The Evolution of Quiet Flight in Owls (Strigiformes) and Lesser Nighthawks (Chodeiles acutipennis)

All locomotion produces sound and flight is no exception. In owls, flight sounds are quieted by three wing and feather features: the leading-edge comb, a modified barb structure that projects dorsally from the front edge of the outermost primary feather (P10), the velvety dorsal surface of flight feathers, and the fringed vane of flight and tail feathers. There are two hypotheses for the evolution of quieting features: stealth and self-masking. Under the stealth hypothesis, we predict quiet flight evolved to aid owls in sneaking up on prey. Under the self-masking hypothesis, we predict quiet flight evolved to aid acoustic hunters in locating prey. To test these hypotheses, we investigated the relationship between leading-edge comb morphology and ecology (Chapter 1), tested the function of the dorsal velvet in Barn Owls (Chapter 2), and investigated the function of quieting features in Lesser Nighthawks, another nocturnal bird (Chapter 3). In Chapter 1, We used phylogenetic generalized least squares (pgls) to test the correlation between comb morphology and the stealth or self-masking scores. We found comb morphology to be correlated with both stealth and self-masking (pgls; DF = 66 test-statistic = -3.92; P-value = 0.0002). In Chapter 2, impairing the dorsal velvet of 10 feathers on 13 barn owls increased broadband sound production and the upstroke increased more than the downstroke, such that the upstroke of manipulated birds was louder than the downstroke, supporting the frictional noise hypothesis. Finally, In Chapter 3, we found Lesser Nighthawks initiated pursuit of prey at a greater distance than the audible detection distance of insects (0.5 m) both when hunting on the ground (1.1 ± 0.2 m; P-value = 0.02) and on the wing (2.5 ± 0.4 m; P-value <0.0001), suggesting they use visual cues to initiate pursuit of prey under these conditions. This dissertation contributes to a research area which has received little consideration: the role of sound and hearing in predation