Fast visual adaptation to dim light in a cavity-nesting bird

Abstract

Many birds move fast into dark nest cavities forcing the visual system to adapt to low light intensities. Their visual system takes between 15 and 60 min for complete dark adaptation, but little is known about the visual performance of birds during the first seconds in low light intensities. In a forced two-choice behavioural experiment we studied how well budgerigars can discriminate stimuli of different luminance directly after entering a darker environment. The birds made their choices within about 1 s and did not wait to adapt their visual system to the low light intensities. When moving from a bright facility into an environment with 0.5 log unit lower illuminance, the budgerigars detected targets with a luminance of 0.825 cd m −2 on a black background. When moving into an environment with 1.7 or 3.5 log units lower illuminance, they detected targets with luminances between 0.106 and 0.136 cd m −2 . In tests with two simultaneously displayed targets, the birds discriminated similar luminance differences between the targets (Weber fraction of 0.41–0.54) in all light levels. Our results support the notion that partial adaptation of bird eyes to the lower illumination occurring within 1 s allows them to safely detect and feed their chicks.

Decision rules for egg-color-based rejection by two cavity-nesting hosts of the brown-headed cowbird

“…Unlike open nests, cavity nests may not always provide enough light to stimulate the four single cone photoreceptors that are necessary to mediate color vision in birds (Olsson et al, 2015;Toomey et al, 2016), which could affect a host's ability to distinguish colors in a cavity nest. Nonetheless, there are some examples of cavity-nesting birds that respond to dramatic differences in eggshell colors in the context of foreign egg rejection (Grim et al, 2011;Hauber et al, 2014;Zhang et al, 2021) and birds can adapt their visual systems from bright to dim illumination within 1 s (Chaib et al, 2023). However, there are fewer examples of how hosts of avian brood parasites discriminate between fine-grained variation in eggshell coloration within dark cavity nests, where poorer light conditions could make egg color discrimination more challenging (Endler, 1993;Honza et al, 2014).…”