A multidimensional approach to comparative avian visual systems

Bret Alan Moore, Purdue University

Abstract

Since the birth of visual ecology, comparative studies on how birds see their world have been limited to a small number of species and tended to focus on a single visual trait. This approach has constrained our ability to understand the diversity and evolution of the avian visual system. The goal of this dissertation was to characterize multiple visual dimensions on bird groups that are highly speciouse (e.g., Passeriformes), and test some hypotheses and predictions, using modern comparative tools, on the relationship between different visual traits and their association with visual information sampling behaviors. First, I developed a novel method for characterizing quantitatively the retinal topography (e.g., variation in cell density across the retina) of different bird species in a standardized manner. Second, using this method, I established that retinal configuration has converged particularly in terrestrial vertebrates into three types of retinal specializations: fovea, area, and visual streak, with the highest, intermediate, and lowest peak and peripheral ganglion cell densities, respectively. The implication is that foveate species may have more enhanced visual centers in the brain than non-foveate vertebrates. Third, forest passerines that form multi-species flocks and belong to an insectivore niche differ in their visual system configuration, which appeared associated to behavioral specializations to enhance foraging opportunities: species that searched for food at steep angles had relatively wide binocular fields with a high degree of eye movement right above their short bills, whereas species that searched for food at shallower angles had narrower binocular fields with a high degree of eye movement below their bills. Eye movement allows these species to move their fovea around to visually search for food in the complex forest environment. Fourth, I studied the visual system configuration of nine species of closely related emberizid sparrows, which appear to maximize binocular vision, even seeing their bill tips, to enhance food detection and handling. Additionally, species with more visual coverage had higher visual acuity, which may compensate for their larger blind spots above their foveae, enhancing predator detection. Overall, the visual configuration of these passive prey foragers is substantially different from previously studied avian groups (e.g., sit-and-wait and tactile foragers). Finally, I studied the visual system configuration and visual exploratory behavior of 29 North American bird species across 14 Families. I found that species with a wider blind spot in the visual field (pecten) tended to move their heads at a higher rate probably to compensate for the lack of visual information. Additionally, species with a more pronounced difference in cell density between the fovea and the retinal periphery tended to have a higher degree of eye movement likely to enhance their ability to move their fovea around to gather high quality information. Overall, the avian visual system seems to have specializations to enhance both foraging and anti-predator behaviors that differ greatly between species probably to adjust to specific environmental conditions.

Degree

Ph.D.

Advisors

Fernandez-Juricic, Purdue University.

Subject Area

Morphology|Ecology|Behavioral psychology

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