The topsy-turvy weather of 2011 had a roller-coaster effect on British wildlife, it has now become clear. The freezing winter, the hot spring, the cool summer and the near-record warm autumn of the year just past produced a mixture of striking but very different effects on birds, mammals, insects and plants, first devastating their populations, then allowing them to rebuild at record rates, and in cases such as butterflies, letting them linger on far longer than normal.
Small birds in particular suffered badly from the icy start of 2011, the coldest winter for 20 years, as the smaller you are, the more quickly you lose heat and energy, and food sources such as worms are impossible to access in frozen ground.
According to records from the British Trust for Ornithology, populations of dunnocks or house sparrows, song thrushes and reed buntings fell to their lowest-ever recorded levels in the great freeze, song thrushes dropping in number by 28 per cent nationally compared to the average of the last five years, reed buntings by 23 per cent and dunnocks by 21 per cent.
Other small species experienced drops which, while not records, were even greater, with tiny wrens plunging by a massive 31 per cent and robins by 30 per cent, while greenfinches and chaffinches also suffered big drops, of 24 and 15 per cent respectively.
Yet they made up for it. For when spring came, unseasonably warm unseasonably early – it was the hottest Easter holiday on record – many of these species, the Trust’s records show, were able to build their numbers back up rapidly, and in some cases had breeding seasons which in terms of productivity were also records.
Dunnocks and chaffinches had their best breeding seasons ever, while robins and wrens had their best seasons since 1987, song thrushes their best since 1985, and greenfinches since 1990.
Some larger birds, especially birds of prey, also did very well out of the spring, with kestrels, barn owls and tawny owls all producing broods of chicks which were larger than normal.
This is thought to be because the small mammals such as field voles on which all three feed were particularly plentiful last spring, perhaps because through parts of the cold winter they had been protected by a blanket of snow lying on the ground. “Then the warm, dry spring weather provided perfect hunting conditions,” the BTO said.
Kestrels in particular, which have been falling in numbers, had a very good 2011, showing a 15 per cent increase above recent averages in the numbers of fledglings produced per nest.
But birds were not the only group experiencing unusual fortunes from the untoward meteorological mix. Butterflies and moths were “bamboozled” by the weather, according to the charity Butterfly Conservation, with many species appearing much earlier and later than in a typical year.
The hot, dry spring combined with the second warmest autumn on record – 1 October was the hottest-ever October day in Britain – saw butterflies on the wing from early March to mid-December, the charity said. Threatened species such as the pearl-bordered fritillary and grizzled skipper benefited from extended flight periods by emerging weeks ahead of their normal dates, as spring temperatures soared.
The endangered black hairstreak typically emerges in June. but was seen in May – the earliest emergence on record – while the Lulworth skipper, which is restricted to southern Dorset, was also on the wing seven weeks earlier than normal.
Several single-brooded species were recorded much later than normal – a rare marsh fritillary being seen in mid-September, almost eight weeks after the butterfly should have disappeared for the year.
The hot autumn also saw a huge influx of migrant moths from southern Europe with exotic species such as the crimson speckled and vestal moths recorded into October. The UK recorded the largest number of rare flame brocades for 130 years, with a colony discovered at a secret location in Sussex, while the spectacular humming-bird hawk moth is thought to have enjoyed its best ever year in the UK with more than 9,000 records sent to Butterfly Conservation, beating the previous 2006 high of 6,500.
“The weather is a matter of life and death for butterflies and moths, and 2011 has been a year of extremes,” said Butterfly Conservation’s surveys manager Richard Fox.
“It’s too soon to tell exactly how the UK’s butterflies and moths have fared but the signs are that spring species, including many threatened butterflies, benefited from the hot weather in April and May. In contrast, most summer-flying species struggled to survive in the cold and damp. Autumn brought a reprieve for our beleaguered butterflies and moths, with many native species able to extend their flight periods or squeeze in an extra brood, as well as the arrival of marvellous migrant moths from overseas.”
As the year ended, the warm weather of Christmas week was actually tricking flowers into flowering early, with the bright yellow lesser celandine, the spring’s first flower which is usually seen in March, already out in parts of Devon.
Marsh fritillary (Euphydryas aurinia)
This charming butterfly is normally on the wing from May to the end of the first week in July, but this year one was spotted in September – a virtually unheard-of occurrence.
Lesser celandine (Ranunculus ficaria)
This yellow star is normally the first spring flower (snowdrops being winter flowers) and normally appears in early March, but parts of December were so mild that it was flowering in the west country.
Wren (Troglodytes troglodytes)
Among the smallest and most vulnerable songbirds, wrens saw their populations plunge by nearly a third in the cold winter, but they built them back up in the warm spring.
Field vole (Microtus agrestis)
Field voles seemed to be particularly abundant in the spring, perhaps because the winter snow kept them hidden from predators such as barn owls and tawny owls.
Hummingbird hawkmoth (Macroglossum stellatarum)
More of these spectacular insects are being seen in Britain, but the warm autumn brought in a record influx from the continent, with more than 9,000 being recorded.
Kestrel (Falco tinnunculus)
Kestrels have been declining in recent years and giving concern to conservationists, but they had a very good breeding season in 2011, thanks to an abundance of prey.