This blog from the British Trust for Ornithology makes interesting reading!
The stop, start nature of this spring’s migration continues. The quiet spells in the weather provide windows of opportunity for migrants heading north through France and Spain, only for them to be closed by the next front to move through. This bank holiday weekend there are some opportunities on Friday but a weather front later in the day and going into Saturday will have a blocking effect on birds heading north across the Channel. Wheatear is trailing well behind its historic average in BirdTrack. Hopefully, they are just held-up and will flood in as soon as conditions allow but, you have to wonder what sort of winter they have had. Read more on the BTO Migration Blog