Bill shape and sexual shape dimorphism between two species of temperate hummingbirds: black-chinned hummingbird and ruby-throated hummingbird (A. colubris)/Formal del pico y dimorfismo sexual entre dos especies de picaflores de la zona templada: Archilochus alexandri y A. colubris

Sexual size dimorphism occurs throughout the animal kingdom, and its ecological and evolutionary causes and implications have been intensively studied. Sex-specific differences in bill curvature are known in several species of birds, including some tropical hummingbirds. Despite the importance of bi... Full description

Sexual size dimorphism occurs throughout the animal kingdom, and its ecological and evolutionary causes and implications have been intensively studied. Sex-specific differences in bill curvature are known in several species of birds, including some tropical hummingbirds. Despite the importance of bill shape for foraging, comparative studies of sexual dimorphism of bill shape are few. We quantified bill shape in two temperate hummingbird species, Black-chinned Hummingbird (Archilocus alexandri) and Ruby-throated Hummingbird (A. colubris) and compared patterns of sexual shape dimorphism. Several commonly used bill- curvature indices yielded contrasting results; one found differences between species and sexes, a second identified no differences in curvature, and a circle-curvature approach revealed shape differences between species and between the sexes. By contrast, landmark-based geometric morphometric methods identified significant differences in sexual shape dimorphism and also revealed that Ruby-throated Hummingbirds exhibited significant sexual differences in shape, whereas Black-chinned Hummingbirds did not. Female Ruby-throated Hummingbirds exhibited relatively greater bill curvature than males, a pattern consistent with observations of some tropical hummingbirds. Although the causes of differences in bill-shape dimorphism between Black-chinned and Ruby-throated hummingbirds remain unclear, we hypothesize that ir may be attributable to differences in the structure of the community in which each species breeds and the interplay between inter- and intraspecific competition for resources in these communities. Finally, we recommend that future studies of bill shape include geometric morphometric approaches because they are better suited than univariate approaches for identifying more complex shape differences within and among species. Received 26 October 2009, accepted 10 March 2010. Key words: Archilocus alexandri, A. colubris, bill morphology, Black-chinned Hummingbird, geometric morphometrics, Ruby-throated Hummingbird, sexual dimorphism, Trochilidae. El dimorfismo sexual en el tamano se presenta a traves del reino animal, y sus causas e implicancias ecologicas y evolutivas han sido intensamente estudiadas. Las diferencias especificas de los sexos en la curvatura del pico son conocidas en varias especies des aves, incluyendo algunos picaflores tropicales. A pesar de la importancia de la forma del pico para forrajear, los estudios comparativos de dimorf