Hummingbirds can tell by one taste how concentrated the sugar is in a flower's nectar. If the nectar is very concentrated, the bird will only drink a little. Drinking too much would mean taking on more weight than it needed. If the nectar is dilute, then it will drink more. If the nectar is too dilute, then the bird flies on to another flower.
The same is true for synthetic nectar in hummingbird feeders. Most sources recommend a solution of four parts water to one part sugar. The solution should be boiled to prevent yeast and mold from growing, and the feeder should be cleaned and refilled frequently.