Nature Notes

August is the time for Lammas growth, when trees defoliated by larval insects sprout new leaves.  New adult insects will be laying eggs for a new generation, swifts that have eaten their fill of insects, and raised a family of one to three young, will fly back to Africa.  Swallows and house martins (hirundines), however, may raise a second brood before they return.  Swifts are long lived, and does not need to raise multiple broods yearly to be sure of replacing themselves.  Hirundines are quite short lived, and need to take every opportunity to breed.

All the above mentioned birds are insectivores, eating large numbers of insects in flight.  Surprising numbers of spiders also fly and are eaten by these birds.  However, the spiders do not have wings, but fly using gossamer threads as a kind of paraglider.  So why are all these mini-beasts up in the sky?  There are several reasons; many, like aphids and flies, are simply dispersing to new areas.  Others are on their nuptial flights, such as the many ants bees and wasps; these are within a group known as Hymenoptera (membrane wings).  The birds trawl the sky collecting vast numbers.  During cool or windy weather, they fly lower, especially over water collecting insects emerging from their aquatic juvenile forms such as mayflies, stoneflies and caddis flies.