a category i have been trained and shown lots of pictures with them without people in them. the pigeon clearly didn't memorize the pictures but learned something abstract from them recognizing a human being and it can apply that to any picture. but. the team of neuroscientists observe the pigeons learning behavior in what's known as a skinner box the process of learning how to distinguish between pictures with and without humans appears to follow certain principles in order to investigate them the tests are carried out according to strict rules and you see it's always close to the food. the pigeon picks out a yellow patch then if it successfully distinguishes between humans and say animals it gets corn feed as a reward if it gets it wrong the light is briefly switched off.