|Red crossbill or common crossbill (Loxia curvirostra)|
Crossbills are specialist feeders on conifer cones. The unusual bill shape is an adaptation to get seeds from cones. These birds are typically found in higher northern hemisphere latitudes, where their food sources grow. They move ("erupt") out of the breeding range if the cone crop fails. Crossbills breed very early in the year, often in winter months. This is the time to get the most cones.
They put their beak slightly open between the scales of conifer cones. Then they close their beaks and the tips push the scales apart. This lets them get at the seed (usually two seeds per scale). The point is that, as cones mature, they tend to open when dry and close when wet. Eventually they fall, and lie on the ground. As they dry the scales naturally open, and any animal can get at the seeds. With their beaks, crossbills can get at the seeds much earlier than any other animal.
Each species' bill shape is optimised for opening seeds from different species of conifer. Their preferred food sources are:
- Parrot crossbill, Loxia pytyopsittacus
- Scots pine Pinus sylvestris
- Scottish crossbill, Loxia scotica (formerly treated as a race of parrot crossbill)
- Red crossbill or common crossbill, Loxia curvirostra
- Two-barred crossbill or white-winged crossbill, Loxia leucoptera
- Hispaniolan crossbill, Loxia megaplaga (previously treated as a race of two-barred crossbill)
- Hispaniolan pine Pinus occidentalis
The relationships between crossbill species has been much studied.