Food supplies are limited, water sources are more scarce and freezing temperatures are difficult for many birds to weather.
Previous spells of bad weather in the UK have had devastating effects on bird species. Following the 1962-63 ‘Big Freeze’, large declines were seen in wrens, pied wagtails, mistle thrush and song thrush. Even in years of less inclement weather, but still with a lot of frost such as 1978-79, the mortality of pied wagtails and wrens was higher than normal.
Small bodied birds such as wrens, goldcrests and long tailed tits can be particularly badly hit by extreme cold and frosty weather. Birds below 10.5g have showed significant reductions in numbers following a harsh winter, as they do not have such high levels of fat reserves and struggle to keep warm.
To survive the harsh weather, songbird species have been known to put on weight to prepare for the cold winters, with many species reaching a peak during December and declining afterwards.
Wren: During severe winter weather, the wren suffers high levels of mortality as is evident by recovery following the cold winter of 1978. Survival rates for wrens are much lower the more snow days that they undergo during the winter months.
Dartford Warbler: They are very site loyal and have been observed to stay in territories even when it is very snowy and other birds leave to find shelter. This same site fidelity has also been witnessed when the moors where they were resident caught fire and they still did not leave. They prey on invertebrates which become inaccessible in the soil when snow covers the ground. When their numbers bounce back following a harsh winter, they can settle in sub-optimal territories, where they suffer more during future winter conditions.