Crossbills rely heavily on Seeds from Pine and Spruce trees, but they are cyclical in their production. Some taking four years to mature. Because of this they travel great distances in search of food. They can live in some areas for quite a while, switching from Spruce to Larch, when conditions are favourable, but recent problems with Larch may limit the time they spend in an area. I have seen them occasionally feed on other plant material such as Oak apples.
Footnote: Recently over the past few years, there have been problems with Larch trees in commercial plantations, resulting in large blanket felling. This I am sure will have a big impact on Crossbill populations within Britain.

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