Spring is arriving early with swallows, frogspawn and unexpected perfume as temperatures soar up to 20C above this time last year when Britain was blasted by the “beast from the east”.

Rooks are nesting, ladybirds are mating and dozens of migratory swallows have been spotted along the south-west coast – more than a month ahead of their normal arrival.

The 121-year-old record February temperature in Scotland was broken on Thursday with a new high of 18.3C recorded at Aboyne in the Highlands. British springs appear to be leaping ahead of climate change: Met Office data shows mean spring temperatures have risen from 7.1C between 1961 and 1990 to 8.1C between 2008 and 2017.

“We’re in a synoptic sweet spot,” said Grahame Madge of the Met Office. “We’ve got this dome of high pressure sat across continental Europe which is giving us settled conditions. The high pressure is shielding us from the worst of the low pressure but encouraging a flow of warm air from the low pressures.”

Daytime maximums of -3C were commonplace in the Midlands and Severn Valley during last year’s “beast from the east”, while temperatures this weekend are expected to reach 18C.

A plume of warm southerly air from the Canary Islands is sweeping migratory birds back to Britain unusually early. Swallows have been sighted in Cornwall, Devon, Dorset, Hampshire and Wales, and a house martin has also been recorded.

The first swallow of the year to be seen at the bird observatory at Portland, on the Dorset coast, was photographed on Thursday. The average arrival date for swallows in the UK is 29 March while house martins are not normally seen until 8 April.

The air is also unusually perfumed this spring, according to Guy Barter, chief horticulturalist at the Royal Horticultural Society. Winter-flowering non-native shrubs, such as daphnes, shrubby honeysuckles and witch hazels, thrived after unusually warm sunny weather last year, growing bigger flowers this spring, which cast forth more scent.

“Winter-flowering plants need lots of scent to attract the relatively few pollinators that are around in winter,” said Barter. “Winter scent is very agreeable and walking around our gardens in Harlow Carr, Yorkshire, Hyde Hall, Essex, and Wisley in Surrey, people are saying, ‘Good heavens, there’s an absolute cloud of scent.’”

The Met Office expects the fine spring weather to continue for another week but there is a risk that more traditional March temperatures and weather – unsettled, wet and windy – will return after that.

Naturalists fear for these early spring species if March does turn markedly colder and wetter.

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Tony Whitehead of the Royal Society for the Protection of Birds said the early-arriving swallows were taking a big gamble to obtain the best nesting sites.

“The ones that return soonest are going to get the best nesting territories but it’s a bit of a gamble that the birds are taking,” he said. “If it stays like this there are quite a few insects around so they’ll be able to eke out a living. If it turns cold like last year, they will need to find food every day simply to survive the night. If it does turn, the outlook isn’t brilliant for these birds.”

Spring 2019 actually started in November and December 2018 according to the Woodland Trust, which compiles Nature’s Calendar, a phenological dataset of reports of spring flowering and other events across the country.

The first flowering snowdrops were spotted in Southampton on 30 November, a month earlier than expected. Early daffodils were widely reported flowering in the south-east in late December.

Early spring records submitted to Nature’s Calendar include ladybirds mating on Valentine’s Day, with widespread reports of bumblebees, brimstone butterflies and frog spawn, as blackbirds, robins and dunnocks began to build nests.

Judith Garforth, citizen science officer for the Woodland Trust, said: “Over this last week the sunshine has come out and we’ve had lots of records – insects such as queen wasps and brimstone butterflies coming out of hibernation because the sun has woken them up. Hopefully they’ve haven’t woken up too early.”

The naturalist Matthew Oates said spring had returned to normal after a cold end to January but was now leaping ahead again.

“Celandines are only just flowering, which is about right; the rooks kicked off nest-building on time, around St Valentine’s Day, as they always do,” he said. “Pinky-white flowering garden cherries are just starting, sallows are silvering and alders are flowering, just about on time. And those who suffer hay fever from the tree pollen are suffering.”

Oates is fearful for early-emerging species.

“It may be that February and March have swapped places. That happens with months – they do swap places in spring,” he said. “The track record for early springs is that they invariably end in tears – you get a month like this and then the jet stream jumps south and then there is six to eight weeks of wet weather and that writes off the birds that were nesting and spring insects and everything that depends upon them.”