The BTO winter atlas showed that Marsh Tits were present in the majority 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 over 8 birds.
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