The geometry of visual signalling in a bird-of-paradise

Yseult Héjja-Brichard , Thomas MacGillavry, Leonida Fusani

Effective visual signalling in animals requires an appropriate viewing geometry, which depends on both the visual system of signal receivers and the spatial context in which the signal is viewed. For example, visual acuity determines the distance at which fine-scale patterns can be resolved, while the visual field defines the space around the audience open to influence from visual signals, particularly when viewing displays at close range and from a predictable location and direction. During courtship, male Victoria’s riflebirds (Ptiloris victoriae) perform two distinct displays: the circular-wings pose at a distance and the alternate wing-clap display at close range. Using avian vision modelling, we show that the circular-wings pose enhances detectability by producing a highly circular, optical “black hole” against the rainforest background when viewed from afar. When females approach the display perch, males switch to alternating movements of the wings at a short distance from the female’s eyes, intermittently occluding ~60% of her visual field and thereby generating a dynamic visual background that likely accentuates flashing colour patches. Thus, black feather morphing displays are highly detectable at long range and shift to function primarily as a visual background at close range. Our findings illustrate how different components of a courtship phenotype can be repurposed across viewing contexts to form an integrated signalling system in which signal efficacy is modulated by the spatial context in which the display is viewed.