Interacting effects of canopy geometry and display activity timing shape conditions for visual signalling in Victoria’s riflebird

Thomas MacGillavry , Amy Carboon, John A. Endler

Lighting conditions in forests are highly variable in both space and time and depend on the interacting effects of canopy geometry, weather, and the time of day. This has strong implications for visual signalling, because different species sharing the same forest habitat may display in dramatically different microhabitats favouring disparate signal phenotypes. However, relatively little is known about the lighting conditions occurring during visual communication in most animal species. Using hemispherical canopy photography and behavioural data, we investigated lighting conditions of the courtship signal irradiance microhabitat in male Victoria’s riflebirds (Ptiloris victoriae) — a bird-of-paradise (Aves: Paradisaeidae) endemic to sub-tropical Australia. Despite no detectable differences in crude rainforest canopy features between male display perches and nearby microhabitat control sites, display sites always featured a small gap directly overhead. We then show that males typically display to females in the early morning, especially within the first two hours after civil dawn. Integrating canopy features and display times, we then simulated under-canopy solar irradiance at exact display times and show that males generally display under dim lighting conditions in the absence of direct sunlight. Thus, rather than utilising sun flecks to enhance plumage brightness and colour contrast, male riflebirds prefer to display in dim, diffuse light. These results contrast with findings in some other species in which courting males utilise direct sunlight to enhance visual display efficacy. Instead, some species may be specialised for visual signalling in dim light.