SUCH is the ever-quickening decline in bird populations that their migration is now "a shadow of what it once was".
So writes American birder Scott Weidensaul in his important new book, A World on The Wing - The Global Odyssey of Migratory Birds.
But he quickly adds: "That shadow is still mighty enough to leave us slack-jawed and awestruck."
"Awestruck"? The same could definitely be said of Weidensaul's book. It is an endlessly captivating study of both what we know - or think we know - about avian passage and how, thanks to technology and the dedication of birders like him, we are steadily making crucial new discoveries.
Among the fascinating sections is one on a New World species, the Kirtland warbler, which migrates from a small corner of Michigan where it breeds to a couple of islands in the Bahamas where it spends winter.
Attaching tiny transmitters, weighing a fraction of a gram, to certain individuals, it has proved possible both to track their passage and to identify what is their most precious food.
To his credit, Weidensaul joined researchers in the field, even at the cost of enduring fierce sunshine and of being bitten to pieces by mosquitoes and other insects.
The information has provided invaluable in stimulating habitat-creation projects which have brought this delightful warbler back from the brink extinction.
Moving to the Old World, the author is very strong in his assessment of how drought in the savannahs of the Sahel, on the southern fringes of the Sahara, have created an insect apocalypse to the detriment of cuckoos, bee-eaters, shrikes and warblers that winter there before migrating to Europe.
If they are not getting enough to eat, the chances of these calorie-starved birds completing the long journey ahead is greatly reduced - as are their chances of breeding if they ever get to arrive.
Weidensaul is also red-hot on the migration of shorebirds such as semipalmated sandpiper, whimbrel and bar-tailed godwit.
And he makes a point of contributing his own expression of concern about how "agricultural intensification has crowded out Nature in favour of chemically-soaked crop monocultures".
Says he: "The evidence is mounting that the very foundations of the planet's ecosystems are crumbling."
He also highlights the problems of persecution - songbird slaughter in the Mediterranean and shorebird hunting in the Caribbean islands where, in Barbados some 34,000 waders - about 19,000 of them lesser yellowlegs - are shot every year.
But there are also positives. For instance, the author highlights the promise of creating "pop-up wetlands" whereby farmers can be induced, out-of-season, to create temporary stop-off habitat for migrating waterfowl.
It should be said that, though rich in research detail, A World on The Wing, is not stuffy or overly academic. The narrative is pacey, vibrant and never less than entertaining.
There is something quaintly touching about how Weidensaul describes his peck-every-three-seconds observations of a palm warbler, speculating what might happen if, through lack of food, this rate increased to a peck every four seconds.
"Sounds like a minor difference, but that's a 25 per cent decrease in what the warbler can consume over the course of a full day - an enormous deficit."
And that deficit in calories and energy, he deduces, could have serious consequences.
"On such razor-thin energetic margins does success or failure hinge for a migratory bird."
* A World on The Wing is published in the UK by Picador at £20 and available wherever books are sold.
