Placing a solution of coffee and sugar in a hummingbird feeder can discourage hummingbirds from feeding. Hummingbirds routinely drink flower nectar and tree sap, and conservationists recommend that hummingbird enthusiasts who wish to feed these birds use a solution of 4:1 ratio of water to sugar in a hummingbird feeder.
In addition to drinking nectar and tree sap, hummingbirds subsist on a diet of small insects and arachnids. If the hummingbird were to drink from a hummingbird feeder laced with coffee, the stimulant effect of caffeine could increase the bird's heart rate to potentially lethal levels. When one considers that the wings of an average hummingbird flap at a rate of between 60 and 80 beats per second, the introduction of a stimulant such as caffeine could pose a serious threat to these tiny animals.