Today, after the storm, a large flock of needle-tailed swifts zoomed about overhead. Needletails are famous as the original thunderstorm chasers. And as typical swifts they fly huge distances and sleep on the wing, not landing except to nest.
Why are they chasing summer storms?
Thunderstorms generally build up from high temperature applied to a background of high humidity. Warm air rises and as it rises it condenses to form clouds. But the warm air also carries flying insects, many of which are too flimsy to escape from the updraught. This is the reason the swifts are darting around – they are feeding on flying insects caught by thermals. And it's also why the strandline on the beach may be covered in insects when a hot north-westerly is blowing – they’ve been blown out to sea.
Thinking about this raises two interesting issues. I've talked about colourful insects being poisonous and therefore avoided by visual predators. But if you are flying at 120km an hour, which needletails do, you aren't likely to be too fussy about breakfast – at that speed the insect will be in your stomach before you’ve even thought about it.
Sometimes that might not be too bad – aphids are nice and juicy – but sometimes it might be something poisonous, like a small ladybird. Long ago, in Ireland, my school had nesting house martins (a bird that feeds like a swift) and they made conspicuous messes on window ledges under their nests. One day I scraped some of the bird poo up and examined it under a microscope – and was surprised to find it full of bits of colourful ladybirds. So beggars can't be choosers – if you feed on flying insects, you take what you get. The corollary is that these birds must have tough stomachs, or iron guts, as we say about a family who never get food poisoning.
Another issue is dispersal. There was once an enormous battle between biologists over the origins of species and indeed the origins of whole faunas. The prevailing wisdom was that everything evolved in and dispersed from centres of origin, and amazingly Europe just happened to be the main centre of origin. So the first mammals, platypus and echidnas, walked all the way from Europe to Australia, then newer, more ‘efficient’ mammals evolved in Europe, which replaced the boring old mammals but couldn’t reach Australia because the tide was in, and so on.
The discovery of continental drift blew that up and soon another school of thought developed – species originate through the splitting of populations (combined with genetic drift).
One of the leading dispersalists was obsessed with the dispersal of insects to remote Pacific Islands. So to prove his point he flew at 7000 metres above the open ocean towing a net – and found the same sorts of insects that our swifts were no doubt feeding on.
As is typical of biology, this was not an ‘either/or’ problem, and scientists are now polite to each other on this subject.
