According to the Merck Veterinary Manual, chickens should not eat the flesh, pits, skins, or leaves of the avocado plant, all of which which contain persin, which is a fungicidal toxin. 

The leaves are where the persin is most concentrated, and they should be avoided at all costs. But all parts of the plant are toxic.

Eating avocado can cause a lack of oxygen and eventual heart attacks in mammals and birds.

In addition to chickens and turkeys, fish, mice and rats, guinea pigs, ostriches and sheep are also susceptible to persin poisoning.

Can Chickens Eat Avocado? 

No, chickens shouldn't eat avocado.

Avocados - all parts of the plant - are bad news. Just 5% of one avocado - just a few nibbles, really - can kill a small bird in 48 hours. 

Seriously, don't even think about feeding your chickens avocado.

Here is a comprehensive list of foods that can be toxic for chickens.

And my list of healthy treats for chickens is here.