values reflected more northerly wintering areas than those of females, indicating shorter latitudinal migration distances for males, but the magnitude of the sex difference varied across years. The distribution of wintering latitudes was generally consistent among years, except for the 2015–2016 winter, which had unusually negative δ2Hc values. Values of δ2Hc were repeatable for males but not for females, suggesting that winter-site fidelity could differ between sexes. The data presented here emphasize the importance of tracking migratory populations across multiple years to uncover factors affecting population dynamics.
Study Population and Field Methods
We captured Song Sparrows breeding at a long-term study site near Newboro, Ontario, Canada (44.66°N, 76.22°W), on land owned by the Queen’s University Biological Station. This population has been studied for >15 yr; thus, age and breeding history is known for most individuals and standard field methods have been established. Fieldwork was conducted during April and the first week of May 2012 through 2017 (Table 1). We captured birds in seed-baited Potter traps, which we checked each hour between 0630 and 1030 hours. We determined sex based on the presence (male) or absence (female) of a cloacal protuberance, the presence of a brood patch (incubating female), supplemented by measurements of unflattened wing chord, measured to the nearest 0.1 mm using dial calipers. We outfitted birds with a Canadian Wildlife Service aluminum band for individual identification. We inferred age from previous years’ capture and banding records and considered previously unbanded adults to be 1 yr of age (in their second year = SY; after second year = ASY) at first capture (Lapierre et al. 2011). Before release, we clipped a small sample of claw tissue (~2.5 mm; Figure 1) from the distal portion of each hallux claw for stable isotope analysis. Animal procedures were approved by the Animal Use Subcommittee of Western University (protocols 2008–054 and 2016–017) and federal approval from Environment and Climate Change Canada (permit 10691).
Stable Isotope Analysis
We soaked, agitated and rinsed claw samples in 1.6 mL of 2:1 chloroform-methanol solution to remove any dirt or oils that might contaminate isotope analyses. We removed excess chloroform-methanol and dried claw samples overnight in a fume hood. Samples were stored at room temperature in a sterile 96-well microplate for 2 wk to equilibrate to laboratory conditions. Once dry, we weighed nails to 350 ± 10 µg (Mettler Toldeo MX5 Microbalance PSU30A-3, Griefensee, Switzerland) and crushed samples in silver capsules.
We analyzed samples for δ2H for the period 2012–2015 at the Stable Isotope Laboratory of Environment Canada, Saskatoon, Canada, and 2016 and 2017 at Western University's LSIS-AFAR isotope laboratory. In each laboratory we derived δ2H values for the nonexchangeable hydrogen portion of claws using online continuous-flow isotope mass spectrometry (CF-IRMS). In Saskatoon, we combusted samples at 1,350°C using pyrolytic (glassy carbon) combustion in a Hekatek furnace interfaced with a Micromass Isoprime mass spectrometer (Micro-mass UK, Manchester, UK). At the LSIS-AFAR laboratory, we combusted samples at 1,020°C using a chromium-based reactor in a Flash Elemental Analyser interfaced with a Thermo Delta V Plus isotope ratio mass spectrometer (Thermo Instruments, Bremen, Germany). At both laboratories, we corrected the influence of exchangeable hydrogen by performing δ2H analyses using the comparative equilibrium method using 2 keratin standards (Caribou Hoof Standard [CBS]: –197‰, and Kudu Horn Standard [KHS]: –54.1‰; ). All keratin δ2H results are reported in units of per mil (‰) and normalized on the Vienna Standard Mean Ocean Water-Standard Light Antarctic Precipitation (VSMOW-SLAP) standard scale. Based on within-run replicate analyses of each keratin standard, the analytical precision was estimated to be ±2‰.