Sleep consolidation index for all seasons. Hourly average ratio of sleep bout length to wake bout length was calculated for birds during each season. The blue line represents a smoothed curve through these data. The time during which the smoothed value of the ratio of sleep bout length to wake bout length exceeded 1.00 was used to define the "sleep consolidation index" for each season. In winter, hour 10 is first hour in which the ratio exceeds 1 and this ratio falls below 1 at hour 21, resulting in a sleep period of 11 hours. In summer, the sleep period begins at hour 18 and ends at hour 23. It is noteworthy that this sleep period begins prior to the onset of darkness. A similar phenomenon can be observed in the sleep times plotted in Figure 2. There was no sleep consolidation during the migratory seasons, spring and fall.