title: Seasonal Time Keeping in a Long-Distance Migrating Shorebird.
abstract: Because of the complications in achieving the necessary long-term observations and experiments, the nature and adaptive value of seasonal time-keeping mechanisms in long-lived organisms remain understudied. Here we present the results of a 20-year-long study of the repeated seasonal changes in body mass, plumage state, and primary molt of 45 captive red knots Calidris canutus islandica, a High Arctic breeding shorebird that spends the nonbreeding season in temperate coastal areas. Birds kept outdoors and experiencing the natural photoperiod of the nonbreeding area maintained sequences of life-cycle stages, roughly following the timing in nature. For 6 to 8 years, 14 of these birds were exposed to unvarying ambient temperature (12 °C) and photoperiodic conditions (12:12 LD). Under these conditions, for at least 5 years they expressed free-running circannual cycles of body mass, plumage state, and wing molt. The circannual cycles of the free-running traits gradually became longer than 12 months, but at different rates. The prebreeding events (onset and offset of prealternate molt and the onset of spring body mass increase) occurred at the same time of the year as in the wild population for 1 or several cycles. As a result, after 4 years in 12:12 LD, the circannual cycles of prealternate plumage state 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 high body mass became longer under unvarying conditions. We see these differences between the prebreeding and postbreeding life-cycle stages as evidence for adaptive seasonal time keeping in red knots: the life-cycle stages linked to the initiation of reproduction rely mostly on endogenous oscillators, whereas the later stages rather respond to environmental conditions. Postbreeding stages are also prone to carryover effects from the earlier stages.
authors.affiliation: NIOZ Royal Netherlands Institute for Sea Research, Department of Coastal Systems and Utrecht University, Den Burg, Texel, the Netherlands jkaragicheva@nioz.nl.
authors.forename: Julia
authors.initials: J
authors.lastname: Karagicheva
authors.affiliation: NIOZ Royal Netherlands Institute for Sea Research, Department of Coastal Systems and Utrecht University, Den Burg, Texel, the Netherlands Department of Vertebrate Zoology, Biological Faculty, Lomonosov Moscow State University, Moscow, Russia.
authors.forename: Eldar
authors.initials: E
authors.lastname: Rakhimberdiev
authors.affiliation: NIOZ Royal Netherlands Institute for Sea Research, Department of Coastal Systems and Utrecht University, Den Burg, Texel, the Netherlands.
authors.forename: Anne
authors.initials: A
authors.lastname: Dekinga
authors.affiliation: NIOZ Royal Netherlands Institute for Sea Research, Department of Coastal Systems and Utrecht University, Den Burg, Texel, the Netherlands.
authors.forename: Maarten
authors.initials: M
authors.lastname: Brugge
authors.affiliation: NIOZ Royal Netherlands Institute for Sea Research, Department of Coastal Systems and Utrecht University, Den Burg, Texel, the Netherlands.
authors.forename: Anita
authors.initials: A
authors.lastname: Koolhaas
authors.affiliation: NIOZ Royal Netherlands Institute for Sea Research, Department of Coastal Systems and Utrecht University, Den Burg, Texel, the Netherlands.
authors.forename: Job
authors.initials: J
authors.lastname: Ten Horn
authors.affiliation: NIOZ Royal Netherlands Institute for Sea Research, Department of Coastal Systems and Utrecht University, Den Burg, Texel, the Netherlands Conservation Ecology Group, Groningen Institute for Evolutionary Life Sciences (GELIFES), University of Groningen, Groningen, the Netherlands.
authors.forename: Theunis
authors.initials: T
authors.lastname: Piersma
mesh_terms: D025041:Animal Migration; D000818:Animals; D001683:Biological Clocks; D001717:Birds / Q000502:physiology*; D001835:Body Weight; D001947:Breeding; D002940:Circadian Rhythm; D004777:Environment; D008018:Life Cycle Stages / Q000502:physiology*; D017440:Photoperiod; D012098:Reproduction; D012621:Seasons; D013696:Temperature
keywords: Calidris canutus; R code for continuous wavelet transformation; annual cycle; circannual rhythm; dissociation of life-history stages; life-cycle stage; mother wavelet; plasticity; seasonal environmental change