The visual control of pecking by pigeons (Columba livia) has latterly been thought to be restricted to the fixation stops interrupting their downward head movements because these stops prevent interference by motion blur. Pigeons were also assumed to close their eyes during the final head thrust of the peck. Here we re-examine their pecking motions using high-speed video recordings and supplementary provisions that permitted a three-dimensional spatial analysis of the movement, including measurements of their pupil diameters and eyelid slit width. The results confirm that pigeons do not close their eyes completely during the presumed optically ballistic phase of pecking. Instead their eyelids are narrowed to a slit. The width of this slit is sensitive to both the ambient illumination levels and the visual backgrounds against which seed targets have to be detected and grasped. There is also evidence of some interaction between pupil diameter and eyelid slit width. We surmise that besides being an eye-protecting reflex, the partial covering of the pupil with the eyelids may increase the depth of focus, enabling pigeons to obtain sharp retinal images of peck target items at very close range and during the beak-gape ‘handling’ of food items and occasional grit particles.
Eyelid squinting during food-pecking in pigeons
J.O. and J.D.D. were members of the former “Experimentelle Psychologie” unit at Universität Konstanz at the time the experiments were carried out. The investigations described in this paper were begun in 2000, and a first draft of the paper was ready by 2005. Because of intervening adverse circumstances, the re-editing that led to the present paper only began in 2019.
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Joachim Ostheim, Julia A. M. Delius, Juan D. Delius; Eyelid squinting during food-pecking in pigeons. J Exp Biol 2020; jeb.223313. doi: https://doi.org/10.1242/jeb.223313
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