A Population Bottleneck Did Not Affect Polymorphism Rates in California Swainson's Hawks

Abstract

ABSTRACT Raptor polymorphisms have played an important role in understanding how evolutionary forces within and between populations operate. However, within a population little is known about the history of the polymorphism, the effects of any population bottlenecks, and the overall stability of the polymorphism. We investigated the stability of a melanin-based color polymorphism in Swainson's Hawks (Buteo swainsoni) in California over a 100-year period. In the mid-20th century Swainson's Hawks in California declined by 90%. Therefore, we examined the morphs of breeding individuals collected before 1950 and compared them to images from eBird taken between 2008–2019 and from a Google Images search, because a disproportionate survival of rare alleles after a population bottleneck could lead to changes in current clines. Between the two periods, we did not find differences in plumage morphs, nor did we find evidence of latitude or longitudinal clines over this relatively small spatial scale. Thus, despite a large population decline, this polymorphism has remained consistent over time. Our results suggest that the relatively high occurrence of dark morphs in this population is not simply a result of a bottleneck.