Long‐term defence of fruit by Mistle Thrushes Turdus viscivorus

Abstract

In an area of south‐central England Mistle Thrushes Turdus viscivorus regularly defend fruit‐hearing plants (four kinds of tree, ivy and mistletoe) against intruders from autumn to early spring. Holly Ilex aquifolium is by far the most important of the plants defended. To be suitable for defence, a tree must be free‐standing and of moderate size. Observations were carried out over three winters, one mild, one very severe and one intermediate. During the mild winter hollies were successfully defended until the Mistle Thrushes gave up active defence with the onset of breeding activities. During the very severe winter nearly all defended hollies were overwhelmed by invading flocks of other thrush species during the very cold spells. Mistle Thrushes in possession of a fruit supply conserve it by feeding on undefended fruit near‐by if it is available, and to a large extent on the ground when conditions are suitable. Defended fruit trees are generally avoided by other species living in the vicinity. Hence depletion of fruit from defended trees is much slower than from undefended trees, and consequently the fruit may last for several months and provide a food supply in dry weather in spring and early summer for Mistle Thrushes and other species.Fieldfares and Blackbirds show similar, but less well developed, fruit‐guarding behaviour. Fruit‐defence is clearly related to the bird's size, but other physical and behavioural adaptations may be involved. The most important characteristic of holly fruit making it suitable for defence, apart from the dimensions and spatial relationship of the tree, is its ability to last for many months without significant deterioration.