The sugar content of flower nectar varies quite a bit from one kind of flower to another. How do hummingbirds know how much to drink in order to get enough energy, without picking up too much weight?
When a hummingbird finds a new flower, it takes a sip of nectar, and then waits a minute or so. As the nectar is digested, its blood sugar rises. This tells the bird how much sugar is in the nectar, so it does not fill up on low calorie nectar or over eat high calorie nectar.