Crossbill

The crossbill (Loxia curvirostra) is a member of the finch family that demonstrates very clearly the principle that Charles Darwin developed from his study of Galapagos finches, namely that species adapt in order to survive in their particular environment. In the case of the crossbill, not surprisingly, the adaptation is the crossed bill after which it is named. 

The crossbill has evolved to survive in coniferous forests, mostly in upland areas of Europe, such as the Alps, the Pyrenees and Scandinavia. In the British Isles it is most likely to be seen in the Scottish Highlands and also in lowland forested areas such as Breckland in East Anglia, the New Forest and the Forest of Dean. Numbers in particular areas can fluctuate considerably from year to year, depending on the availability of conifer seeds (the crossbill’s main diet) in its usual breeding areas. 

In North America, the crossbill is known as the common or red crossbill. There are several variant sub-species, including the Scottish crossbill (Loxia scotica) that is, as its name suggests, only seen in the eastern Scottish Highlands. The only significant difference from the “non-Scottish” crossbill is the shape of the bill, which is deeper and with blunter tips in the case of “Scottish”. However, it is the main species that is more likely to be seen in most of Scotland. 


Appearance 

The crossbill is 16-17 centimetres (6.5 inches) in length. It is thickset and with a large head. The bill is short and pointed, with the ends crossed over. There is no general rule as to whether the upper bill crosses to the left or the right. 

Male crossbills are distinctively coloured with crimson plumage that can also have shades of orange. The colours are more prominent on the crown of the head, the rump and the underparts, and are also brighter during the summer months. The wings and tail are brown. 

Female crossbills are yellow-green in colour, tending to grey on the throat and chest, but some individuals may have a more golden colouring. Wings and tail are brown, as with male crossbills. 


Behaviour 

Crossbills have a fast and undulating flight, often at treetop level. They generally only come to the ground for water. They will sidle along thin branches in a parrot-like fashion, sometimes using the bill for support. Crossbills hang down to reach a cone, holding on to it with one foot and prising the scales apart with the bill so that the seed can be removed at the end of a long tongue. The crossed tips are therefore vital for exerting enough pressure to do the job. 

Apart from conifer seeds, crossbills will also take insects and berries. 


Breeding 

Nest building and breeding takes place earlier than for many bird species, the first brood being raised between January and April, with a second brood possible any time up to June. This is done to take advantage of abundant conifer seeds at this time of year. The nest is built by the female crossbill alone, high in a conifer. Three or four eggs are laid and incubated by the female alone, for 14 or 15 days. The male crossbill feeds her during this time with food passed from his crop. 

For the first few days after hatching the male crossbill feeds the whole family, as the female has to provide constant shelter for the chicks in what is still likely to be winter at a northern latitude, and in a mountainous location. 

Young crossbills hatch with straight bills, with the crossing only taking place after they reach three weeks or so. By this time they will have left the nest to roam the countryside with their parents in search of food.

© John Welford