1. Light-level geolocator tags use ambient light recordings to estimate the whereabouts of an individual over the time it carried the device. Over the past decade, these tags have emerged as an important tool and have been used extensively for tracking animal migrations, most commonly small birds.

Please refer to our instructions for authors for full details on manuscript preparation. Please note that this template should act as a guide on content, but doesn't necessarily need to be followed for style and formatting. Our typesetters will set your manuscript into house style after acceptance.*Author for correspondence (Susanne.akesson@biol.lu.se). †Present address: Department of Biology, Lund University, Ecology Building, SE-223 62 Lund, Sweden 2 Summary Migratory birds regularly perform impressive long-distance flights, which are timed relative to the anticipated environmental resources at destination areas that can be several thousand kilometres away. Timely migration requires diverse strategies and adaptations that involve an intricate interplay between internal clock mechanisms and environmental conditions across the annual cycle. Here we review what challenges birds face during long migrations to keep track of time as they exploit geographically distant resources that may vary in availability and predictability, and summarise the clock mechanisms that enable them to succeed. We examine the following challenges: departing in time for spring and autumn migration, in anticipation of future environmental conditions; using clocks on the move, for example for orientation, navigation, and stopover; strategies of adhering to, or adjusting the time program while fitting their activities into an annual cycle; and keeping pace with a world of rapidly changing environments. We then elaborate these themes by case studies representing long-distance migrating birds with different annual movement patterns and associated adaptations of their circannual programs. We discuss the current knowledge on how endogenous migration programs interact with external information across the annual cycle, how components of annual cycle programs encode topography and range expansions, and how fitness may be affected when mismatches between timing and environmental conditions occur. Lastly, we outline open questions and propose future research directions. Non-technical summaryMigratory birds perform impressive long-distance flights, timed relative to the availability of resources in different geographical areas. To manage this birds use different strategies including an interplay between internal clock mechanisms and environmental conditions across the annual cycle. Here we review what challenges birds face during long migrations to keep track of time as they exploit geographically distant resources that may vary in availability and predictability, and summarise the clock mechanisms that enable them to meet these challenges. We discuss how range expansions affect components of annual cycle programs, and how mismatches between timing and environment may affect reproductive success.3

Under climate warming, migratory birds should align reproduction dates with advancing plant and arthropod phenology. To arrive on the breeding grounds earlier, migrants may speed up spring migration by curtailing the time spent en route, possibly at the cost of decreased survival rates. Based on a decades-long series of observations along an entire flyway, we show that when refuelling time is limited, variation in food abundance in the spring staging area affects fitness. Bar-tailed godwits migrating from West Africa to the Siberian Arctic reduce refuelling time at their European staging site and thus maintain a close match between breeding and tundra phenology. Annual survival probability decreases with shorter refuelling times, but correlates positively with refuelling rate, which in turn is correlated with food abundance in the staging area. This chain of effects implies that conditions in the temperate zone determine the ability of godwits to cope with climate-related changes in the Arctic.

Summary1. Solar geolocators are relatively cheap and simple tools which are widely used to study the migration of animals, especially birds. The methods to estimate the geographic positions from the light-intensity patterns collected by these loggers, however, are still under development. 2. The accurate reconstruction of the annual schedules and movement patterns of individual animals requires analytical methods which provide estimates of daily locations, distances between the locations and the directions of movement, with measures of their uncertainty. 3. The new R package FLIGHTR meets all these requirements. It enables refined and statistically validated estimations of movement patterns of birds. Here, we present main features of this advanced package.

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.

No abstract

In the original HTML version of this Article, the order of authors within the author list was incorrect. The consortium VRS Castricum was incorrectly listed after Theunis Piersma and should have been listed after Cornelis J. Camphuysen. This error has been corrected in the HTML version of the Article; the PDF version was correct at the time of publication.

The ecological reasons for variation in avian migration, with some populations migrating across thousands of kilometres between breeding and non‐breeding areas with one or few refuelling stops, in contrast to others that stop more often, remain to be pinned down. Red Knots Calidris canutus are a textbook example of a shorebird species that makes long migrations with only a few stops. Recognizing that such behaviours are not necessarily species‐specific but determined by ecological context, we here provide a description of the migrations of a relatively recently described subspecies (piersmai). Based on data from tagging of Red Knots on the terminal non‐breeding grounds in northwest Australia with 4.5‐ and 2.5‐g solar‐powered Platform Terminal Transmitters (PTTs) and 1.0‐g geolocators, we obtained information on 19 route‐records of 17 individuals, resulting in seven complete return migrations. We confirm published evidence that Red Knots of the piersmai subspecies migrate from NW Australia and breed on the New Siberian Islands in the Russian Arctic and that they stage along the coasts of southeastern Asia, especially in the northern Yellow Sea in China. Red Knots arrived on the tundra breeding grounds from 8 June onwards. Southward departures mainly occurred in the last week of July and the first week of August. We documented six non‐stop flights of over c. 5000 km (with a maximum of 6500 km, lasting 6.6 days). Nevertheless, rather than staging at a single location for multiple weeks halfway during migration, piersmai‐knots made several stops of up to a week. This was especially evident during northward migration, when birds often stopped along the way in southeast Asia and ‘hugged’ the coast of China, thus flying an additional 1000–1500 km compared with the shortest possible (great circle route) flights between NW Australia and the Yellow Sea. The birds staged longest in areas in northern China, along the shores of Bohai Bay and upper Liaodong Bay, where the bivalve Potamocorbula laevis, known as a particularly suitable food for Red Knots, was present. The use of multiple food‐rich stopping sites during northward migration by piersmai is atypical among subspecies of Red Knots. Although piersmai apparently has the benefit of multiple suitable stopping areas along the flyway, it is a subspecies in decline and their mortality away from the NW Australian non‐breeding grounds has been elevated.