The effect of constant light and phase shifts on a learned time-place association in garden warblers (Sylvia borin): hourglass or circadian clock?

Garden warblers are able to learn an association between time of day and feeding place. In constant dim light and constant food availability, the learned feeding pattern (successive visits to four feeding rooms for approximately 3 hr each) persisted for at least 1 day in three birds and for at least 6 days in one bird. The free-running feeding rhythm had a period of slightly greater than 23 hr. In response to a 6-hr phase advance of the light-dark cycle, the birds advanced their learned feeding pattern by 2.6 hr on the first day, whereas a 6-hr phase delay had no significant effect. These results are consistent with the hypothesis that the birds use a circadian clock rather than an hourglass mechanism of timing. This conclusion is further supported by the response of birds to forced interruptions of the daily feeding pattern (Krebs and Biebach, 1989).