Phylloscopus trochilus trochilus
Common summer visitor, and abundant passage migrant, arrives last week in March.
He also mentions, under the section of the book on Land-Birds:
The first Willow-Warblers are trapped at Skokholm during the last week of March. This is one of the largest small bird migrations in Pembrokeshire, and for the whole month between mid-April and mid-May there are days when the county swarms with these charming singers.