The analysis of numbers of juveniles trapped in 2 large trap in the postbreeding period (before 10-20 August) on the Courish spit (the Baltic Sea) revealed higher numbers in the 1960s and 1980s than in the 1970s and 1990s (Fig. 1, 2). The increase of juvenile numbers in the 1960s and 1980s was significant in the majority of species (Table 1). The estimate of juvenile numbers in local population correlated well with the mean April and May temperatures in 18 species (Table 2). Trapping in the study area revealed that high temperature in the spring months influenced the increase in juvenile numbers. In 14 species the numbers of juveniles correlated negatively with the timing of post-juvenile dispersal (i.e., timing of breeding): early timing was associated with high numbers (Table 2). Why does the population density increase in years with warm spring and early breeding? The favourable weather conditions in spring that are often associated with high temperatures can limit the mortality of arriving birds. The number of breeding pair in many species is controlled by spring mortality. In a number of species in the study area it has been shown, that the clutch size in early breeders is significantly higher than in late ones. It is possible that in early years the total number of eggs laid by the local birds is higher. There are data suggesting that in cold and rainy years (e.g., 1987 was exceptionally cold and rainy in our area--Fig. 3) embryo and nestling mortality can be substantially higher than in early warm years. There are data suggesting that in cold and rainy years quite a number of juveniles die during 2 weeks after fledging. Indirectly this has been confirmed by data on catches of juveniles ringed as nestlings in the postbreeding period (Fig. 5). Finally, after favourable years more first-year birds appear at the study area, than after a bad years (Fig. 4). It increases the local populations and the number of spring migrants (Fig. 4).
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