Crossbill | |
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Red crossbill or common crossbill (Loxia curvirostra) | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Aves |
Order: | Passeriformes |
Family: | Fringillidae |
Subfamily: | Carduelinae |
Genus: | Loxia Linnaeus, 1758 |
Type species | |
Loxia curvirostra | |
Species | |
Loxia curvirostra Contents |
Crossbills are birds of the genus Loxia within the finch family (Fringillidae), with six species. These birds are characterized by the mandibles with crossed tips, which gives the group its English name. Adult males tend to be red or orange in color, and females green or yellow, but there is much variation.
Crossbills are specialist feeders on conifer cones, and the unusual bill shape is an adaptation which enables them to extract seeds from cones. These birds are typically found in higher northern hemisphere latitudes, where their food sources grow. They irrupt out of the breeding range when the cone crop fails. Crossbills breed very early in the year, often in winter months, to take advantage of maximum cone supplies.
The genus Loxia was introduced by the Swedish naturalist Carl Linnaeus in 1758 in the 10th edition of his Systema Naturae . [1] The name is from the Ancient Greek loxos, "crosswise". [2] The Swiss naturalist Conrad Gessner had used the word Loxia for a crossbill in 1555 in his Historiae Animalium . [3] The type species was designated as Loxia curvirostra (red crossbill) by George Robert Gray in 1840. [4] [5]
Analysis of mitochondrial cytochrome b sequence data indicates that the crossbills and redpolls share a common ancestor and only diverged during the Tortonian (c. 8 mya, Late Miocene). [6] The research suggests that the genera Loxia and Carduelis might be merged into a single genus, for which the name Loxia would then have priority. But this would imply changing the name of a large number of species, and given that the adaptations of the crossbills represent a unique evolutionary path (see Evolutionary grade), it seems more appropriate to split up the genus Carduelis as it had already been done during most of the 20th century. Unfortunately, the fossil record is restricted to a Late Pliocene (c. 2 mya) species, Loxia patevi, found at Varshets, Bulgaria.
The species of crossbills are difficult to separate, and care is needed even with the two-barred and Hispaniolan crossbills, the easiest. The other species are identified by subtle differences in head shape and bill size, and the identification problems formerly led to much taxonomic speculation, with some scientists considering that the parrot and Scottish crossbills and possibly the Hispaniolan and two-barred crossbills are conspecific.
The identification problem is least severe in North America, where only the red and white-winged species occur, and (possibly) worst in the Scottish Highlands, where three species breed and the two-barred is also a possible vagrant.
Work on vocalization in North America suggests that there are eight or nine discrete populations of red crossbill in that continent alone, which do not interbreed and are (like the named species) adapted to specialize in different conifer species. While several ornithologists seem inclined to give these forms species status, no division of the American red crossbills has yet occurred. [7] Preliminary investigations in Europe and Asia suggest an equal, if not greater, complexity, with several different call types identified; [8] [9] these call types being as different from each other as from the named species of the parrot and Scottish crossbills - suggesting either that they are valid species, or else that the parrot and Scottish crossbills may not be.
Genetic research on their DNA failed to reveal any difference between any of the crossbills (including the morphologically distinct two-barred), with variation between individuals greater than any difference between the taxa. This led to the suggestion that limited interbreeding between the different types prevented significant genetic differentiation, and enabled each type to maintain a degree of morphological plasticity, which may be necessary to enable them to feed on different conifers when their preferred food species has a crop failure. Research in Scotland, however, has shown that the parrot and Scottish crossbills are reproductively isolated from each other and also from the red crossbill, despite irruption of that species into their ranges, and the diagnostic calls and bill dimensions have not been lost. They are, therefore, good species. [10]
Currently accepted species [11] and their preferred food sources are:
Image | Scientific name | Common name | Food source | Distribution of species |
---|---|---|---|---|
Loxia curvirostra | Red crossbill | Spruce (Picea) species; some populations (distinct species?) on various pine (Pinus) species and (in western North America) Douglas fir | Eurasia, North Africa, and North America | |
Loxia leucoptera | Two-barred crossbill | Larch (Larix) species, particularly L. sibirica, L. gmelinii, L. laricina and (in North America) also hemlock (Tsuga) | Eurasia and North America | |
Loxia megaplaga | Hispaniolan crossbill | Hispaniolan pine (Pinus occidentalis) | Hispaniola (Haiti and the Dominican Republic) | |
Loxia pytyopsittacus | Parrot crossbill | Scots pine (Pinus sylvestris) | Northern, Eastern, and Central Europe | |
Loxia scotica | Scottish crossbill | Scots pine (Pinus sylvestris) and larch (Larix) species (particularly plantations of L. decidua) | Scotland | |
Loxia sinesciuris | Cassia crossbill | Isolated population of the lodgepole pine (Pinus contorta latifolia) | South Hills and Albion Mountains, Idaho, United States | |
Originally, the chestnut-backed sparrow-lark (Eremopterix leucotis) was also classified as belonging within the genus Loxia. [12]
The different species specialize in feeding on different conifer species, with the bill shape optimized for opening that species of conifer. This is achieved by inserting the bill between the conifer cone scales and twisting the lower mandible towards the side to which it crosses, enabling the bird to extract the seed at the bottom of the scale with its tongue.
The mechanism by which the bill-crossing (which usually, but not always, occurs in a 1:1 frequency of left-crossing or right-crossing morphs) is developed, and what determines the direction has hitherto withstood all attempts to resolve it.
It is very probable that there is a genetic basis underlying the phenomenon (young birds whose bills are still straight will give a cone-opening behavior if their bills are gently pressed, and the crossing develops before the birds are fledged and feeding independently), but at least in the red crossbill (the only species which has been somewhat thoroughly researched regarding this question) there is no straightforward mechanism of heritability.
While the direction of crossing seems to be the result of at least three genetic factors working together in a case of epistasis and most probably autosomal, it is not clear whether the 1:1 frequency of both morphs in most cases is the result of genetics or environmental selection. Populations that feed on cones without removing or twisting them will likely show a 1:1 morph distribution, no matter what the genetic basis may be: the fitness of each morph is inversely proportional to its frequency in the population. Such birds can only access the cone with the lower mandible tip pointing towards it to successfully extract seeds, and thus a too high number of birds of one morph will result in the food availability for each bird of this morph decreasing. [13]
They can utilize other conifers to their preferred, and often need to do so when their preferred species has a crop failure, but are less efficient in their feeding (not enough to prevent survival, but probably enough to reduce breeding success).
Loxia patevi was described from the late Pliocene of Varshets, Bulgaria. [14]
The Eurasian bullfinch, common bullfinch or bullfinch is a small passerine bird in the finch family, Fringillidae. In Anglophone Europe it is known simply as the bullfinch, as it is the original bird to bear the name bullfinch.
The Scottish crossbill is a small passerine bird in the finch family Fringillidae. It is endemic to the Caledonian Forests of Scotland, and is the only terrestrial vertebrate species endemic to the United Kingdom. The Scottish crossbill was confirmed as a unique species in August 2006, on the basis of having a distinctive bird song.
The parrot crossbill is a small passerine bird in the finch family Fringillidae. It breeds in pine forests in northern and northeastern Europe.
The red crossbill or common crossbill is a small passerine bird in the finch family Fringillidae. Crossbills have distinctive mandibles, crossed at the tips, which enable them to extract seeds from conifer cones and other fruits.
The two-barred crossbill or white-winged crossbill is a small passerine bird in the finch family Fringillidae. It breeds in the coniferous forests of North America and the Palearctic.
The pine siskin is a North American bird in the finch family. It is a migratory bird with an extremely sporadic winter range.
The palm cockatoo, also known as the goliath cockatoo or great black cockatoo, is a large smoky-grey or black parrot of the cockatoo family native to New Guinea, Aru Islands, and Cape York Peninsula. It has a very large black beak and prominent red cheek patches.
The plum-headed parakeet is a species of parakeet in the family Psittacidae. It is endemic to the Indian Subcontinent and was once thought to be conspecific with the blossom-headed parakeet before being elevated to a full species. Plum-headed parakeets are found in flocks, the males having a pinkish purple head and the females, a grey head. They fly swiftly with twists and turns accompanied by their distinctive calls.
The chestnut-backed sparrow-lark is a passerine bird which is a resident breeder in Africa south of the Sahara Desert.
The blue-crowned hanging parrot is a parrot species endemic to southern Burma and Thailand, Malaya, Singapore, and Indonesia. These parrots are 12cm in height and weight 28g and have a longevity of 14 years. They are recognized by their green plumage, black beak and characteristic blue feathers arranged like a crown on their head.
The citril finch, also known as the Alpine citril finch, is a small songbird, a member of the true finch family, Fringillidae.
The Hispaniolan crossbill is a crossbill that is endemic to the island of Hispaniola, and the only representative of the Loxia genus in the Caribbean.
The British finches are made up of several species of finch which were formerly very popular as cage birds in Great Britain. They are not currently commonplace, but are still kept by a few dedicated fanciers.
The thick-billed parrot is a medium-sized parrot endemic to Mexico that formerly ranged into the southwestern United States. Its position in parrot phylogeny is the subject of ongoing discussion; it is sometimes referred to as thick-billed macaw or thick-billed conure. In Mexico, it is locally called guacamaya ("macaw") or cotorra serrana. Classified internationally as Endangered through IUCN, the thick-billed parrot's decline has been central to multiple controversies over wildlife management. In 2018, the estimated wild population in Mexico was 1,700.
The cardueline finches are a subfamily, Carduelinae, one of three subfamilies of the finch family Fringillidae, the others being the Fringillinae and the Euphoniinae. The Hawaiian honeycreepers are now included in this subfamily. Except for the Hawaiian honeycreepers which underwent adaptive radiation in Hawaii and have evolved a broad range of diets, cardueline finches are specialised seed eaters, and unlike most passerine birds, they feed their young mostly on seeds, which are regurgitated. Besides this, they differ from the other finches in some minor details of their skull. They are adept at opening seeds and clinging to stems, unlike other granivorous birds, such as sparrows and buntings, which feed mostly on fallen seeds. Some members of this subfamily are further specialised to feed on a particular type of seed, such as cones in the case of crossbills. Carduelines forage in flocks throughout the year, rather than keeping territories, and males defend their females rather than a territory or nest.
The Newfoundland red crossbill is a member of the crossbill genus which has its crossed bill adapted for prying open the tightly closed spruce or pine cones in order to extract the seeds found abundantly on the island of Newfoundland.
The yellow-billed amazon, also called the yellow-billed parrot or Jamaican amazon, is a species of parrot in the family Psittacidae. It is a predominantly green parrot with a short tail and pink throat and neck. It is endemic to Jamaica, where its natural habitats are subtropical or tropical moist lowland forests, subtropical or tropical mangrove forests, subtropical or tropical moist montane forests, plantations, and rural gardens. It is threatened by habitat loss and illegal trapping of wild birds for the pet trade.
The golden-backed bishop is a species of bird in the family Ploceidae. It is found in western Angola and São Tomé Island.
The Cassia crossbill is a passerine bird in the family Fringillidae. It is endemic to the South Hills and Albion Mountains in southern Idaho. Cassia crossbill rarely interbreeds with other call types that move into the South Hills of Idaho yearly, and can be considered to represent a distinct species via ecological speciation. The Cassia crossbill have specialized beaks to access the seeds of the lodgepole pine cones in this region, but are poorly adapted to other pine cones in surrounding regions.
In the 10th edition of Systema Naturae, published in 1758, the Swedish naturalist Carl Linnaeus described 554 species of bird and gave each a binomial name.