How do Spring Migrants Cope?

We are experiencing very variable weather at the moment. Things are changing in the range of three seasons in one day. Yet for the last week, I have been regaled on my early morning walks with the constant reminder of the arrival of a chiffchaff. Singing its repetitive notes from high in the tree tops, this bird appears to be ready to settle in the wood and establish a territory. But how does such a small seemingly delicate warbler survive in these conditions? Presumably it is accustomed to meeting with very cold night time temperatures on the African continent during its winter sojourn there, but there you would expect it to be able to find ample food sources during daylight hours. Here it is sometimes meeting winter conditions during the day and you would imagine that there are few insects around. Certainly they appear should the sun emerge. Bumblebees have been out and about for some weeks now and clouds of midges appear with the sun but on some days, my feeble eyes are unable to see any minute life. Clearly the chiffchaff has its strategies to survive, but it is for me one of the surprises of nature.