Swifts and Weather Movements
Here is a very brief resume as I see it relative to what we are seeing now.
As Swifts dont normally breed until they are several years old there is thought to be a large transient population of immature birds and I would suggest it is some of these we are seeing now as "moving" birds. They say the immature birds are the last to arrive at these latitudes.
As they are aerial feeders they are very sensitive to micro weather changes that might affect food availability and therefore they tend to track the weather best for feeding. Studies by David Lack a good number of years ago proved that to do this they skirt round approaching depressions before the arrive, always into the wind to reach better weather sooner. During my studies I have nearly always found this to hold good and nearly always most intense as the season wears on.
Sometimes later in the season they say breeding Swifts even leave the colonies to do this as bad weather approaches to follow the food, returning later as conditions improve.
Check this out to see what you find and let me know!