Page:The British Warblers A History with Problems of Their Lives - 1 of 9.djvu/45

 the same colouring, or whether the variations are sexual or sporadic, further investigation can alone show.

Owing to certain peculiar characteristics of this species, naturalists acquainted with its habits are comparatively few, and consequently our knowledge of its distribution is incomplete. We may, therefore, find that certain features in this distribution, which at the present time appear to us anomalous, may, as our knowledge increases, be partly or possibly wholly explained.

Great Britain is apparently one of the principal breeding grounds, although over the whole area it can only be described as locally distributed. We find it rare in Cornwall, but in the remainder of the southern counties, including Devon, a regular but local visitor. Further north it appears to be more common, and in parts of Oxfordshire. Cambridgeshire, Gloucestershire, Worcestershire, Shropshire, Yorkshire, Cumberland, Durham and Northumberland it is numerous.

The greater part of Wales is unsuited to its habits, consequently it is very local; I have, however, frequently heard it in Anglesey. Proceeding north, over the border, we find it gradually becoming scarce; south of an imaginary line drawn from the mouth of the River Clyde to the Firth of Forth locally distributed, but north of this line rare, and we lose sight of it at Arisaig on the mainland and in the north-west of Skye. In the Orkneys, Shetlands and Western Isles there is no record of its occurrence. Westward we find it occurring in the Isle of Man, and then we come to a large breeding ground in Ireland, where it is generally distributed, but especially numerous in the counties of Antrim, Dublin, Wexford and Waterford.

The task of forming a systematic distribution outside the British Isles is by no means an easy one, and the difficulties face us directly we cross the Channel.