Ingrained city rhythms: flexible activity timing but more persistent circadian pace in urban birds.

Urbanization dramatically increases the amount of light at night, which may disrupt avian circadian organization. We measured activity patterns of great tits breeding in the city and forest, and subsequently measured two clock properties of these birds under controlled conditions: tau (endogenous circadian clock speed) and after-effects (history dependency of the clock relative to previous conditions). City and forest birds showed a high repeatability of activity onset (0.60 and 0.41, respectively), with no difference between habitats after controlling for date effects. Activity duration and offset showed more variance, without a difference between birds from the two habitats. Tau did not differ between city and forest birds, however, city birds showed stronger after-effects, taking more days to revert to their endogenous circadian period. Finally, onset of activity was correlated with clocks speed in both habitats. Our results suggest that potential differences in activity timing of city birds is not caused by different clock speeds, but by a direct response to light. Persistence in after-effects suggests a reduced sensitivity of the clock to light at night. Urbanization may select for clock properties that increase the inertia of the endogenous circadian system to improve accuracy of activity rhythms when exposed to noisier lighting cues.