Minnesota's Red-tailed Hawks: Probabilistic Origins of B.j. abieticola and Dark-morph Migrants

Abstract

The Red-tailed Hawk (Buteo jamaicensis) is one of North and Central America’s most common, polymorphic raptor species, with an extensive geographic range divided into 12 putative subspecies ranges. In North America, plumage polymorphism occurs along a clinal gradient with dark-morph individuals becoming less prevalent east of the Rocky Mountains. Polymorphism and other plumage traits can be used to identify individuals to a subspecies, but high levels of intergradation and individual variation can complicate identification. Duluth, Minnesota, USA is a migratory hotspot well known for phenotypically diverse Red-tailed Hawks, including B.j. abieticola and dark-morphs plumages. Due to atypical plumage traits of B.j. abieticola and dark-morphs, subspecific origins of Minnesota’s migratory individuals are not always resolved. Genetic data was collected from Duluth’s migratory Red-tailed Hawk population and known subspecies populations, B.j. calurus and B.j. borealis, to determine the probabilistic subspecific origins of B.j. abieticola and dark-morph migrants. Twelve microsatellite markers were used to analyze and compare genetic diversity and population structure within and among breeding populations and the migratory individuals. Bayesian statistics were also performed to determine probabilistic assignments of migratory individuals to putative subspecies. Supplemental spatial data was also collected from two presumed adult dark-morph B.j. abieticola. Pairwise FST revealed the B.j. abieticola and dark-morph migrants were both more genetically similar to B.j. borealis than B.j. calurus. Population assignment probabilities supported that these migratory individuals were more closely related to B.j. borealis than B.j. calurus. Furthermore, preliminary satellite transmitter data from one presumed adult dark-morph B.j. abieticola migrant revealed the individual spent at least one summer east of the Rocky Mountains. These findings suggest Minnesota’s B.j. abieticola and dark-morph migrants have a higher probability of originating from B.j. borealis, a Red-tailed Hawk subspecies historically known to only present light-morph plumage, than B.j. calurus.