Published version [Abstract:]Reductions in body size are increasingly identified as a response to climate warming. Here we present evidence for a case of such body shrinkage, potentially due to malnutrition in early life. We show that an avian long-distance migrant (red knot Calidris c. canutus), experiencing globally unrivaled warming rates at its high-Arctic breeding grounds, produces smaller offspring with shorter bills during summers with early snowmelt. This has consequences half a world away where short-billed individuals have reduced survival on their tropical wintering grounds. This is associated with these molluscivores eating fewer deeply buried bivalve prey and more shallowly buried seagrass rhizomes. We suggest seasonal migrants experience reduced fitness at one end of their range due to a changing climate at the other end. Published version 3Phenological changes and geographical range shifts represent well-known responses to climate change (1). A third broadly observed response to global warming appears to be shrinkage of bodies (2-5). It has been hypothesized that body shrinkage is a genetic micro-evolutionary response to warming due to smaller individuals being better able to dissipate body heat due to a larger body surface/volume ratio (e.g., Bergmann's rule (2)). Conversely, it has been put forward that climate change may disrupt trophic interactions, potentially leading to malnutrition during an organism's juvenile life stage (6, 7). As poor growth may not be compensated for later in life (8), this would lead to smaller bodies (i.e., shrinkage as a phenotypically plastic response).Under climate change, some regions are warming up faster than others. Especially in the Arctic, warming has been observed at unprecedented rates (9, 10). Hence, body-size reductions would be expected to be most pronounced in the world's most northerly region (6). Many Arctic-breeding avian species, however, are long-distance migrants spending the northern winter at lower latitudes (11), where the impacts of climatic change are less obvious.Here, based on the analysis of satellite data, we show that over the past 33 years, snowmelt has occurred progressively earlier on the high-Arctic breeding grounds of the red knot (Calidris canutus canutus) at Taimyr Peninsula ( Fig. 1; 76-78°N), changing at a rate of about half a day per year ( Fig. 2A; R 2 = .32, F1,31 = 14.77, P < .001; see Table S1 and Figs. S1-S3). During these three decades, 1,990 juvenile red knots were caught and their body sizes measured in Gdańsk Bay, Poland, during their first southward migration to the West-African nonbreeding grounds (Fig. 1). These juvenile birds were smaller after Arctic summers with an early snowmelt, notably with respect to body mass ( Fig. 2B; AIC c = 14775.24, P < .0005; Table S2), bill length ( Fig. 2C; AICc = 7610.48, P < .005; Table S3), and overall body size (PC1 on bill, tarsus, and wing; Table S4; AICc = 5925.22, P < .05). The models best explaining variation in bill length and overall body size additionally included breedin...

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.

Lemmings are a key component of tundra food webs and changes in their dynamics can affect the whole ecosystem. We present a comprehensive overview of lemming monitoring and research activities, and assess recent trends in lemming abundance across the circumpolar Arctic. Since 2000, lemmings have been monitored at 49 sites of which 38 are still active. The sites were not evenly distributed with notably Russia and high Arctic Canada underrepresented. Abundance was monitored at all sites, but methods and levels of precision varied greatly. Other important attributes such as health, genetic diversity, and potential drivers of population change, were often not monitored. There was no evidence that lemming populations were decreasing in general, although a negative trend was detected for low arctic populations sympatric with voles. To keep the pace of arctic change, we recommend maintaining long-term programs while harmonizing methods, improving spatial coverage and integrating an ecosystem perspective.

Seasonal declines in breeding performance are widespread in wild animals, resulting from temporal changes in environmental conditions or from individual variation. Seasonal declines might drive selection for early breeding, with implications for other stages of the annual cycle. Alternatively, selection on the phenology of nonbreeding stages could constrain timing of the breeding season and lead to seasonal changes in reproductive performance. We studied 25 taxa of migratory shorebirds (including five subspecies) at 16 arctic sites in Russia, Alaska, and Canada. We investigated seasonal changes in four reproductive traits, and developed a novel Bayesian risk-partitioning model of daily nest survival to examine seasonal trends in two causes of nest failure. We found strong seasonal declines in reproductive traits for a subset of species. The probability of laying a full four-egg clutch declined by 8-78% in 12 of 25 taxa tested, daily nest survival rates declined by 1-12% in eight of 22 taxa, incubation duration declined by 2.0-2.5% in two of seven taxa, and mean egg volume declined by 5% in one of 15 taxa. Temporal changes were not fully explained by individual variation. Across all species, the proportion of failed nests that were depredated declined over

Tundra-breeding birds face diverse conservation challenges, from accelerated rates of Arctic climate change to threats associated with highly migratory life histories. Here we summarise the status and trends of Arctic terrestrial birds (88 species, 228 subspecies or distinct flyway populations) across guilds/regions, derived from published sources, raw data or, in rare cases, expert opinion. We report long-term trends in vital rates (survival, reproduction) for the handful of species and regions for which these are available. Over half of all circumpolar Arctic wader taxa are declining (51% of 91 taxa with known trends) and almost half of all waterfowl are increasing (49% of 61 taxa); these opposing trends have fostered a shift in community composition in some locations. Declines were least prevalent in the African-Eurasian Flyway (29%), but similarly prevalent in the remaining three global flyways (44-54%). Widespread, and in some cases accelerating, declines underscore the urgent conservation needs faced by many Arctic terrestrial bird species.