Sand Martins and Swallows feed on airborne insects, which in the falling temperatures become harder to find, and whilst these early arrivals can survive two or three days of cold, inclement weather, by day four they are in serious trouble. Bad weather at this time of the year is normally associated with strong winds, which will prevent these pioneer migrants from just turning around and heading back south in search of better conditions. Unfortunately, during prolonged spells of poor weather many of these early birds will perish.
According to BTO BirdFacts, the average arrival date in the UK for the Sand Martin is 25 March, by which time the weather might generally be more settled and favourable.
So, these early birds are taking a huge gamble, if the weather is kind to them then the gamble will pay off and they will be back at their breeding sites securing the nest hole ahead of those birds arriving later, however, when the weather is bad they pay a high price indeed.
The strong winds forecast for the next few days will almost certainly put migration on hold, however, with lighter winds mid-week we might see the first Wheatears arrive.