The endemic Scottish crossbill is a threatened species, but restoration measures for the Caledonian Forest should ensure that it flourishes in greater numbers again in future.
The Scottish crossbill is endemic to Scotland, and is the only bird which is restricted exclusively to the UK. However, there is some taxonomic uncertainty as to whether it is in fact a species in its own right, or whether it is a variety of one or other of two closely-related species. These are the common or red crossbill (Loxia curvirostra), which is found in coniferous forests in North America, Europe and Asia, and the parrot crossbill (Loxia pytyopsittacus) which occurs throughout Scandinavia and western Russia. All three species are identical in plumage, and the Scottish crossbill is intermediate in physical size between the smaller common crossbill and the larger parrot crossbill. While further studies to clarify its taxonomic status are being carried out, the Scottish crossbill is treated by scientists as a distinct species.
Distribution in Scotland
The Scottish crossbill is confined to the Highlands of Scotland, where it occurs in the pinewood remnants of the Caledonian Forest, and in conifer plantations which are 100 years or older in age. The population has been estimated at 1,500 adult birds, but because of the difficulties in distinguishing it from the common and parrot crossbill, the actual numbers of Scottish crossbills are unknown. Work is currently underway on differentiating between the 3 species by analysing recordings of their calls, and if this is successful it should lead to a more accurate population estimate.
Before the native pinewoods were reduced to their present figure of just 1% of their original extent, the Scottish crossbill must have been much more numerous and widespread, with a population between 10 and 100 times that of today.
The Scottish crossbill is included in Annex I of the European Community's Birds Directive, which lists Europe's most threatened birds. It is a protected species in the UK, and the government has drafted a Species Action Plan for it. This aims to maintain the current population by conserving and restoring the native pinewoods on which the species depends.
Physical characteristics and behaviour
The Scottish crossbill is a sparrow-sized member of the finch family, measuring about 16 centimetres in length, and is closely related to goldfinches and canaries.
Males and females are quite different in colouration, with the male having a bright orange-red, brick-coloured plumage, while the female is a dull green-yellow, which provides her with good camouflage when she is sitting on her nest. Both males and females have dark brown wings and tails.
All crossbills are instantly recognisable by the curved mandibles which cross over when their bills are closed - they are the only type of bird which exhibits this characteristic. The mandibles cross either to the left or the right, and enable the bird to pry open the tight scales of cones and extract the seed from within them. It is the differences in bill sizes between the Scottish crossbill and its close relatives which led to it first being identified as a separate species. The common crossbill, which feeds mainly on spruce seeds contained in relatively small cones, has a slender bill, whereas the parrot crossbill has a much larger bill for opening the tougher cones of Scots pine. The bill of the Scottish crossbill is in between the others in size.
The Scottish crossbill is a gregarious species, and is often seen in flocks or groups. This behaviour is thought to have arisen partly as a result of the bird's diet, which consists almost exclusively of the seeds of Scots pine (although this has been augmented more recently by seeds of introduced exotic conifers such as European larch). As the cones of Scots pines take 2 years to ripen and cone production varies considerably from year to year, the birds have to vary their feeding grounds, depending on where the cones are abundant, and flocking may be a natural consequence of them converging on cone-laden trees.
The crossbill uses a variety of different calls and sounds, including a loud piercing cheeping call whilst in flight and a deep toop call to express a range of emotions, such as alarm or aggression.
The crossbill feeds on pine seeds either by pulling a cone off a branch and then holding it with its feet while it uses its bill to extract the seeds, or it acrobatically moves around the cone, extracting the seeds without removing the cone from the branch. The location of a feeding crossbill can often be determined by the floating seed cases and occasional falling pine cones which result from its foraging. When crossbills are nesting, they will often favour particular pines near their nest which are heavily-laden with cones and will return to them repeatedly to feed.
Although pine seeds form the vast majority of their diet, crossbills occasionally feed on small shoots and buds, while in spring the females frequently feed on insects, to provide the extra protein needed to produce their eggs. Males feed on insects to a lesser extent than the females, but insects, including the larvae of the pine sawfly (Neodiprion sertifer), are sometimes brought to the young in the nest.
Courtship amongst crossbills begins in late winter or early spring when the males in a flock sing loudly and in chorus, with each individual seeking to broadcast his fitness for mating. They become aggressive towards each other, and will often fight for the right to mate with a female. When a female accepts a male, she will allow him to touch her bill with his, and the male will then feed her to confirm their partnership. Mating often takes place during the process of nest-building, which is done almost exclusively by the female, although males sometimes help in the initial stages of construction.
Nests are usually situated high up in pine trees, 10-15 metres above the ground, although occasionally a stunted bog pine, no more than 5 metres tall, may be used. The nest itself is made from a base of twigs, upon which grass, straw and lichen are built up, followed by a lining of moss, feathers and animal hair or fur. Nesting has been observed in all months between February and June, with March and April being the main months when eggs are laid.
The clutch size varies from 2 to 6, with 4 eggs being the most common size. The key factor which determines the size of the clutch is the availability of pine seeds, and in years of poor cone production crossbill pairs may fail to breed at all.
The female broods on the eggs for 13 to 15 days until they hatch, and during this period the male will feed her. Both birds feed the young, which leave the nest about 3 weeks after hatching. It is usually a further 10 days before the young birds' bills become crossed, so they still depend on their parents to provide them with pine seed during this period. After this, the family group will split up, although the young may stay with one or other of their parents, and, with them, may become part of a flock.
Ecological relationships of the Scottish crossbill
Although it depends so heavily on the Scots pine for its food, the Scottish crossbill does not appear to have a significant effect on the tree's ability to reproduce. The flocks concentrate in forest stands where there is a good cone crop, and in those situations a healthy mature pine can produce over 3,000 cones. Not all of these are tackled by the birds, and even in those which they do feed upon, not all the seeds are removed. Thus, some seeds always survive, and given the right conditions, will germinate and grow into new trees.
Great spotted woodpeckers (Dendrocopos major) and red squirrels (Sciurus vulgaris) also feed on pine seeds, but the former are relatively few in number and the squirrels are unable to move so freely to forest areas where cones are abundant, so the crossbill does not face much competition for its food supply, especially during the breeding season.
Nest predation is, however, quite common, with carrion crows (Corvus corone corone), hooded crows (Corvus cornix) and red squirrels all preying on both the eggs and young birds. The Scottish crossbill seems particularly vulnerable in this regard, because it nests earlier in the year than many other birds, and consequently there are few other sources of food for the predators. Parasites of the crossbill include a louse (Philopterus curvirostrae), a tape worm (Anonchotaenia globata) and a fluke (Brachylaemus mesostoma).
Although it is considered threatened at present, the future looks brighter for the Scottish crossbill, as all the work currently underway to restore the native pinewoods of Scotland will provide the habitat for an expansion of its population in years to come.
Alan Watson Featherstone
Published: Caledonia Wild! Winter 2000-01
Last updated: 25 August 2010