My readings in this area do make it clear that hummingbirds do prefer red. The problem with some of the cited research is that it is indeed anthropomorphic by reason of ignoring or not controlluing for colors outside the human visual spectrum. Many apparently blue flowers are QUITE red when seen through a humming bird's eyes, and birds and insects do learn rapidly when motivated by hunger. What WE see is not what they (insects and birds) see. There are a few illustrated articles (in both color and black and white) that show the differences in what appears to insects' and birds' eyes . A few were published in one of the iris-related publications about 10 years ago. Sorry, I cannot provide a reference right now. (Maybe also National Geographic.)