Fra forlagets egen omtale:
Except during the breeding season, almost the whole of a bird’s waking hours can be directed, if necessary, towards the search for food. By winter, any migratory journeys have been undertaken and, with no eggs to incubate or young to nurture, individuals are able to forage over a much wider area than in summer, although some move little. In summer, however, adult birds not only need to feed themselves but also find sufficient food for their young. The principal distinction between winter and summer feeding lies in the type of food available and its distribution, and the abundant populations of insects present only in summer in mid and high latitudes form the food resource of a very large number of species.
The amount of foraging or hunting that diurnal species can accomplish is controlled by daytime light intensity, which may be modified by meteorological factors such as thick cloud. The importance of day length in winter increases with latitude, since at 66°N, for example, midwinter daylight (including twilight) is only 62% of that at 45°N. At this time it follows that any reduction in the ability of an individual to find sufficient food may prove fatal if the interruption is prolonged. It is the length of a period of adverse weather, rather than the intensity of the contributory element, that is significant. Provided that an adequate supply of food is readily available, and that foraging can continue, the survival of an individual and its offspring (in a meteorological context) is more or less assured.
It is in winter that the weather has the greatest influence on food gathering. The effects of low temperature, snow cover, frozen water and ground and the icing of vegetation and other food sources are compounded if prolonged, particularly since low temperature increases a bird’s maintenance needs, i.e. more fuel is required to maintain its metabolic rate. In the current scenario of changing climates, in which warmer and wetter winters are becoming more frequent in northwest Europe, prolonged severe wintry weather is becoming less of a problem for birds in this region. Nevertheless, such conditions remain prevalent in subarctic and continental winters.
