Figure 1.

Variation in subjective onset, end and length of day in one rural and two urban sites. We used activity data recorded on rural male European blackbirds in combination with data from light loggers to determine the average light intensity at the time when rural birds started their activity in the morning or ceased it in the evening. We then used these two values as thresholds to calculate the time at which light intensity passed these thresholds in the morning and evening, and defined those times as onset (a) or end (b) of that day. The ‘subjective daylength’ was calculated as the difference between the onset and end of day (c). We standardized the measurements to the natural variation in daylength, the horizontal line at time 0 (a, sunrise; b, sunset; c, total daylength hours). Each dot represents the individual mean time of onset or end of day (a and b, respectively), and the individual subjective daylength (c). Error bars are s.e.m. Sample sizes: rural forest n = 9, urban parks n = 11, business district n = 12.