The Bohemian Waxwing is an irruptive species whose appearances are governed by the berry supply in the north.
Irruption years: when the northern berry crop fails, large numbers push south and can then be seen throughout the country.
When: the winter flocks arrive in October–November and move back north again in March–April.
Unpredictable: some winters the species is very numerous, others it is almost absent.
Origin: many of the waxwings that reach Sweden in irruption winters come from the east, from Russian and Finnish coniferous forests, and the movement is driven by food rather than by day length as in true migrants.