Its nor bother with a hover

OUTSIDE the double-glazing, the summer spiders weave a third veil of gossamer, so that the morning after a calm, midgey night finds scores of little black diptera, even more than a spider can eat, suspended as take-away food for nesting birds.

Blue-tits and robins are both adept at hovering for a few seconds, picking off the midges with a click of bill-tip against glass. But far and away the most accomplished hoverer is the little, pale-bellied "willow wren", one of the several sorts of warbler (the chiff-chaff is another) that come to Ireland from Africa to breed.

The first time I saw a willow warbler hovering, to pluck an insect from a leaf, I took it to be a piece of improvisation. But now I see it is a deliberate and regular behaviour. The bird-books, unaccountably, make little of this accomplishment, so the whirring at the window is just something the warbler and I know about, displayed and admired in an exchange of knowing looks through the glass.

Peering into its blur of wings I see that, as with the hummingbirds of the New World, they are beating to and fro like little doors on hinges, their are parallel with the ground and all their lift directed upwards. This is also how the lark, a trembling speck in the sky, holds its singing-station. The hummingbird's tiny wings are much more narrowly shaped to work as a vertical propeller, and it can even fly backwards (out of the flower it is feeding at) by reversing the pitch of its primaries, the finger-like feathers at the tips of the wings.

These furiously energetic hoverings could not be more different from the seemingly effortless wheeling of swallows around the eaves of the house and in and out of the trees. Their long wings and streamlined shapes keep them aloft at the minimum of metabolic cost and they glide to save energy, especially in cool weather when flying insects are scarce.

Knowing a bit about bird flight can add to the pleasure of watching an aerial performance, whether of a duck doing a vertical take off from a lake, or a flock of curlews spilling the wind from their wings to land, feather-light, on the shore.

Most people have absorbed the notion of the ideal wing, as borrowed from birds for aircraft: the curved upper surface and the flattened underside that together create lift when the wing is pushed through the air. In aircraft, it is a propeller or jet which drives the fuselage forward; in birds, the thrust comes from the sweep of the hand section of the wing, with its bunch of flexible, primary feathers.

As they separate and spread on the downstroke, each of the first few primaries twists like the blade of a propeller. On the upstroke, they reverse their twist and snap together again, helping to push the bird forward. While these movements provide the engine power, the main section of the wing is maintaining lift and providing stability.

The spreading of the primaries also helps the bird when it swivels its wings out of the horizontal to steepen its "angle of attack". This manouevre can bring it to the stalling angle, especially during landing and take-off. By spreading the primaries and pushing forward the allula, the little "thumb" of feathers at the joint of the wing, the bird restores a smooth flow of air over its upper surface.

Movements such as these, reproduced ponderously by wing-slots and ailerons, can be glimpsed - sometimes uneasily - from your seat at the window of a Boeing-747. By winding down the flap at the trailing edge of the wing, the pilot can slow his flight, while maintaining lift, without stalling. The bird can do this by spreading and lowering its tail. A swan landing on a lake, or an eagle on its eyrie (enormous wings cupping the air, tail spread and feet thrust forward), shows the same massive precision of aerial control.

At extremes of adaptation, wings are selected for highly specialised purposes. Swifts spend almost their whole lives aloft even sleeping in flight, and their knife-like wings are swept back for speed at the lowest possible toll in energy. Albatrosses, too, on a very different scale, have long, narrow-wings that maintain the smoothest-laminar flow of air and take the most energy (in the form of lift): out of the ocean winds, but that would be quite a handicap in other settings.

In our gardens, or the leafy countryside, compromises have to be made. Here, most birds need explosive acceleration, to avoid predators, but also compact, rounded wings that don't use too much space as they whir past obstructions At the same time, they must have full manoeuvrability in difficult conditions. Next time there's a gale, watch a robin landing on a moving twig. In that last split-second of adjustment and co-ordination you have flight at its ultimate refinement.

In a recent column, I discussed the puzzle set by the observation of a reader in Co Meath. In the long, hot summer of 1995, a willow tree in her garden was "alive with wasps". A dark, burnt-looking patch appeared on the grass under the tree, grew to about 10 inches across, and attracted red admiral butterflies in such numbers that "the patch was covered by shimmering colour".

The black patch was obviously sugar in some form, and in teasing out some possible explanations, I considered honey-dew, the sweet excreta produced by aphids. Wasps have been known to sip at this, but I thought its effect would probably be too dispersed.

However, James O'Shea writes from Killarney: "That same summer, here in Derrynane National Park, I saw the same phenomenon, also on a sally (but a young one), low down and clearly observed. The young branches were covered densely with aphids (dark grey) who were all ejecting honey-dew at quite a rate, into the air. A large number of wasps were feasting on it, catching it in mid-air or sucking it from the trunk of the tree. Beneath, on the grass, a large dark patch had formed (from the honeydew) where many red admirals were feeding, with the occasional peacock or tortoiseshell."

Thus does Eye on Nature advance the frontiers of entomology . . .