About 230 cuckoos sing in Sussex but the number declines. When you see the problems these harbingers of spring have in Timbuktu and the surrounding desert landscapes which they have to cross from the Congo to the English Channel, you wonder that any get here at all.
One bird with a radio tag was found to have travelled at 62mph across the Sahara this spring so these canny old cuckoos know about thermals and tail winds.
Birders assemble in droves in Gibraltar in autumn and wait for that moment when all the European raptors, from honey buzzards to ospreys, take the airlift off the Iberian plains and cross the Straits in one vast thermal of ascent.
Glider pilots were taught the basic principles by condors and even greater black-backed gulls. My brother John, who had in his day circled with both these birds among scores of others, was even able when on board an RAF Shackleton escorting the Queen Mother on a trip out of Africa, to advise the pilot on uplift to clear the Atlas Mountains.
John pointed out Egyptian vultures climbing rapidly ahead; the Shackleton joined the thermal and quickly added 5,000 feet to its struggling performance.
The fact that several cuckoos refuelled in the South of France this spring shows how difficult that Sahel/Saharan route has now become for them. They had lost every gram of body fat and were running on empty, even burning muscle to keep airborne.
With the Sahel again overgrazed by goats as it was in the 1960s/70s during previous droughts, we can presumably expect another crash in migrant numbers, notably whitethroat and lesser whitethroat warblers, also nightingales and grasshopper warblers.
It took years for these species to return in the numbers we have again enjoyed in recent years. Quite apart from air travel, cuckoos and warblers have to put up with a slightly less friendly UK landscape they nowadays find.
Two million deer in the UK have stripped the shrub layer in woods leaving less habitat, especially brambles, which for cuckoos means less host species such as dunnocks (hedge sparrows).
To an extent they can compensate by using reed warblers in reedbeds courtesy of the RSPB nature reserves.
The huge number of badgers and foxes effectively hoover up many ground-nesting species too, thus the decline in tree pipits (as well as curlews, lapwings) while the successful spread and back-from-the-brink return of peregrines, sparrowhawks and buzzards just has to be part of the equation loosely called the balance of nature.
Gain some, lose some: heigh-ho!