Gelter, H.P., Tegelstrom, H. & Gustafsson, L. (1992) Evidence from Hatching Success & DNA Fingerprinting for the Fertility of Hybrid Pied x Collared Flycatchers Ficedula hypoleuca x albicollis. Ibis, 134(1), pp62-68
Essentially there is a set of conditions specific to the islands of southern Scandinavia where it is profitable for female Collared Flys to mate with male Pied Flys in order to gain a healthy mate that will raise offspring. Offspring that could well be the result of pure albicollis parents due to extra pair copulation. There is a chance of hybrid offspring which is more desirable for a female than be spending a season unmated as male offspring are fertile and often successful in passing on their genes. Female offspring are sterile and their reduced fitness often leads to death in the first winter.
There is an interesting flycatcher at Spurn at the moment. Wing length too long for Pied but rump pattern not conclusive for Collared. Mild, K. (1994) Field identification of Pied, Collared and Semi-collared Flycatchers – Part 3: 1st winters and non-breeding adults. Birding World, 7(8) 325-334 mentions that 1st winter Ficedula flycatchers are all diagnosable on rump pattern. This bird isn't. Why? I am no expert on black and white flycatchers so I wont be dissecting the id from pics but my inclination is to believe at present this is a 1st w hybrid albicollis x hypoleuca. As a tick hungry tart I would usually bury my head in the sand but this is my reading of the situation.
As for other birds recently I managed a seawatch on sat which produced 2 Sooty Shearwaters, 2 Arctic Skuas, 1 Great Crested Grebe, 2 Whimbrel and a few other bits and pieces. I met Jim and we went on the quiet boat out of brid for a princely single Arctic Skua, nectarine and Purple Sandpiper combo. Dipped the local crane twice but I will see it.