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International audience; In western Europe, common pochard populations have experienced a sharp decline over the last two decades, together with an increasing proportion of males. Both of these changes were suggested to result from decreasing survival of nesting females (i.e. survival of adult females) owing to increasing predation pressure. To test this hypothesis, we used capture–mark–recapture/recovery data of common pochard ringed during autumn–winter (October–February) in three countries of western Europe (Switzerland, United Kingdom and France). We found no evidence for decreasing survival of individuals ringed in the United Kingdom or in Switzerland over the long term (1977–2011). In France, adult males and juvenile females experienced significant decreasing survival over a shorter interval (2004–2017). Overall, females displayed lower survival than males, although this was only weakly supported by the French dataset. In contrast, only sex differences and no age differences in survival rates were recorded in the UK and Switzerland (females 0.67 ± 0.03 and 0.69 ± 0.03; males: 0.81 ± 0.01 and 0.75 ± 0.01, respectively), while both age and sex differences were recorded for France (adult females 0.62 ± 0.07, adult males 0.66 ± 0.07, juvenile females 0.49 ± 0.08, juvenile males 0.54 ± 0.08). Therefore, decreasing survival of adult females was unlikely the underlying cause of the decline of common pochard populations in western Europe. Using an age-structured two-sex matrix population model, we show that when adult males experience higher survival than adult females (as it is the case for common pochards), decreasing survival of nests and/or juveniles can trigger decreasing population size and increasing proportions of males at the same time.

We used ringed-recoveries to assess: 1) the connectivity (movements of individuals) between the three putative flyways (north–west Europe, central Europe, south–west Asia) of common pochards in the Western Palearctic, 2) possible spatio-temporal variations in the distribution of the species, and, 3) temporal evolution in spring and autumn migration dates. Based on winter counts of common pochards in the north–west European flyway, we distinguished three periods in the analyses (1960–1990, 1991–2000 and 2001–2016), which correspond to successive periods of population increase, peak and decline, respectively. Whatever the season (wintering or breeding) and period of ringing, large probabilities of ring-recoveries outside the flyway of origin (ringing) were recorded, suggesting a high connectivity between the three putative flyways. There was a significant trend towards an earlier departure from the wintering area, and an eastward shift of spring recoveries over periods. In contrast, neither autumn/winter recovery locations, nor departure dates from the breeding area in Latvia and Russia changed over periods. The latter results do not support the hypothesis that short-stopping (i.e. a reduction of fall migration distances/delaying of departure dates from the breeding area) could explain the observed decline of wintering common pochards in the north–west European flyway. Indirect recoveries support that large proportions of individuals wintering in western Europe may originate from the south–west Asia flyway and more particularly from a region in Siberia located in the Ob river catchment area. Considering trends in numbers for the three flyways together confirmed the “vulnerable” IUCN status of common pochard in the Western Palearctic as a whole, with a 35% decline over the last decade. The important connectivity between the northwest, central European and southwest Asian flyways call for considering such conservation problems at a much broader scale than the regional flyway.

Common pochard (Aythya ferina) is a harvested diving duck that has experienced a decline in population size over the past 20 years in Europe. This decline has been hypothesized to result from reduced breeding success in strongholds of Central Europe. Thus, assessing factors underlying breeding success is a priority. To evaluate determinants of nesting success, we modelled daily survival probabilities of nests on Grand-Lieu Lake, France. Nest survival rates varied with female age and annually (range, 5% for the worst year in yearling females to 74% for the best year in adult females). The effect of year was entirely absorbed by the onset of laying, computed separately by age, with higher survival rates observed in both adults and in years of early onset of nesting. Nest survival was also positively correlated with clutch size and female body condition. Nest survival was related negatively to numbers of neighbouring nests destroyed. Area of open water, wet meadows and sedge tussocks had a positive influence on nest survival; in contrast, that of willows had a negative influence. Spring temperatures on one hand and cumulative rainfall and water levels on the other were positively and negatively correlated to the annual start of nesting, respectively, although these factors were not retained in the best-approximating models for daily survival probabilities. Nest survival seemed to depend most on female quality, including timing of nesting, and to variables affecting predation risk. Management actions targeting these variables could therefore be the best way to improve nesting success in this species.