Greig outlined their stand against capitulation. Hummingbirds graze on flowers that have only a little nectar. Greig: "They need to keep going back." "They have evolved to defend areas of food-rich flowers."
For hummingbirds, a nectar feeder is "like a huge, never-ending flower." They eliminate rivals because they are unaware that it is unending. Spare resources must be preserved at all costs.