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As individual tracking devices and year‐round genetic sampling become more accessible, research on the historically understudied nonbreeding period has exploded in the past decade. These studies are revealing tremendous inter‐ and intraspecific variation in migratory, molting, and other nonbreeding strategies, thereby informing efforts to protect bird populations throughout the entire annual cycle. However, we still have much to learn about where and when nonbreeding adaptive variation influences reproductive isolation and speciation. Previous work has demonstrated that some adaptations to conditions in different nonbreeding areas or migratory routes can fuel diversification by precluding opportunities for diverging lineages to interbreed or, in instances where lineages do interbreed, manifesting as disadvantageous phenotypes in hybrids. In this paper, we provide an overview of both established and speculative processes through which the primary nonbreeding events in the avian annual cycle (i.e. molt, migration, and overwintering) may interact to regulate gene flow between avian lineages. Although the relatively few but well‐described examples of divergence in nonbreeding phenotypes contributing to reproductive isolation suggest nonbreeding divergence is a common mode of speciation in birds, a growing number of population genetic studies reporting nonbreeding divergence in the absence of reproductive isolation seemingly suggest the opposite conclusion. We outline processes that could result in this apparent contradiction and propose general comparative frameworks to test factors that may predictably mediate the relationship between nonbreeding divergence and reproductive isolation. In the past, a shortage of nonbreeding natural history and population genetic data have impeded our ability to test these predictions in more than just a few systems. We urge evolutionary biologists to pay closer attention to conservation‐oriented studies, which are rapidly filling these knowledge gaps and presenting opportunities to better understand the true role of nonbreeding divergence in avian diversification. 

Abstract Describing how hybrid zones respond to anthropogenic influence can illuminate how the environment regulates both species distributions and reproductive isolation between species. In this study, we analyzed specimens collected from the Passerina cyanea×P. amoena hybrid zone between 2004 and 2007 and between 2019 and 2021 to explore changes in genetic structure over time. This comparison follows a previous study that identified a significant westward shift of the Passerina hybrid zone during the latter half of the twentieth century. A second temporal comparison of hybrid zone genetic structure presents unique potential to describe finer-scale dynamics and to identify potential mechanisms of observed changes more accurately. After concluding that the westward movement of the Passerina hybrid zone has accelerated in recent decades, we investigated potential drivers of this trend by modeling the influence of bioclimatic and landcover variables on genetic structure. We also incorporated eBird data to determine how the distributions of P. cyanea and P. amoena have responded to recent climate and landcover changes. We found that the distribution of P. cyanea in the northern Great Plains has shifted west to track a moving climatic niche, supporting anthropogenic climate change as a key mediator of introgression in this system. 

Abstract Natural history collections are repositories of biodiversity specimens that provide critical infrastructure for studies of mammals. Over the past 3 decades, digitization of collections has opened up the temporal and spatial properties of specimens, stimulating new data sharing, use, and training across the biodiversity sciences. These digital records are the cornerstones of an “extended specimen network,” in which the diverse data derived from specimens become digital, linked, and openly accessible for science and policy. However, still missing from most digital occurrences of mammals are their morphological, reproductive, and life-history traits. Unlocking this information will advance mammalogy, establish richer faunal baselines in an era of rapid environmental change, and contextualize other types of specimen-derived information toward new knowledge and discovery. Here, we present the Ranges Digitization Network (Ranges), a community effort to digitize specimen-level traits from all terrestrial mammals of western North America, append them to digital records, publish them openly in community repositories, and make them interoperable with complimentary data streams. Ranges is a consortium of 23 institutions with an initial focus on non-marine mammal species (both native and introduced) occurring in western Canada, the western United States, and Mexico. The project will establish trait data standards and informatics workflows that can be extended to other regions, taxa, and traits. Reconnecting mammalogists, museum professionals, and researchers for a new era of collections digitization will catalyze advances in mammalogy and create a community-curated trait resource for training and engagement with global conservation initiatives.