sirsweetporcelain:

Chocolate is a highly palatable food. Some of us know that all too well. Unfortunately, many birds seem to love chocolate almost as much as people. Chocolate is not bad for birds just because it is high in calories and fat though, it contains a compound called theobromine. Theobromine is toxic to dogs and cats as well, but birds, again due to their smaller size and more rapid metabolism, may be even more susceptible to its toxic effects.

Generally, the sweeter the chocolate (more sugar), the less theobromine present (milk chocolate contains about 1-2 mg of theobromine per gram, dark unsweetened chocolate may contain as much as 15-16 mg per gram). Despite this, no chocolate should be given to birds. Signs of chocolate toxicity include: hyperactivity, vomiting, diarrhea, heart arrhythmias, seizures, dark colored droppings and death.