Seasonal Time Keeping in a Long-Distance Migrating Shorebird

Because of the complications in achieving the necessary long-termobservations and experiments, the nature and adaptive value of seasonal timekeepingmechanisms in long-lived organisms remain understudied. Here wepresent the results of a 20-year-long study of the repeated seasonal changes inbody mass, plumage state, and primary molt of 45 captive red knots Calidriscanutus islandica, a High Arctic breeding shorebird that spends the nonbreedingseason in temperate coastal areas. Birds kept outdoors and experiencing thenatural photoperiod of the nonbreeding area maintained sequences of life-cyclestages, roughly following the timing in nature. For 6 to 8 years, 14 of these birdswere exposed to unvarying ambient temperature (12 °C) and photoperiodicconditions (12:12 LD). Under these conditions, for at least 5 years theyexpressed free-running circannual cycles of body mass, plumage state, andwing molt. The circannual cycles of the free-running traits gradually becamelonger than 12 months, but at different rates. The prebreeding events (onset andoffset of prealternate molt and the onset of spring body mass increase) occurredat the same time of the year as in the wild population for 1 or several cycles. Asa result, after 4 years in 12:12 LD, the circannual cycles of prealternate plumagestate had drifted less than the cycles of prebasic plumage state and wing molt.As the onset of body mass gain drifted less than the offset, the period of highbody mass became longer under unvarying conditions. We see these differencesbetween the prebreeding and postbreeding life-cycle stages as evidencefor adaptive seasonal time keeping in red knots: the life-cycle stages linked tothe initiation of reproduction rely mostly on endogenous oscillators, whereasthe later stages rather respond to environmental conditions. Postbreedingstages are also prone to carryover effects from the earlier stages