Patterns of spring arrival dates differ in two hirundines

We examined the first arrival dates in the UK over a 56 yr period of 2 hirundines, sand martin Riparia riparia and barn swallow Hirundo rustica, in relation to temperatures along migration routes and at destination. Changes in arrival dates have been much greater for sand martin than for barn swallow, and the arrival order of the 2 species now appears to have reversed. There appear to be 2 reasons for this: a greater response to temperatures in SW Europe and a different response to temperature in recent years, leading to earlier arrival at the same temperature than formerly. We argue that this may involve evolutionary change, but conclude that it is difficult to be definite about this without information on individual animals.