The screaming comes first. Then they wheel into view.
The tiny birds with wings like a crescent moon and a voice that belies their size scythe through leaden skies above the Victorian Gothic tower of Oxford’s Museum of Natural History.
Our ancestors called them the devil’s birds, in part because their screeching sounded like the screams of the departed. Their nightly pirouette higher-and-higher until they disappeared into the sky was labelled the ‘vesper flight’ and the swifts thought to be the souls of the dead ascending to heaven.
Some of these swifts circling the museum on an overcast Monday morning could have – and most likely will have – flown the city’s skies before. Once, a swift used the same nest box in the museum’s roof for a decade before disappearing into the ether.
George Candelin has monitored the museum’s swift colony for more than two-and-a-half decades, beginning in the mid-90s when then Keeper of Swifts Roy Overall enlisted his help to ‘ring’ the birds – attaching a small ring to their leg which, like a passport, linked them to Oxford. After a couple of years away he returned about a dozen years ago, this time as Keeper.
Why do you keep coming back, I ask, as the birds yell and whistle overhead.
“I think it’s just the swifts,” he grins.
“There’s a fascination to them. Once you get into them, you don’t want to stop.”
There is no sign of stopping.
Started in 1947 by David Lack, the director of Oxford University’s field ornithology institute, and his soon-to-be bride Elizabeth, this is said to be one of the oldest continuously-running studies of a single bird species in the world.
Every summer in the three-quarters of a century since then the colony has been monitored, with researchers noting down every week how many nest boxes are being used, the number of eggs laid and chicks fledged. In 1956, Lack wrote a famous book, Swifts in a Tower, about the study.
“From the first Monday in May through to when the swifts go away, every week I come up here on a Monday morning at 10 o’clock, go round all of the 147 nests and take a note of progress,” Mr Candelin says.
“Through the season I monitor adults, young, eggs, whatever else is happening.”
The process is painstaking. Swifts scare easily and it can take weeks for them to settle. Mr Candelin has to be careful to avoid disturbing the birds as he carries out his weekly checks only aided by a torch with a red light - although he says he often forgoes the torch and uses whatever residual natural light there is in the museum’s loft.
“It can be very noisy. When you think about late June, early July, all the youngsters are clamouring to be fed and all the adults are screaming around bringing in food.”
Unlike the rest of the city, there is no shortage of housing for swifts in the museum. Of the 147 nest boxes, around 40 will be occupied.
Mr Candelin says: “They’re site faithful. Once they have a box the male will consider that territory his and they’re back every year.
“Anything they use [to build] the nest has been captured on the wing. Bits of grass, tissue paper from the May Day, rose petals, and feathers are a great favourite, because woodpigeon and blackbirds shed feathers everywhere.”
The advantage of studying the same colony for so long is the ability to note long-term trends. Like the Great Tits monitored at Wytham Woods, west of Oxford, the birds seem to be nesting a few weeks earlier in the year.
Numbers are lower than they were. “If we go back to the 2000s, we’d have had 90 of the boxes occupied. Probably 100 plus chicks. The minimum we’d have would be about 80. Now, on a good year I can get 70 something.”
The population crashed around 2010. There were events held in front of the museum which disturbed them, in 2010 a sparrowhawk snatched many of the adult birds and two years later – a year when there were just 14 chicks - the city suffered its wettest June on record.
This year the birds arrived late. The first swift arrived on May 12. Storms in Spain and cool temperatures in the UK earlier this month were blamed.
They will be gone some time in August. The parents are the first to go, leaving behind their chicks for several days before the youngsters fledge.
Once airborne, the chicks will spend months airborne without once touching the ground – even sleeping in the air.
The preparation for their 4,000 mile journey to their wintering grounds in sub-Saharan Africa begin at about a month-old, when they start doing ‘press-ups’ in the nest.
“When the wing is long enough and has enough wing area for their weight, they come out the hole and that’s the last time they’ll see the box, because they’re off,” Mr Candelin says.
Only one chick in a hundred returns to the Museum of Natural History as an adult. Relatively few colonies are monitored as closely as the Oxford one.
The researchers know from rings found around the ankles of the birds that one of the swifts was discovered near Aberdeen.
Another Oxford swift was found in Cambridge. “We don’t allow that now,” Mr Candelin jokes.
Find out more about the Museum tower swifts on the Museum of Natural History's website: oumnh.ox.ac.uk/learn-swifts-diary.
This story was written by Tom Seaward. He joined the team in 2021 as Oxfordshire's court and crime reporter.
To get in touch with him email: Tom.Seaward@newsquest.co.uk
Follow him on Twitter: @t_seaward