Keeping nectar fresh is important, as it can ferment and get hummingbirds drunk, and bacteria and fungus may start to grow on improperly stored or room-temperature nectar.
Homemade hummingbird nectar has the shortest shelf life since its only preserving agent is sugar. Store it in a bottle with an airtight seal on the lowest shelf in the fridge.
For store-bought nectar, consult the manufacturer's directions and best-by dates to determine how to store them. As a general rule, treat them as you would fresh, homemade nectar.
Other best practices for keeping your nectar safe include avoiding products with preservatives, sticking to a sufficient water-to-sugar ratio, and boiling the water beforehand.
Usable nectar looks like clean water. If you notice your fridge or pantry-stored nectar is cloudy, smells off, or has mold growing on the surface, discard it.