I am a medical reference librarian, retired, and an essential oils sales rep for Doterra, an excellent company with an excellent product line. All of our oils are certified pure, therapeutic grade oils (free range and gluten free ;-).
As others have stated, do NOT use the cheaper oils you find at
Walmart and such. They are made with artificial flavors/scents (like cheap perfumes) and are combined with petroleum based solvents. Because of the unique breathing system of chickens (they don't do it like we do), any use of harsh or artificial products/aromas can quickly compromise their health. But you probably already knew that.
So, I do a lot of research for a lot of clients on essential oils. Recently I had the opportunity to learn about EO's and Chickens.
EOs are used with commercial poultry ALL. THE. TIME. A search of any food or medical database will turn up thousands of articles on the application of EOs to commercial livestock production, including laying hens and poultry. Oregano is the most commonly studied, but new research emerges DAILY. I'm not kidding. Safe food production is a hot topic, and many essential oils already carry a GRAS (Generally regarded as safe.) rating. EVERY COUNTRY with a science team is looking at them.
So, the article I am taking to my doctor's receptionist, who has laying hens, is from the
South African Journal of Animal Science, 2006 36(4) : 215-221. The researchers were looking at how to maintain the health of laying hens during the grueling, hot summers in Africa. They used a special diet, supplemented with mannan oligosaccharides, derived from yeast cell walls. They also added six essential oils: oregano oil (Origanum sp.), laurel leaf oil (Laurus nobilis L.), sage leaf oil (Salvia triloba L.), myrtle leaf oil (Myrtus communis), fennel seed oil (Foeniculum vulgare), and citrus peel oil (Citrus sp). They also tested current and traditional antibiotics and, of course, they had a control group.
FYI: Essential oils can be derived from different parts of plants. That's why the researchers specify which part of the plant that the oil is derived, erg, laurel LEAF oil, citrus PEEL oil. Most citrus oils are derived from the peel, which is maybe why they didn't specify what citruses were used for the citrus peel oil.
Conclusions: verbatim.
The observed results in the summer season indicated that the inclusion of 24 mg EO/kg in a layer diet significantly improved egg production, food conversion rate, and reduced cracked-broken egg ratio of laying hens (FYI: 480 were tested) over a period of 20 weeks.