On a cool morning, a hummingbird will lose heat from its small body, making it harder to maintain body temperature. Warmer flowers would then be preferable since they will provide warmer nectar for the hummingbird as well as, radiating heat for the hummingbird to absorb.
Work Step by Step
Think first about the nature of the hummingbird, its small body and how critical environmental sources of heat are for something so small. Then consider ways in which warmer flowers could help with this.