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Many thanks for replies, especially Milvus. I have investigated further and realise I am rather naïve, I didn't realise that there is an illegal trade in Goldfinches, apparently thin sticks are coated in a very sticky substance and inserted into feeders and the birds are trapped and are then retrieved, if not too damaged and sold or bred from. How horrible is that. The thieves didn't get into our garden but managed to get the feeders through the trellis probably with a hook or something, but thanks Tilly2