My initial speculation is that the hummingbird provides some special service for that particular type of red flower that enhanced its ability to survive and procreate its species. That is the main driving force behind Darwins theory, most appropriately entitled "survival of the fittest". Only the organisms with adaptations that gave them the upper edge, the decisive factor, the thing that split the "haves" from the "have nots", survived as a species. Perhaps the hummingbird helped in the pollination of this particular red flower. Maybe the structure of the flower was very well suited for the hummingbirds rather long beak. Or maybe the rapid oscillation of the hummingbirds wings assisted in the pollination and further nurturation of this particular type of red flower. Maybe it's the color red; might hummingbirds be predisposed to red flowers, as opposed to all other colors of flowers? It could be the particular aroma, or scent, these flowers give off. Take your pick, any or all of these could be the reason the hummingbird seems to have an affinity for this particular flower type.