Brian Busby

Date of Award


Document Type


Thesis Advisor

William M. Shields


Hummingbirds, birds--color, sexual differences in birds


Within the birds there are countless examples of sexual dimorphism, ranging from obvious differences such as in plumage to more discreet variations such as in bill or wing length. The more conspicuous plumage differences are typically attributed to sexual selection, but the evolutionary cause of subtle dimorphisms is much more unclear, with sexual selection and ecological causation both being valid possibilities. Therefore the question arises, are subtle dimorphisms more correlated with species that are already sexually dichromatic, or do both plumage dimorphic and plumage monomorphic species have an equal likelihood of displaying discreet gender differences? To answer this we captured and analyzed fifteen different species of Ecuadorian hummingbirds, six of them being dichromatic, and nine of them being monochromatic. We measured four subtle traits in both genders across all species: weight, wing length, tail length, and culmen (bill) length, and used ANOVAs to determine if there were any significant differences between genders. Our results revealed that dichromatic species do have a greater chance of displaying subtle dimorphisms, with 83% of species having gender differences in at least one subtle trait as opposed to only 44% in monochromatic species. This indicates that there is a correlation between obvious gender differences like plumage and more discreet dimorphisms, although the cause for this, be it sexual selection of ecological causation, remains unclear. However, because some monochromatic species do display significant subtle dimorphisms, we have now opened the door to differentiating gender in these species in the absence of sexual structures.

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Ornithology Commons