There’ll be bluebirds over the white cliffs of Dover tomorrow, just you wait and see. So runs the first line of a popular world war 2 song. The singer, Dame Vera Lynn, is of course referring our summer visiting swallows, true bluebirds indeed.

However, when watching swallows swooping over the river they appear very dark coloured above especially on dull days.

Poet Matthew Arnold writes "where black-winged swallows haunt the glittering Thames".

But the accompanying photo, taken in bright sunlight, shows how completely transformed is their exquisite plumage radiating a vibrant glossy almost iridescent quality.

The swallow illustrated, a male, (note the diagnostic long tail streamers) perched along the towpath at Kingston, allowed me to approach to within six feet.

This is because the birds are so used to people thronging the towpath and walking above their nest sites under the boardwalk that they have become almost tame and indifferent to the hustle and bustle.

Fledglings remain in the nest for some time before flying as do several species whose nests are built in inaccessible places.