Determining how animal populations are linked in space and time is important for identifying factors influencing population dynamics and for effective conservation and management. Arctic-breeding migratory passerines are declining and at risk due to forecasted climate change, but are a challenge to monitor due to their inaccessible breeding locations, long-distance migration routes and small body size. For the first time, we combine sub-gram geolocator technology and stable-isotope analysis with mark-recapture (banding) and citizen science data to determine patterns of migratory connectivity for multiple populations of a declining North American Arctic-breeding passerine, snow bunting (Plectrophenax nivalis). We show strong evidence for an east-west parallel migratory system, with Hudson Bay acting as a migratory divide. While band recoveries suggest strong migratory connectivity among eastern wintering populations (more than 95% of band recoveries reveal connections between western Greenland and eastern North America), novel application of geolocators and stable-hydrogen isotope analysis to a Canadian breeding population revealed a high degree of migratory connectivity within western North American wintering populations. Our results also show distinct differences in migratory distance between eastern and western populations, and illustrate how applying multiple techniques can effectively be used to track migration patterns of remote populations. Differences in annual distribution and migratory distance suggest that separate consideration of eastern and western wintering populations may improve future conservation and management efforts for this species.

There are many pairs of related western and eastern avian taxa in North America, and for many of these, little is known about their interactions in sympatry. One example is provided by MacGillivray's warblers Oporornis tolmiei and mourning warblers Oporornis philadelphia. There have been occasional reports of range contact and hybridization between these forms, but recent authors have doubted these reports. We show that these two species do in fact come into extensive range contact in the southern Peace Region of British Columbia, just east of the Rocky Mountains. We analyze whether patterns of variation in morphometric traits, eye‐arcs, a mitochondrial DNA marker (COI), and a Z‐chromosome marker (CHD1Z) are consistent with reproductive isolation or hybridization in this contact zone. Each trait shows strong differences between allopatric MacGillivray's warblers and allopatric mourning warblers, yet in the contact zone there are many birds with a combination of traits typical of both species. This is clearly seen in the molecular markers, for which 18 of 50 birds genotyped in the contact zone have both western and eastern alleles. These patterns strongly indicate the presence of an extensive hybrid zone between MacGillivray's and mourning warblers. Variation in each of the four traits is explained well by a single sigmoidal cline, with a width of roughly 150 km (or 130 km based only on the molecular markers). This is only the fourth hybrid zone known among North American wood‐warblers (Parulidae).

The costs of reproduction are an important constraint that shapes the evolution of life histories, yet our understanding of the proximate mechanism(s) leading to such life-history trade-offs is not well understood. Oxidative stress is a strong candidate measure thought to mediate the costs of reproduction, yet empirical evidence supporting that increased reproductive investment leads to oxidative stress is equivocal. We investigated whether territory quality and offspring provisioning increase oxidative stress in male snow buntings (Plectrophenax nivalis) using a repeated sampling design. We show that arrival oxidative stress is not a constraint on territory quality or the number of offspring fledged. Nevertheless, owners of higher-quality territories experienced an oxidative cost, with this cost increasing more rapidly in younger males. Males that provisioned offspring at a high rate also experienced increased oxidative stress. Together, these findings support the potential role of oxidative stress in mediating life-history trade-offs. Future work should consider that reproductive workload is not limited to offspring care, and other activities -including territory defence -may contribute significantly to the costs of reproduction.