The population genetics of nonmigratory Allen’s Hummingbird (Selasphorus sasin sedentarius) following a recent mainland colonization

Abstract

Allen's Hummingbird comprises two subspecies, one migratory (Selasphorus sasin sasin) and one nonmigratory (S. s. sedentarius). The nonmigratory subspecies, previously endemic to the California Channel Islands, apparently colonized the California mainland on the Palos Verdes Peninsula some time before 1970 and now breeds throughout coastal southern California. We sequenced and compared populations of mainland nonmigratory Allen's Hummingbird to Channel Island populations from Santa Catalina, San Clemente, and Santa Cruz Island. We found no evidence of founder effects on the mainland population. Values of nucleotide diversity on the mainland were higher than on the Channel Islands. There were low levels of divergence between the Channel Islands and the mainland, and Santa Cruz Island was the most genetically distinct. Ecological niche models showed that rainfall and temperature variables on the Channel Islands are similar in the Los Angeles basin and predicted continued expansion of nonmigratory Allen's Hummingbird north along the coast and inland. We also reviewed previous genetic studies of vertebrate species found on the Channel Islands and mainland and showed that broad conclusions regarding island–mainland patterns remain elusive. Challenges include the idiosyncratic nature of colonization itself as well as the lack of a comprehensive approach that incorporates similar markers and sampling strategies across taxa, which, within the context of a comparative study of island–mainland relationships, may lead to inconsistent results.

Quantifying phenology and migratory behaviours of hummingbirds using single-site dynamics and mark-detection analyses

“…Seasonal movements among the hummingbirds in our study areas are therefore more complex than previously presumed. Overall, we present evidence of facultatively controlled migratory behaviour in Allen’s hummingbirds ( figure 1 ), while there is currently little conclusive genetic [ 46 ] or morphological [ 26 , 29 ] evidence of distinct subspecies. We therefore suggest that factors other than subspecies dynamics are primarily driving differences in seasonal movement patterns among populations of Allen’s hummingbirds.…”

“…The migratory behaviours of Allen’s hummingbirds have changed over the past half century since the non-migratory subspecies ( S. s. sedentarius ) purportedly colonized mainland southern California [ 27 ]. Since the colonization of the mainland by this resident subspecies, non-migratory Allen’s hummingbirds expanded their range and it was postulated that the subspecies ranges did not overlap [ 27 , 45 , 46 ]. Our data demonstrate that this is false, as both migratory and resident Allen’s hummingbirds co-inhabit the same territory in our study area ( figure 1 ).…”

Widespread gene flow following range expansion in Anna's Hummingbird

“…While the expectation for the magnitude and direction of gene flow is unknown in C. anna , largely due to enigmatic movement patterns, a previous study suggested high gene flow between three California populations (Engeln, 2013). Another California hummingbird, Allen's Hummingbird ( S. sasin ), was found to have high gene flow among the mainland populations (Myers et al, 2021), potentially indicative of high overall levels of mobility in hummingbirds. If gene flow in C. anna is extremely high, we might expect signatures of selection caused by exposure to novel selective agents during range expansion to be present across the entire species rather than divergent between populations.…”