"One swallow," as Aristotle remarked so very wisely all those years ago, "does not a summer make." But two or three or more are as good a sign that the year is on the turn as daffodils or cuckoos.
And it is also said that they have a modicum of skill as short-range weather forecasters.
"Swallows high, staying dry; swallows low, wet 'twill blow," the saying goes, or as a traditional attempt at rhyme expresses it:
When swallows fleet soar high and sport the air,
You may be sure the weather will be clear.
Insofar as there is any truth in these beliefs, it is certain that the behaviour of swallows is dictated by the conditions pertaining at the time, rather than an indication of any insight into future weather.
These hunters of the air feed while on the wing, darting, gliding, swooping as they as capture succulent morsels from the insect population flying in the air.
Naturally, they ply their trade in that part of the sky which seems to them most bountiful at any given time. It is, therefore, to the distribution of insects in the atmosphere that we must look for an explanation of any swallowly behaviour - the birds simply go where food is most abundant.
There are two main theories on this question. The first is that the insects on which the swallows prey feel happiest at a barometric pressure of about 1010 hectopascals. When the pressure at ground level is higher than this value - which often happens when the weather is set fair - the insects, so the theory goes, congregate at a higher altitude than usual.
To find a palatable pressure in which to buzz around, they must ascend about 250 feet for every 10 hectopascals that the barometric pressure rises.
Now Weather Eye is ill informed on insects and cannot comment on the barometric pressure at which such creatures might be expected to be comfortable. Those who disagree with this pressure theory, however, point out that on a fine, summer day the swallows can be seen soaring as high as several thousand feet, where the pressure might be expected to be around 900 to 950 hectopascals and, therefore, by this reckoning, entomologically unpleasant.
A more plausible explanation, it is argued, is that on warm, summer days thermal activity can carry bubbles of air to heights of many thousands of feet and these rising currents sweep the insects high into the sky. Consequently, on fine days, the swallows have to fly progressively higher to maximise their catch of insects.