It's well worth checking the Migration Watch website.
http://www.bto.org/migwatch/text/info/what.htm #
It provides detailed data on the causes and volume of immigration - for which Sir Andrew should be applauded. Without statistical analysis like this, how can we have a proper debate?
Its review of 2002 is particularly fascinating.
"Migrants have the amazing ability to know when to leave their wintering grounds in Africa, exactly what triggers them to start migrating north is still poorly understood: internal rhythms, poor conditions in the local habitat and day length may all play a part. Weather plays an important role during migration and affects their timing of arrival in Britain and Ireland. 2002 was rather a strange year; there was the usual trickle of Wheatears, Sand Martins and Swallows in early March and then a southerly airflow coming from Iberia during the last week of March brought some migrants to our shores exceptionally early such as Whinchat and Swift (which typically arrive in mid-April) aswell as some European species �over-shooting� like Hoopoe, Alpine Swift and Black-eared Wheatear. Cool easterly and northeasterly winds coupled with �blocking� depressions over Europe slowed migration down during early April and migrants were generally thin on the ground. By mid-April, high pressure and soaring temperatures across Britain and Ireland and much of Europe allowed many migrants to flood in."