I was just reading in the book City Chicks, written by Patricia Foreman (also author of Chicken Tractor, Day Range Poultry, etc.), that mushrooms are good for chickens. She has kept poultry for over 20 years, and she also has a degree in Pharmacy from Purdue University, as well as a degree in Agriculture, animal science, genetics and nutrition to back up her statements.
About mushrooms she says: "Chickens like mushrooms and readily eat the fungi growing on compost or mulch. I've yet to hear about a chicken dying from eating a poisonous mushroom; ... Mushrooms are high in protein - up to 30%. They are also high in sulfur, which is necessary for many metabolic reactions. Fungi can have an anti-bacterial properties and immune-enhancing enzymes." She goes on to tell how you can give them to your chickens.
Chickens have been around for thousands of years, so I imagine if mushrooms would kill them, they would be extinct by now.