Population dynamics of passerine birds

The analysis of numbers of juveniles trapped in 2 large trap in the postbre eding period (before 10-20 August) on the Courish spit (the Baltic Sea) rev ealed higher numbers in the 1960s and 1980s than in the 1970s and 1990s (Fi g. 1, 2). The increase of juvenile numbers in the 1960s and 1980s was signi ficant in the majority of species (Table 1). The estimate of juvenile numbe rs in local population correlated well with the mean April and May temperat ures in 18 species (Table 2). Trapping in the study area revealed that high temperature in the spring months influenced the increase in juvenile numbe rs. In 14 species the numbers of juveniles correlated negatively with the t iming of post-juvenile dispersal (i.e., timing of breeding): cal ly timing was associated with high numbers (Table 2). Why does the population density increase in years with warm spring and early breeding? The favourable weat her conditions in spring that are often associated with high temperatures c an Limit the mortality of arriving birds. The number of breeding pair in ma ny 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 si gnificantly higher than in late ones. It is possible that in early years th e total number of eggs laid by the,local birds is higher. There are data su ggesting that in cold and rainy years (e.g., 1987 was exceptionally cold an d rainy in our area-Fig. 3) embryo and nestling mortality can be substantia lly 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 fledg ing. Indirectly this has been confirmed by data on catches of juveniles rin ged as nestlings in the postbreeding period (Fig. 5). Finally, after favour able 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 sprin g migrants: (Fig. 4).