Your question struck a chord with me as I am half way through reading "Essential Ornithology" by Graham Scott, an excellent book that explains bird biology, ecology and behaviour in a more "layman" way than some "scientific" type books.
Unfortunately I can't give you a concise answer to the question you pose but the below extract from the "Movement: Migration and Navigation" chapter may be of interest;
"It is possible to recognise two distinct categories of bird movements. The first includes those movements that are concerned with a proximate response to an actual resource shortage: foraging trips, commuting, or ranging between patches and dispersal from a natal area to an available local area to establish a home range. These movements conclude when the need for the resource involved is satisfied . The second class of movements are true migrations: triggered by internal rhythms or by a forecast of resource shortage. They are characterised by the physiological suppression of the proximate response to resource need and their conclusion is a result of physiological changes resulting from the movement itself"
For the first category my immediate thought would be Waxwings when there is a food shortage in Scandinavia moving south over to GB. For the second Redstart would be an example.
The chapter goes on to explain the work that has been done on studying Blackcaps (P Berthold, V Querner 1981), which exhibit a range of migratory behaviours. The study concludes that the behaviour of an individual is on a genetic basis. There appears to be quite a large amount of research done in this area (and for quite some time). From the evidence of ringing recoveries Blackcaps over-wintering in GB are mainly from the population of birds that breed in Holland, Belgium and Germany (and the continuous nature of this supports the belief that the migratory direction is genetically passed down).
I think the answer to your questions lies somewhere between "resources" and "genetics" !
There is a 20 page chapter on migration ! The book is well worth a read if you get the chance.
