FOR years, I’ve watched swallows flying over water and dipping the ends of their beaks into the surface. I assumed they were getting a drink on the wing.
House martins and sand martins do it, too. All three species are related and the speed and precision with which they fly puts them in the top ranks of avian aviators. These hirundines (their scientific name), are supposed to feed exclusively on flying insects they catch on the wing. I’ve been watching them more closely this summer, and I think ‘exclusively’ is incorrect.
I don’t think they are drinking when they dip their lower beaks into water, I think they’re feeding. I first suspected this from the frequency with which they do it. This was not consistent with slaking a thirst. I got out the binoculars to have a closer look and went out in a boat to examine the water surface. I found large numbers of chironomids floating on the surface. Chironomids are non-biting midges and their life cycle involves a larval stage that is spent under water; before they pupate, rise to the surface, and hatch into winged insects.
At this point, critical changes take place. They have to pump blood through their wings before they can fly, and have to dry off because there is liquid inside the case. This can happen in seconds or it can take minutes. During this time, they float on the surface film getting everything organised.
The swallows and martins I was watching were exploiting this vulnerable transition period in the insects’ life cycle and stocking up on high-protein midges.
Once I’d established that the birds feed on floating insects, as well as flying ones, I started to notice other odd behaviour. One warm evening, I was sitting on my patio when I noticed a group of a dozen young swallows dive-bombing the crown of a tall ash tree. I couldn’t get up to examine the top of the tree, but I watched the birds through binoculars. They were approaching the tree at high speed and, with incredible skill, picking something off the leaves, then jinking away and gaining altitude for another dive from 20 metres above the tree top.
I’d love to know what they were taking. It could have been some form of fly. Flies will land on leaves near the top of a tree in warm weather, because the leaves transpire moisture that the insects can drink. But it could equally have been caterpillars or spiders.
My next observation was while sitting on the deck of my boat, on a canal, a couple of weeks ago. I was 50 yards from a lovely, 18th century limestone canal bridge and the arch of the bridge was suddenly filled with swallows. There were 30 of them, and how they managed to fly in such a confined space without colliding or crashing into the stone work was little short of miraculous. But they did and they kept it up for half an hour, before I disturbed them by walking up to have a closer look. I can’t be certain, but I think they were picking off spiders. There were a lot of spiders, both web-spinners and hunting spiders, in the rough masonry under the bridge. This is not a great ornithological breakthrough and somebody’s probably observed it before, but I got satisfaction from learning something new about these little birds.
* dick.warner@examiner.ie
a d v e r t i s e m e n t
This appeared in the printed version of the Irish Examiner Monday, August 04, 2008
