@Sally PB Also, I just remembered that you started keeping bees. Just in case your hives are within bee fly's range to your coop : Ivermectin and bees don't mix well. While it certainly wouldn't cause issues with just one chicken, I would think twice about using it systematically on the flock.
Recently we had a case here where a professional bee keeper lost so many hives he had to quit and he obtained a financial compensation from the AG chamber because it was recognised Ivomec for the sheep was responsible for his losses.