by Pajarita » Thu Aug 17, 2017 10:20 am
You are absolutely right that birds migrate for food and not warmth but that doesn't change the fact that all animals evolve to 'match' their environment so nature already took into consideration that the bird migrating for food will be travelling South and will not be exposed to cold weather so it did not give it 'protection' against it. This means that, although they do not migrate to escape the cold, if they don't, they die.
IRNs make it in London because it doesn't really have super cold weather, the coldest months are January and February and the lowest temperature is between 36 and 39 degrees which usually only happen for a couple of hours or so during the night - islands always have milder weather than continents.