Luminance-dependence of spatial vision in budgerigars (Melopsittacus undulatus) and Bourke’s parrots (Neopsephotus bourkii)
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Budgerigars (Melopsittacus undulatus) and Bourke’s parrots (Neopsephotus bourkii) are closely related birds with different activity patterns. Budgerigars are strictly diurnal while Bourke’s parrots are active in dim twilight. Earlier studies show that the intensity threshold of colour vision is similar in both species while Bourke’s parrots have larger eyes with a higher density of rods than budgerigars. In this study, we investigate whether this could be an adaptation for better spatial vision in dim light. We used two alternative forced-choice experiments to determine the spatial acuity of both species at light intensities ranging from 0.08 to 73 cd/m2. We also determined the spatial contrast sensitivity function (CSF) for bright light in Bourke’s parrots and compare it to existing data for budgerigars. The spatial acuity of Bourke’s parrots was found to be similar to that of budgerigars at all light levels. Also the CSF of Bourke’s parrots is similar to that of budgerigars with a sensitivity peak located between 2.1 and 2.6 cycles/degree. Our findings do not support the hypothesis that Bourke’s parrots have superior spatial acuity in dim light compared to budgerigars and the adaptive value of the relatively rod-rich and large eyes of Bourke’s parrots remains unclear.
KeywordsSpatial vision Luminance-dependence Spatial contrast sensitivity Bird Dim light
We would like to thank Stefan Sydoff and Per Vestergren for help with the experimental setup, and the Lund Vision Group for helpful discussions. We also thank the anonymous reviewers for helpful comments on the manuscript. Financial support from the Royal Physiographic Society in Lund, the Royal Swedish Academy of Sciences, and the Swedish Research Council, Stockholm (grants number 2006-4510, 2009-5683), is gratefully acknowledged. The animals were kept in conditions that followed the ethical guidelines from the Swedish Board of Agriculture and the same department approved the experiments (M18-07, M190-10).
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