Population dynamics Sample Clauses

Population dynamics. Data on productivity and survival are scant for both European populations of Long-tailed Duck. No long-term studies have been carried out, with the exception of hunting bag data from Denmark, and few short-term studies have focused on demography. However, information on the ratio of adults to immature birds during the winter comes from a number of sources (see Figure 3) within the Baltic Sea and these data suggest that annual productivity has declined significantly during approximately the 20 years up until the late 2000s. Although Long-tailed Duck productivity has always fluctuated significantly between years, in response to well-established factors such as weather and predator-prey cycles in the Arctic breeding grounds, the peaks in productivity have become less frequent since the early 1990s and thus average productivity has apparently decreased. It is also possible that at some point during this decline the average level of productivity became insufficient to maintain a stable population trend. These data derive from the examination of corpses from harvests, oiling incidents and ▇▇▇▇ net drowning and, since 2008, a more focused effort to estimate the ratio of adult to immature (first-winter) males by photographing large numbers of flocking birds and identifying the age class of each bird on plumage differences. The only continuous dataset of >20 years is the Danish wing survey, which shows a decline in productivity from the mid-1980s to the mid-2000s, and a small increase since then though remaining below average levels of the 1980s. Other temporally shorter datasets (see Figure 3) do not individually demonstrate a decline in productivity, however, they do show similar temporal patterns in the range of annual productivity, with datasets from before the 1990s showing a large range in annual productivity, and those since the early 1990s showing a lower range and overall average. Collectively, data from ▇▇▇▇ net victims in the south Baltic indicate that annual productivity decreased by approximately 75% from 1990 to 2000 (▇. ▇▇▇▇▇▇▇▇▇ in ▇▇▇▇.). Ageing of wintering birds in the field has been undertaken at key wintering areas in the central Baltic Sea and at migration bottlenecks in the Åland archipelago, the Gulf of Finland and in Estonia. Data from Gotland and the Swedish offshore banks indicate that the annual proportion of immature birds averaged 11.4% between 1996 and 2012 (▇. ▇▇▇▇▇▇▇ pers. comm.).