The BTO winter atlas showed that Mute Swans were present in suitable 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 24 birds seen in a day.
The distribution plot highlights the square that includes Pembroke Mill Ponds where the largest concentration in the county occurred.
Graham Rees