![]() ![]() The extreme challenge posed to birds by the shortage of food in winter is illustrated by the impact of bird-tables in urban areas, which supplement their natural diet. It has been estimated that a small bird, such as a great tit, which feeds on seeds and small insects, has to find food items at an average rate of one every ten seconds during daylight hours through the winter. For birds and many mammals, shorter day length in winter reduces the time available for finding food. Reduced environmental temperatures increase the amount of heat that they need to generate internally to maintain a constant temperature, at a time when the amount of food available to them is much reduced. Whilst endothermy allows some birds and mammals to remain active during winter, it also places formidable demands on those animals. Some endotherms, as you will see later, do not maintain a high body temperature at all times, and some ectotherms, such as larger reptiles, maintain a constant body temperature for long periods, even though the temperature of their environment changes. The terms endotherm and endothermy are now often used in preference to homeotherm and homeothermy, which refer to the ability of birds and mammals to maintain a more or less constant body temperature. Birds and mammals are endotherms, meaning that they produce and retain a lot of heat within their own tissues, rather than absorb heat from their environment, as ectotherms, such as insects and reptiles, do. ![]()
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