Songs have been written about it as a magnificent spectacle and poets have used the annual event to signify new seasons.
The arrival of migratory birds from southern and west Africa each spring marks an important point in Britain’s wildlife calendar, but it is one that is now challenged by significant changes in the numbers and observed habits, triggering concern for ecologists.
The birds fly more than 8,000km over seas, mountains and desert to take summer sanctuary in meadows, woodlands and wetlands across the UK. But in recent years they have been appearing in lower numbers, often at unusual times, with some even struggling to breed as effectively as their ancestors once did.
Staff at the WWT Wetland Centre in Barnes, south London, are concerned about the plummeting number of birds who turn up each year to set up a temporary home.
Adam Salmon, manager of the 42-hectare site, said he has noticed that numbers have been on a downward spiral since he started working there 25 years ago.
He cited climate change and habitat losses in the UK as the main reasons driving the trend.
“The spring is gradually coming earlier. It’s starting in February when it used to start in March,” he told The National.
“Migratory birds react to that because if the weather is heating up they think it’s time to go [to Europe]. The plants have not evolved as quickly as climate change is happening. This is the problem with climate change: it’s man-made and it’s not at one with nature.”
The alterations in seasonal change means when many birds arrive in February there is not enough food for them because insects are not as plentiful as they are in March. The lack of nourishment for the birds leads to changes in breeding habits. Those that do manage to find a mate and procreate often produce weaker offspring.
“If they breed too early there’s not enough food and their broods are smaller or they don’t become as well developed as they should,” Mr Salmon said. “We see that a lot of the time.”
'Fory per cent reduction makes for a depressing picture'
Tens of thousands of species that head north from western African countries typically spend about half a year in the UK before departing in the autumn before the colder months set in.
These include sand martins, reed warblers and sedge warblers ― all of which are declining in the UK.
The widespread use of pesticides by gardeners and farmers is thought to be harming the habits of such birds.
Hunting in the Mediterranean has also led to turtle doves becoming a rare phenomenon in British skies.
Mr Salmon said sand martins, part of the swallow family, have been arriving in London in obviously smaller flocks.
“The sand martins are sometimes hunted on their passage [through Europe],” he said. “The cuckoo has also declined massively.
"I would estimate that the number of migratory birds [at the wetlands] has reduced by 30 per cent to 40 per cent in the past 25 years.
“We’re not seeing birds dying, but it’s more the timings [that are concerning]. It’s very sad and very worrying. It’s a very depressing picture."
He put the change in seasons and wildlife habits down to global warming, which he said is having an undeniable effect on wildlife across the spectrum.
“I would say in 20 years it will be a lot worse,” he said. “You can see it happening in front of you.
“It’s everything, it’s CO2 emissions. It can still be [reversed] but it’s going to be very difficult.
“A lot of it does start at home. We need to cut down on household emissions.
“Habitat loss has also been going on for a long, long time. The countryside is being chewed up by all the building.”
'There's next to nothing left'
On a crisp winter’s morning at the wetlands in Barnes, the sight of planes flying across the London sky is a reminder of how much pollution the oasis is subjected to.
Otters could be seen scurrying over rocks around a pond in their enclosure as a group of visitors awaited feeding time.
Ducks waddled along pathways and swans glided across ponds hemmed by pampas grasses.
Jacqui Alder, who has been a frequent visitor to the wetlands for the past 25 years, said the lack of birds is noticeable.
“It’s sad,” she said, scanning a landscape of bare trees dotted with empty nests. “I have noticed fewer birds here compared with 10 years ago.
“Also the ducks. We were here a week ago and there’s next to nothing.”
Mrs Alder, from Ealing, west London, fears if something is not done to alleviate the problem, Britain’s countryside and urban wildlife areas could be completely devoid of migratory birds.
“If they’re declining at such a rate, if you wait two to five years there won’t be anything here at all.
“I think that something should be done, I’m not sure what, but obviously if these birds are arriving and the right kind of insects aren’t here for them when they get here what are they going to do? They’re going to die.”
Staff at the wetlands are taking steps to mitigate some of the challenges posed by problems that are out of their hands.
Angelica Teixeira, marketing manager at the site, said what species find upon arrival is akin to “a posh hotel in London with free food”.
But she admitted the fall in the numbers of birds coming each year was concerning.
Callum Moore, reserve warden at the site, cited drought in African nations as a second contributory factor to the lower numbers of birds arriving from the continent.
Shoveler ducks, who winter in southern Europe, have also been on the decline in the past few years.
He said staff are busy preparing secure nesting areas for the birds to ensure they get the best possible start to their stay, even if they arrive earlier than expected.
“We have ways of deterring the crows and the herons from coming in and taking the eggs and chicks, which has various success each year,” he said.
He urged members of the public to get on board with the fight to save Britain’s wildlife and ensure migratory birds are there for future generations by supporting their local wildlife charities.
“It’s all separate and there are separate things that can be done to address biodiversity issues and climate change issues, but eventually they all kind of converge to the same problems.”
In a report published in 2021, the Royal Society for the Protection of Birds found that about 600 million birds had been lost in Europe since 1980 due to a combination of factors. The study called for "transformative action across society to tackle the nature and climate crises" as a way to help populations recover.
It is a dispiriting twist in a once inspirational phenomenon.