The BTO winter atlas showed that Chiffchaffs were present in 41% of 10km squares during the winters of 1981-82, 1982-82 and 1983-84.
The darker the colour, the higher the relative total count for each 10km square. The darkest blue represents 3 or more birds seen in a day.
However, it should be noted that the figures show a high correlation with the number of recording cards returned (therefore related to recording effort) for each 10km square.
Graham Rees