A primer on the hybrid zone of Jamaican streamertail hummingbirds (Trochilidae: Trochilus)

—The endemic hummingbirds Trochilus polytmus and T. scitulus hybridize in a narrow zone of secondary contact in eastern Jamaica. The cline in bill width across the hybrid zone represents the steepest morphological gradient documented thus far in avian biology. Hindlimb size and skeletal proxies for core body size, however, exhibit incongruent patterns of variation across the same transect. Significant correlation between bill width and skull width indicates that genetic loci that affect bill width have pleiotropic effects on cranial morphology posterior of the craniofacial hinge. This study is the first direct examination of skeletal size variation across an avian hybrid zone.