Crossbills are specialised finches dependent on conifer seed crops, with welfare shaped by the highly variable and unpredictable food supply of different tree species.
Crossbill welfare is fundamentally shaped by the mast year cycles of their conifer food trees. In years when spruce, pine, or larch produce abundant seeds, crossbills can breed successfully and maintain good body condition. In years with poor seed crops, crossbills must irrupt - moving long distances in search of food in an energetically costly and uncertain strategy. Individual welfare during irruptions is challenging, as birds must navigate unfamiliar terrain and may fail to find adequate food. The Scottish crossbill's dependence on native Caledonian pinewood makes it particularly vulnerable, with habitat loss and fragmentation threatening both food supply and nesting habitat.