I've done a bit of research on coffee grounds bedding today. Most commercial coffee bedding is the chaff of the coffee beans, but many people have posted stuff on the internet about how they get dregs from Starbucks or save their own coffee grounds, dry it out and use it in their coops as bedding, and they swear no chickens ever got sick.
It's a given fact that coffee, mostly the caffeine in it, is toxic to chickens. All chickens. However, chickens encounter many plants and substances that are toxic and most do not become sick. Why would that be? The answer is that most chickens will not eat toxic substances, mostly because they taste bad. But this doesn't make coffee safe because there's always one chicken or chick somewhere that eats it regardless and then gets sick.
Chicks especially love to eat things they find on the ground. If their bedding is small particles, they are more likely to eat it. The chick in this thread ate its commercially produced coffee bedding. This chick had all the symptoms of constipation. Usually oil will break up the impaction, and the chick recovers in just a few hours. The chick in this thread did not respond to the oil and remained impacted, and it never recovered. But it's not clear that it suffered toxicity from the coffee. It doesn't matter. It did eat the bedding and it did constipate the chick. Therefore, I conclude that coffee bedding has particles much too small to be safely used in a chick brooder as would any type of bedding with particles too small that a chick mistakes them for food.
