Visual constraints upon avian behaviour

The aim of my PhD research was to investigate how avian behaviours may be constrained by eye design and visual capabilities. As great diversity exists across bird species in the design of the visual system, I utilised this variation to examine and explain the links between vision and behaviour. Several methods were employed in order to gain ocular information from species: including measuring eye size, retinal image brightness and retinal topographic analysis. I applied and developed these methods in studies ranging from a microscopic scale analysis of retinal structure, to studies comparing whole eye size and morphology, to gain an insight into how the timing of different behaviours may be constrained by visual capability, and how artificially modified visual constraints can lead to behavioural plasticity. Specifically, I compared the visual ability of extant diurnally and nocturnally active avian species with that of Archaeopteryx, finding that this early bird was likely to have been diurnal in the timing of its activity. Following on from this, I found that eye size, controlling for body mass, is an important predictor in the timing of onset of dawn song for species around the world. This relationship is stronger in circumstances when twilight length is longer. This study also revealed how species richness may affect the timing of behaviours, suggesting that tropical birds are able to partition their dawn choruses to reduce masking of acoustic signals. When the presence of artificial lighting modifies the visual constraints at twilight and night, I found that European robins Erithacus rubecula correspondingly modified their singing, foraging and mass regulation behaviour. Overall, my research highlights the great diversity of avian visual adaptations, and emphasises that understanding the differences between them is intrinsic to appreciating the way in which behaviour is constrained by visual ability