Invasion of the Crossbills
Many birds migrate annually - it's "in their genes" to fly thousands of miles from one hemisphere of the earth to another. However, some birds such as the Crossbill have evolved to undertake similar feats of stamina in response to their environment.
This year flocks have been turning up where they're not usually seen. We even had 6 birds fly over the WOTM office here in Bristol so Brett got in contact with Pim Edelaar, an evolutionary biologist from Uppsala University in Sweden, to determine the cause of this unexpected invasion of Crossbills.
Why are Crossbills being found in the skies above us where we least expect them? Pim Edelaar explains.
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Crossbills are a colourful and large, almost starling-sized, finch. The males are red and the females olive green and feed on the seeds in pine, larch and spruce cones. They use the crossed tips of their bills like pliers to prise out the flaky seeds and as a result, they can be found in Scandinavia and parts of the British Isles where conifers are common.
Crossbills make the most of these resources in winter and spring because when summer arrives, the pines start to dry out and the seeds become inedible. As Pim Edelaar mentions in the audio above, it seems that this year's poor cone crop is forcing Crossbills to migrate further and further afield, in search of cones.
What makes the Crossbills so interesting to us at WOTM, is that they are not regular migrants. Instead they have adapted to the cyclical variation of conifer seeds and are able to undertake massive flights in response to the availability of resources.
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