POPULATIONS OF TITS AND GOLDCRESTS AND THEIR FOOD SUPPLY IN PINE PLANTATIONS

Abstract

SUMMARY This paper describes a five‐year study in the pine plantations of Thetford Chase, East Anglia, to test the hypothesis that populations of tits (Paridae) and Goldcrests Regulus regulus are controlled by food shortage; and secondarily to assess the impact of bird predation on populations of forest insects. In winter, tits spend most of the day feeding; each bird examines some 1100 trees daily, and must find about 5 mg. dry weight of food, or 24 average‐sized insects, every minute of the day to maintain itself. Coal Tits Parus ater and Willow Tits P. montanus stored much food in spring and autumn; but stored foods formed a very small part of their diet. Interspecific segregation among the feeding birds was less marked than among the same species in broad‐leaved woods. The stock of invertebrate food in the pine foliage in winter ranged from 15–500 mg. dry weight per m 2 of forest. The stock declined steeply during the winter, but increased again slightly in early spring. The level to which it fell in late winter was closely correlated with the mean air temperature at the time. In winter, the birds sometimes ate a substantial proportion of the available stock of food; in particular, they regularly ate about 50% of the stock of the eucosmid Ernarmonia conicolana (Heyl.), which was an important food of Blue Tits (Gibb 1958). The birds probably ate similar proportions of other suitable foods. Intraspecific competition for food was judged to be often severe. In winter, the birds' density was closely related to their stock of food. Coal Tits' survival from the end of the breeding season to the end of September was nearly constant from year to year, and was not correlated with the stock of food; but their survival from October to March was extremely variable and was closely correlated with the stock of food. This confirms that the population of tits was controlled by food shortage; nevertheless their territorial behaviour in autumn was probably also important. The paper ends with a discussion on the impact of bird predation on forest insects. It is concluded that as the birds eat a substantial proportion of the invertebrate stock in winter, they are at least a force to be reckoned with; also, if they help to stablise forest insect populations stepi should be taken to increase their numbers, perhaps most profitably by planting a variety of tree species to provide them with additional winter food.