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Abstract Seasonal migration is performed by taxonomically diverse groups across the planet’s oceans and continents. Migration has been hypothesized to promote speciation through a variety of mechanisms that may initiate reproductive isolation and population divergence, such as temporal or spatial migratory divides, migration “falloffs,” or the colonization of new, geographically isolated breeding areas. Migration has also been implicated in recent population divergence within a handful of bird species; however, it is unknown whether migration is generally associated with higher speciation rates. We sought to test this question in two large clades of New World birds with diverse migratory phenotypes, the suboscines and the Emberizoidea, employing three state-of-the-art comparative methods of trait-based diversification: estimates of tip speciation rates using 1) BAMM and 2) ClaDS, and 3) hidden-state speciation extinction models. Our results differed across methods and across taxonomic scales, suggesting an acute need to corroborate inferences across different frameworks and data sets prior to concluding that a given trait has, in fact, promoted diversification. Overall, and based upon the majority of results across different methods, we conclude that there is no methodologically consistent evidence of faster speciation in migratory lineages in these groups. We discuss the biological implications of this finding, as well as the challenges of inference posed by current trait-based diversification methods.
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This content will become publicly available on September 1, 2026
Incorporating the full annual cycle when studying reproductive isolation and speciation
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.
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- Award ID(s):
- 2305971
- PAR ID:
- 10656705
- Publisher / Repository:
- The Nordic Society Oikos
- Date Published:
- Journal Name:
- Journal of Avian Biology
- Volume:
- 2025
- Issue:
- 5
- ISSN:
- 0908-8857
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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