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 minutes 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 one second 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/m² 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/m². 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 one second allows them to safely detect and feed their chicks.

The threshold data were collected in a forced 2-choice behaviour experiment. Stimulus strength was varied according to a 1-up/2-down staircase proceedure, and a threshold for each session was calculated by averaging the "turning points" for the 20 last choices. 

Time recordings and calculations were created by a simple script in MATLAB. "Timestamps" were noted as a bird passively triggered a breakbeam motion detector or the experimenter actively pressed a key to indicate a specific behaviour of the bird.

The data files were created in Microsoft Excel but can in addition be opened in LibreOffice calc and similar open source programs.