Strange at times, the thoughts that occupy one's head. Yesterday's talk in this column of swallows set me thinking about Tennyson's epic poem The Prin- cess, which, believe it or not, has a pair of cross-dressing adventurers caught up in a campaign to further women's rights.
The swallows appear in Canto IV. Most of this passage appears to be a long conversation between one of the heroes and the Princess Ida of the title, a lady who insists on singing songs. At one point, however, she insists that the narrator must sing one too, at which suggestion he is taken aback. But:
"Then I remember'd one myself had made,
That time I watched the swallow winging south."
And so he sings:
O Swallow, Swallow, flying, flying south
Fly to her, and fall upon her eaves
And tell her, tell her, what I tell to thee . . . and continues in this vein for half a dozen verses.
Now when the swallows head south, as they tend to do around this time of year, we are deprived of what some regard as very reliable predictors of the weather. "Swallows high, staying dry; swallows low, wet 'twill blow," the saying goes. But insofar as there is any truth in this, we can certainly conclude that the behaviour of the swallows is dictated by the conditions pertaining at the time, rather than any insight into future weather.
Swallows feed while on the wing, darting, gliding, swooping as they capture succulent morsels from the insect population flying in the air. Naturally, they ply their trade in that part of the sky that seems to them most bountiful at any given time.
It is therefore to the distribution of flying 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 important question. The first is that the insects on which the swallows prey feel happiest at a barometric pressure of about 1010 hectopascals, and when the pressure at ground level is higher than this value - which often happen when the weather is set fair - the insects, so the theory goes, congregate at a higher altitude than usual.
A more plausible explanation is that on warm summer days thermal activity can carry bubbles of air to heights of many thousands of feet above the ground, and that 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.