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How do birds survive very cold winter nights?

Sent in by Mrs Meredith, Perthshire

Birds, as with all other warm blooded animals, have temperature-regulating mechanisms to keep body temperatures at suitable constant levels.

In any instance of deteriorating light and temperature a bird will adjust behaviourally by selecting a roosting place, and then fluffing up the feathers, placing the head under the scapular feathers and sometimes huddling with other individuals.

To shelter from the harsh weather, some birds creep into the space between loose bark and tree trunks, using both natural and artificial cavities. Other species excavate their own roosting cavity. Sparrows, for example, use thick vegetation, vines next to houses, or available roof spaces. The recent perceptions of intolerance to birds roosting in roof spaces will certainly be a factor which has contributed to recent declines in overall numbers of species such as house sparrows and starlings.

Temperature variation will place a certain demand on the bird because of the body heat being lost. If the demand is low, it may be balanced by the bird’s lowest metabolism rate. When the demand is greater, physiological responses cause regulating mechanisms to act. This causes metabolism to rise – energy originally taken in as food is turned into heat at a greater rate, to meet the greater demand, and body temperature is maintained. If the demand (heat loss) is extreme, body temperature may be permitted to fall, reducing the rate of heat loss. Even so there is then the still the risk that a birds energy reserves will be used up resulting in total lack of temperature regulation which inevitably leads to death during the night.

Severe winter weather can therefore be a major hazard for survival. A bird will loose a substantial proportion of its body weight during one cold winters night, and unless able to replenish its reserves, a prolonged cold spell could be catastrophic for large numbers of birds.  Some species such as tits and crows cache or store food for times of less natural food abundance.

In normal circumstances the fat reserves built up by the bird will keep it going for a few days, but mortality will increase rapidly if the cold spell continues onto a second week. Smaller species such as robins, wrens and goldcrests will be more susceptible to the cold.

Birdtables make a huge difference to improving birds chances of survival through very cold periods. Provide food items such as meal worms, fatballs, crushed peanuts, dried fruit and seeds and grain. For more information please click here

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