If you look for papers by Bo Dalsgaard on my group's page you'll find some studies that may provide an interesting historical context to patterns of flower visitation in hummingbirds:
Gass, C. L. and G. D. Sutherland. 1984. Specialization by territorial hummingbirds on experimentally enriched patches of flowers: energetic profitability and learning. Canadian Journal of Zoology 63:2125-2133.
Kuban, J. F. and R. L. Neill. 1980. Feeding ecology of hummingbirds in the highlands of the Chisos Mountains, Texas. Condor 82:180-185.
Baltosser, W. H. 1989. Nectar availability and habitat selection by hummingbirds in Guadalupe Canyon. Wilson Bulletin 101:559-578.