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Crossbills are finches with large heads and bright colours: the males are red and the females are olive green. What makes them so unusual is that the tips of their beaks are crossed over; allowing them to rip into pine cones and extract the seeds. Different species of crossbills have different sized bills, which have evolved in association with the species of cones they eat. The Common Crossbill is found across the UK all year round and its numbers have been boosted by the planting of commercial conifers such as pine and larch. A real prize for birdwatchers is the larger and much rarer Parrot Crossbill, which has a very deep bill and can tackle the biggest and thickest cones. Presenter Trai Anfield and ornithologist Ian Newton, who has studied the movements of crossbills, take the rare opportunity to track down this flock, which probably irrupted from the breeding forests in Scandinavia.