On the other hand, I found this which seems to cite a good source:
According to the Merck Veterinary Manual, feeding avocados to your pets or any other animal should be avoided at all costs. Animals such as cattle, horses, goats, rabbits, birds, dogs, cats, and even fish can be severely harmed or even killed if they consume the leaves, bark or fruit of the avocado tree.
Levels of toxicity vary between the two types of avocados (Mexican and Guatemalan) that grow in California. Toxicity also tends to vary depending upon the time of year though this and various other factors relating to toxicity are not easily understood by experts. Periodically, cases of avocado poisoning are seen in animals living in Los Angeles County.
Avocados contain a toxic fatty acid derivative known as persin and many animal organizations recommend total avoidance of all parts of the plant. The symptoms include gastrointestinal irritation, vomiting, diarrhea, respiratory distress, congestion, fluid accumulation around the tissues of the heart and even death. Cattle, goats, horses, mice, rabbits, guinea pigs, rats, sheep, budgerigars, canaries, cockatiels, ostriches, chickens, turkeys, and fish are susceptible. Caged birds appear more sensitive to the effects of avocado, while chickens and turkeys appear more resistant. Additionalyy, dried avocado seed can kill mice.
The primary negative effects in humans seem to be limited to those individuals who have an actual allergy to avocados themselves.
Sources: Merck Veterinary Manual