according to rspb its:
The first birds begin to sing about an hour before sunrise. If you listen carefully, you may notice that there is a regular sequence, with some species habitually starting before others. Among the earliest to rise are skylarks, song thrushes, robins and blackbirds, and as they do eat worms there may be some truth to the old saying!
A more relaxed approach is taken by wrens and warblers, that typically appear later. These smaller birds, who are perhaps more sensitive to the coldness of dawn, feed on insects that themselves appear later in the morning.