Surely in this case Pete, it's survival of the fattest.

Adult Swallows and House Martins put their own survival before that of their young.If food starts to run low because of cold weather then Hirundines are known to abandon nearly fully fledged birds and head off south whilst there is enough aerial food to sustain themselves on migration.Survival of the fittest.

i think a few end up not going as far south as they used to and end up wintering in france,and a few in southern england,not too sure but i think ive read a few winter records somewere

Reading todays posts, i see the Swallows at Martin Mere are still feeding young. What are the chances of them making a sucessful migration? I wouldn't have thought the youngsters had much chance but how will the adults fair? having stayed around to bring up the brood, which could be a second or even third brood, will they be too worn out to fly to Africa? It's interesting what's going on here. Are the adults risking their own survival to bring up another brood, which may not survive. Or do the adults realise that they are not going to make it back to Africa so are sacrifising themselves to bring up another brood in the hope that some make it? The instinct to migrate must come into conflict with instinct to feed the young. In this case it seems feeding the young takes priority. I wonder if some broods are abandoned when the urge to migrate kicks in? Dean.