Fig. 2. Nest site light levels at dawn and dusk, in human photometric light units. The upper plots show light levels as a function of time relative to the sun at the horizon. The black trace is the mean, dotted lines show 95% confidence intervals, and gray lines show several sample traces. The lower plots show the magnitude of the 95% confidence intervals. Light changes most when the sun is just below the horizon, by about an order of magnitude in just 10 min. The uncertainty in light intensity is around an order of magnitude for any time around this window.