Abstract
The breeding passerine bird community of 9 km2 of a primeval subalpine birch forest in Swedish Lapland was censused yearly from 1963 through 1999. The community density remained stable, fluctuating around a mean value of 403 ± 85 territories/km2. Species richness increased by an average of one species every four years. Twenty species made up 98% of the community and Phylloscopus trochilus (40%) and Fringilla montifringilla (19%) remained the dominant species. The species turnover rate between successive seasons was on average 30%. The mean temperatures in June and July were positively correlated to the community density two years and one year later, respectively, less so for the densities of the individual species. Fringilla montifringilla increased in density during the first two years of each Epirrita autumnata caterpillar outbreak, whereas the other species responded only weakly or not at all. All species tended to fluctuate in parallel (no significant opposite trends) which together with the other results suggests that the investigated bird community is far from saturated.
Highlights
Having analysed the classical census works by e.g. Palmgren (1930) and Merikallio (1946) and having observed the appearance of more efficient census methods, Lauri Siivonen (1948) pointed to the possibility of designing investigations to follow shortterm fluctuations in passerine bird populations
This highlighted the need for long-term investigations aimed at discovering long-term trends in bird densities, which would provide sufficient data that might help to identify communitycontrolling factors. The latter expectation has been only partly fulfilled and the interpretations are controversial.After reviewing the field, Wiens (1989a, b) recommended to continue to examine how communities vary in nature by approaching the patterns of variation as something of interest in their own right. From the outset this has constituted the guideline for our project, which has provided a great deal of descriptive information regarding the bird fauna in the subalpine birch forests in Swedish Lapland
The lowest densities amount to less than 50% of the maximum levels
Summary
Having analysed the classical census works by e.g. Palmgren (1930) and Merikallio (1946) and having observed the appearance of more efficient census methods, Lauri Siivonen (1948) pointed to the possibility of designing investigations to follow shortterm fluctuations in passerine bird populations. Thereafter population fluctuation studies soon developed into one of the main streams in the field investigations on birds, some initiatives had been taken place at a much earlier stage (Kendeigh 1982) This highlighted the need for long-term investigations aimed at discovering long-term trends in bird densities, which would provide sufficient data that might help to identify communitycontrolling factors. The latter expectation has been only partly fulfilled and the interpretations are controversial.After reviewing the field, Wiens (1989a, b) recommended to continue to examine how communities vary in nature by approaching the patterns of variation as something of interest in their own right. The density figures for the passerine populations over the first 37 years (1963–1999) are presented and analysed from selected aspects in this report
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