How do they know? I put out a bowlful of sunflower hearts and the finches came from nowhere, in their masses. They fought and squabbled, came into the camera in gangs and as I had put the extra battery pack on the photo booth I thought I'd have loads of amazing photos to choose from. I brought the camera in and it told me there were no photos, I put it back out and it told me the card was full. I repeated this, then I turned it off and on, then I gave up and reformatted the card, I can only think it was overwhelmed!! Lesson learnt, with sunflower hearts it goes out for twenty minutes at a time because the finches are busy and hugely good fun.