Cold
- Many birds and mammals have a system of cooling the blood flowing to their extremities to reduce heat loss – this is why ducks can stand for hours on ice-covered ponds.
- Mammals develop a thick winter coat. Its effectiveness as an insulator increases with the length of the fur, which serves to hold a layer of inert air close to the skin.
- The further north you go, the longer the fur on animals such as foxes. In fact, the problem faced by foxes is not staying warm but overheating when they are active. When resting, a fox curls its tail over its nose and forefeet, where the fur is shortest.
- Small mammals cannot grow long winter coats, so they spend more time underground. Voles tend to stay under snow, which provides an insulating blanket; wood mice tend to run around on the snow surface.
- Deer stay on slopes where the snow is thinnest, thereby reducing the depth to which they sink when walking and making it easier to forage.
Wind
- Wind poses problems by accelerating heat loss, so most birds and mammals seek shelter during windy conditions.
- Foxes and other carnivores are nervous in the wind, since they cannot rely on their hearing. This means they are reluctant to move away from cover, particularly in strong wind.
Lack of food
- Small mammals and birds have high energy demands that increase in winter to help keep them warm.
- On a cold night, small birds can lose up to 10 percent of their bodyweight trying to keep warm. Since small birds and mammals cannot reduce their food intake, a prolonged spell of bad weather can lead to high levels of mortality.
- Small rodents store seeds and nuts to keep them going through winter.
- Species that forage on earthworms and invertebrates have the greatest problems during cold winters – badgers eat little or no food for about six weeks after Christmas and live off their fat reserves.
Rain
- For many mammals, rain poses a bigger problem than cold, since wet fur loses its heat insulation.
- Harvest mice rapidly lose heat and die in wet conditions. During winter they avoid rain by living at ground level (rather than climbing in the grass stalks as they do in summer) and seek sheltered dry banks and hay ricks to overwinter.
- In the wetter west, brown hares are smaller and breed less successfully than in the east, because they expend more energy on staying dry and warm in the rain.
