Rob Lensink & Robert Kwak (2024) Irruptions of Jay Garrulus glandarius in the Netherlands in 1975-2021: size, timing and origin. LIMOSA 97 (3): 119-128.

The Jay is one of the irruptive species in Europe: in some years high numbers migrate through the Netherlands and in other years the species is absent at migration watchpoints. In this paper we shed light on four aspects of irruptions of Jays in the Netherlands: size, timing, scale and origin. We first use data on visible bird migration sampled on two sites in the Netherlands, Winterswijk (1975-2021) and Arnhem (1981-2003), both in the eastern part of the country. Each autumn between 30 and 120 early morning counts (2.5 hours) were conducted (total 2440) on the first site, on the second site around 100 each autumn (total 2300). We also used data on the number of migrants in southern Germany (Bavaria; Gatter 2000). Furthermore, we used data on the mast of acorns in the Netherlands (1950- 2021), Bavaria (Germany, 1960-2003) and Poland (1950- 1999). The number of migrants on the three sites varied enormously. For the Dutch sites, a negative relationship with the number of acorns in autumn was found, as well as for the German site with the Bavarian data. Massive eruptions in 1965, 1972, 1977, 1983, 1996 and probably 2019 and 2021 coincided with lack of acorns in larger parts of West- and Central-Europe. Most large eruptions started in the first half of September with the maximum halfway September whereas smaller eruptions reach their maximum one or two weeks later. Jays were seen flying in different directions, especially about SW, NNW and NE. The first two directions could be linked to nearby breeding areas and/or an Scandinavian/Eastern- and CentralEuropean origin. A NE flight direction indicates return migration. Shortage of acorns seem to be the primary factor in the irruptions of Jays. Some irruptions (1965, 1972, 1996) seem supported by massive mast in the preceding year (1964, 1971, 1995) resulting in large Jay populations. Irruptions appear to have a primarily regional origin, but if acorns are lacking over larger geographical areas movements over longer distances can occur.

[pdf only for members] [dutch summary]