Autumn irruptions of Eurasian Jay (Garrulus glandarius) in Norway in relation to acorn production and weather.

In Scandinavia, the Eurasian Jay (Garrulus glandarius) is a resident forest-dwelling species, which in some years show eruptive movements in the autumn. Such irruptions seem to be related to the variation in seed crops of oaks (Quercus spp.). When there has been a high seed crop, termed mast, Jays use hoarded acorns not only during winter, but also as food for their young in the succeeding spring. However, whether this leads to an irruption in a succeeding year may also depend on other factors that affect survival and reproduction. A Jay hunting index (1995-2015) and a Jay irruption index (observations at a coastal ornithological station 1976-2015) from southern Norway peaked in years with mast failure, but there also was a significant negative relationship with mean snow depth in January-February and precipitation in April-May. Much snow is assumed to reduce winter survival, by making hoarded acorns less available, whereas heavy rain in spring is known to have a negative effect on passerine breeding success in general.