Figure 2.

Differences in rhythmicity index and circadian period length between forest and city blackbirds. Forest (n = 14) and city (n = 14) birds were held in constant dim light of 0.3 lux and their activity was continuously recorded for at least a week (10 ± 2 days). From these recordings, we estimated (a) robustness of the rhythmicity and (b) period length τ. Since five city birds showed a statistically non-significant rhythmicity index, they were excluded from the analysis of period length. For box plot specification see figure 1.