GASTEREN H VAN, MOSTERT K, GROOT H & RUITEN L VAN (1992) The irruption of the Coal Tit Parus ater in the autumn of 1989 in The Netherlands and NW-Europe. LIMOSA 65 (2): 57-66.

A huge Coal Tit invasion occurred in Europe in the autumn of 1989. In The Netherlands, visible bird migration was recorded at twelve observation sites (four of which situated along the North Sea coast) during I 409 hours from September to December, 1989 (tab. I, fig. I). Comparable information from observation and ringing sites elsewhere in Europe was gathered. Notably large numbers of Coal Tits were recorded in The Netherlands in the third decade of September (dozens of birds per site per hour, all over the country). Two waves of birds passed in the course of October. The first occurred during the first week of that month (300 birds/ h along the coast, 35 birds/h at inland sites), the second peaked on October 17 (1 100 birds/h along the coast, 50 birds/h at inland sites, fig. 2, a and b). The first wave consisted mainly of Coal Tits, but during the second wave they were accompanied by Blue and Great Tits. The two waves are considered to have consisted mainly of adult (first) and juvenile (second) birds. The first flocks were observed about half an hour before sunrise, and migration usually peaked between 07hOO and 08hOO (fig. 5). Mean flock size correlated positively with mean number of birds observed per hour, except for the last two hours of the day when relatively large flocks were observed. It appears that two separate Coal Tit-irruptions affected Europe in autumn 1989: one originated from the taiga zone, the other from northern Germany plus western Poland, and the two differed with respect to timing. Examination of Coal Tits caught in The Netherlands at the time (wing-length, back colour, moult of greater coverts) showed that these birds originated from northern Germany and Poland (Wijker 1991). This irruption coincided with one of Goldcrests and Firecrests, which may have come from the same geographical area. The Coal Tit irruption to western Europe reached France, the northern Alps and Austria. Mean travelling speed of these birds is calculated to have been about 30 km per day. It is estimated that c. 725 000 birds passed through The Netherlands, and c. 2.3 million through western Europe. The other irruption, originating from the taiga zone, reached southern Finland, the Baltic republics and Poland. In this irruption birds travelled approximately 40 km per day. It is not possible to estimate the number of birds that participated in this irruption, since quantitative data are lacking. Generally, food shortage and unusually' high population density (following a mild winter, a high breeding population density and/ or a successful breeding season) are considered the causes of irruptions. Food shortage probably affects taiga zone Coal Tit populations especially. This may result in irruptions occurring relatively early in autumn, and characterised by a high overall percentage of adult birds (order 20-50%), and two separate migration peaks (adults first, juveniles later). The 1989 Coal Tit irruption in western Europe, however, was characterised by a relatively late timing, and (overall) a very small fraction of adult birds (3%). These features are considered characteristic of an irruption resulting from a high population density. Indeed, breeding population density (Netherlands, FRG, Switzerland, Estonia) and breeding success (Netherlands) are known to have been high in 1989, and, furthermore, conifer seed production was at least normal (FRG) that year. It is therefore concluded that the 1989 Coal Tit irruption in western Europe resulted from unusually high population densities (northern Germany, Poland) that year. Some earlier Coal Tit irruptions in western Europe are known to have been characterised by either low (1959, 1972) or high (1967, 1969) percentages of adult birds (Scherrer 1972, Winkler 1974). We consider these irruptions to have been caused by high population densities, and by food shortage (probably in the taiga zone), respectively. Coal Tit Periparus ater

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