Studies of West Palearctic Birds 192

Studies of West Palearctic birds 192. Bullfinch* Ian Newton n many respects, the handsome Bullfinch Pyrrhula pyrrhula is quite distinct I from most other finches of the West Palearctic region. Its coloration is striking, more different between die sexes than with other finches, and both adult and juvenile plumages lack any hint of streaking. The feathers themselves have a soft, silky texture, quite unlike those of other finches. The bill structure is also unique, both the rounded shape and the pattern of grooves on the palate, which function in feeding. For while, like other finches, the Bullfinch cats mainly seeds, it includes a much greater proportion of fleshy fruits and tree buds in its diet than do the other species. Behaviourally, the Bullfinch has been described as 'quiet and unobtrusive'. It remains inconspicuous, even in the nesting season, and, as explained later, it has unusual courtship and breeding behaviour. Another peculiarity, which is well known to bird-ringers, is that it seldom struggles when handled, but usually lies limply with open bill. No other European finch behaves in this way. Despite the bright plumage of the males, Bullfinches are never easy to see. They usually betray their presence by tiieir piping calls or by the flash of white rumps as a small party is flushed from a feeding site. The piping call is quite unlike the multisyllabic twittering calls of Carduelis finches, and is uttered mainly by isolated individuals, apparently wishing to re-establish contact, or by startled birds, suddenly flushed from a feeding site. It is also given in flight. Otherwise, Bullfinches in a group maintain contact with one another using a faint pipping note, which precedes or accompanies short moves within a tree or bush. In distribution, the Bullfinch extends from Ireland, across Eurasia, to Japan. In general, individuals are larger and brighter towards the nordi and towards the tops of mountain ranges. The British birds are given subspecific rank (P. p. nesd) because they are smaller than P. p. pyrrhula from northern Europe and *This paper, and others in this long-running British Birds series, will be published in a forthcoming HarperCollins book. 638 [Brit. Birds 86: 638-648, December 1993] Studies of Bullfinch 639 Siberia, and darker and duller than other continental forms (notably P. p. coccined). The most curious race is P. p. murina of the Azores Islands, in which the male has lost die bright coloration typical of other Bullfinches, and looks like the female (Bibby et al. 1992). Over most of their Eurasian range, Bullfinches breed at low density in forests dominated by coniferous trees, but in western Europe they also extend into broadlcaved woodland. In Britain, they are widespread, breeding in woodland undcrgrowtii, thickets, shrubberies and tall straggling hedgerows, and in the parks and gardens of towns. In various parts of Europe, densities of up to five pairs per knr have been recorded, rising to more than 20 pairs per km2 in patches of especially good nesting habitat, such as scrub or thicket spruce. They arc usually seen singly or in pairs, but, at favoured feeding sites, parties of up to a dozen or more are not infrequent, especially in autumn and winter. Such groups are only loosely bound, however, and throughout the day individuals continually arrive at and leave the feeding site, apparently behaving largely independently of one another. When disturbed, they retreat rapidly into the nearest cover. When numbers of Bullfinches arc feeding together, for example in a fruiting tree, aggression between them is frequent. In the typical threat display, one crouches with its tail twisted to one side, head feathers sleeked and open bill directed towards an opponent, uttering a hoarse braying sound, which carries only about 20 m (Hinde 1955). Where feeding positions arc scarce, diere are also frequent supplanting attacks, in which one bird flies direcdy at another, which flees, allowing the attacker to perch in its place. In extreme cases, the attacker may chase its victim for a few metres, but such chases are more often associated with pair formation than with feeding. Feeding behaviour For most of the year, as mentioned above, Bullfinches feed on the fruits of certain trees and herbaceous plants, switching from one favoured species to another as each in turn comes into crop. During the growing season, many seeds are eaten in a soft, unripencd state. In English woodland, preferred seeds include those of dog's mercury Mercurialis perennis, wych elm Ulmus glabra, birch Betula, meadow-sweet Filipendula ulmaria, bramble Rubus, common netde Urtica dioica, and ash Fraxinus excelsior, while on cultivated land the seeds of many common weeds are eaten, starting in spring with chickweed Stellaria media and dandelion Taraxacum officinale, and followed in turn by various buttercups Ranunculus, sorrel Rumex acetosa, sow-diisdc Sonchus okraceus, redshank Polygonum persicaria, fat-hen Chenopodium album, common nettle and various docks Rumex (Newton 1967, 1972). Towards the end of the growing season, Bullfinches turn increasingly to the seeds that will sustain them through die autumn and winter, initially those of netde, birch and rowan Sorbus aucuparia, and later those of bramble, dock and ash (and, in upland areas, headier Calluna vulgaris). Where it is available, ash forms a major winter food, but Bullfinches feed chiefly from certain individual trees, sometimes completely stripping them during the course of a winter, while avoiding the majority of still-laden trees in the vicinity. Such highly selective feeding occurs because ash seeds contain poisonous phenolic 640 Studies of Bullfinch compounds (Grcig-Smith & Wilson 1985), which, to the human palate, give the seeds a bitter taste. The seeds from preferred trees have lower phenolic content than those from other trees, and also tend to have higher fat contents. As seed supplies arc not replenished during the winter, the size of the initial (Tops, and the rate at which they are depicted, influence the date when they run out, and Bullfinches have to switch from seeds to buds. In southern England, in years of good ash crops, Bullfinches can continue to feed on seeds until February or March, before switching wholly to buds. In years with no ash crop, however, the seeds of other food-plants arc usually eaten by January, and the switch to buds occurs earlier. In these poor seed years, Bullfinches can suffer heavy mortality because buds are often still small then, and poor in nutrients. In January, on a diet of buds alone, the birds may rapidly lose weight, and only later in the year, when buds arc larger, can they subsist on buds alone (Newton 1964). The implication is that, in years of poor seed crops and late bud-swell, food supply can be an important factor limiting Bullfinch numbers (but sec later). Among the buds of wild trees that arc eaten in spring, favourites include blackthorn Prunus spinosa and hawthorn Crataegus monogyna, and, where available, also crab apple Mains sybestris. It is only the centres of the buds which are swallowed, the parts otherwise destined to become fruit. The outer parts are peeled off in the bill and discarded, littering the ground below. Within any one tree species, the birds tend to return repeatedly to those individual trees whose buds arc most advanced. In late spring, as buds open, Bullfinches turn to the flowers, shedding the outer parts and swallowing the ovules. Flowers of sallow Salix, oak Querents and crab apple arc especially favoured, and form the bulk of the diet until fresh seeds become plentiful again. For fruit-growers and gardeners, the Bullfinch creates problems, because it cats the buds from fruit and ornamental trees, reducing the blossom and hence the crop. Cultivated fruit trees have buds of the size most acceptable to Bullfinches, and arc derived from wild species whose buds arc preferred in natural conditions. Various ornamental trees, such as forsythia Forsyihia, arc also attacked in spring, partly because their early flowering requires early bud- swell, ahead of native trees. A single Bullfinch can remove the buds from fruit trees at 30 or more per minute, and in winter the feeding is remarkably systematic, as a bird works along each branch taking all but the terminal buds (Newton 1972). As Bullfinches enter orchards from the adjacent woods and hedgerows, they attack the nearest trees first, penetrating farther into the orchard as the days go by. For this reason, damage is often most marked on the edges of orchards, and declines towards the centre. In general, among cultivated fruits, the* buds of plum and pear arc most preferred, with gooseberries and currants next, followed by apples and cherries, but within any one type of fruit certain varieties are preferred to others. Among pears, for example, 'Conference' is preferred to 'Cornice', so that, in a mixed orchard at blossom time, 'Conference' trees may be almost denuded of flower, while the adjoining 'Cornice' are in full bloom (Newton 1964). The preferences for certain varieties are again linked with earlier A donation from HarperCollins has subsidised the publication of plate 232 in colour. Studies of Bullfinch 641 bud-swell, with higher nutrient content, notably protein and fructose, and in some varieties with lower concentrations of offensive chemicals (Summers & Jones 1976; Greig-Smith 1985a). In January, captive Bullfinches lose weight less rapidly when fed on 'Conference' pear buds than when fed on 'Cornice' pear or wild hawthorn buds (Newton 1964; Summers 1982). In England, the Bullfinch was considered an orchard pest as long ago as the sixteenth century, when one penny was offered in reward for 'everic Bulfynche or other Byrde that dcvoureth the blowthe of fruit'. In southeast England, however, Bullfinch numbers increased enormously in the 1950s, making this for a time the biggest problem that the fruit-growing industry had to contend with.

Load more