Date of Award
Doctor of Philosophy (PhD)
J. Albert C. Uy
anthropogenic disturbance, communication, Eastern bluebird, urban noise
Signals that are efficiently transmitted and easily detected in their signaling environment are favored by natural selection. Anthropogenic disturbance can rapidly alter the signaling environment, and recent studies have shown that acoustic and visual signals change in response to these altered habitats. Although these studies provide important insight into the effects of urbanization on animal signals and have served as experiments testing the role of the environment in shaping signal design, several key aspects of how signal design can be influenced by environmental changes remain unclear.
Previous studies have focused on signals used between adults, such as those used in mate choice, yet other signals should be similarly affected by anthropogenic disturbance. Thus, one facet of my research examines how anthropogenic disturbance can influence parent-offspring communication. I tested whether nestling mouth coloration in Eastern bluebirds (Sialia sialis) was a signal of quality, and if a parents ability to discriminate among the mouth coloration of their nestlings was affected by level of human disturbance. I found that the perceived color contrast of nestling mouths against its nest was significantly correlated with nestling body condition, suggesting that it may be signal of nestling quality. Additionally, I found that the parent's ability to perceived a difference in color contrast of a nestling's mouth among nest-mates was lower in disturbed habitats, than in undisturbed habitats, showing less discriminability among nestlings in disturbed habitats. These results suggest that parent-offspring communication can be affected by anthropogenic disturbance which may reduce a parent's ability to preferentially invest in high quality young.
Past research on anthropogenic disturbance and signaling has focused on the response of single signals, yet most organisms communicate using signals from multiple sensory systems (i.e., multimodal signals). Thus, I examined how anthropogenic disturbance can simultaneously influence components of multimodal signals in Eastern bluebirds. I measured the visual and acoustic environment at different disturbance levels and related them to male plumage and song characteristics. I found that in areas with high levels of anthropogenic noise, males sing at a higher minimum frequency, presumably to avoid overlap with low frequency background noise. I also found that the visual background is altered in disturbed sites; however, plumage characteristics did not covary with the altered habitats. These results suggest that human disturbance is interfering with both visual and acoustic signals, yet only acoustic signals have responded to the changes.
Few studies on anthropogenic disturbance directly explore the explicit evolutionary mechanisms underlying the changes in signal design. Thus, in the final chapter of my dissertation, I explored how selection on traits varied across habitats with different levels of disturbance. To do this, I determined paternity of nestlings using microsatellite. Then, I determined the major factors influencing rates of extra-pair paternity (i.e., proportion of nestlings within a nest that were sired by other males), and tested whether these factors varied with disturbance levels. I found that the minimum frequency of song, and the brightness of the male's chestnut breast are important predictors of extra-pair paternity rate across all disturbance levels. Additionally, I found an interaction between disturbance level and the minimum frequency of song in relation to extra-pair paternity. This interaction effect was due to differences in selection pressure on the minimum frequency of song in relation to habitat disturbance. Males that sing in higher minimum frequencies have lower rates of extra-pair paternity, in disturbed areas, but higher rates of extra-pair paternity in undisturbed areas. These results suggest that selection on signals vary across disturbance levels and this could drive the observed changes in the design of signals. Potential consequences of these changes include the possibility of long-term differentiation between bluebird populations living in disturbed and undisturbed habitats, and a shift in important traits across the entire species.
Wisner, Ellen Michale, "The Consequence of Anthropogenic Disturbance on Communication and the Operation of Sexual Selection in the Eastern Bluebird (Sialia Sialis)" (2011). Biology - Dissertations. 75.