This could be anything that interests you in nature but here are some ideas to get you started:
Include any information that interests you. There is an example from author Robert MacFarlane and illustrator Jackie Morris in the link below. They have used a painting and a poem but you can design your poster however you want - a fact file, a story, pictures and labels, diagrams.
https://www.johnmuirtrust.org/assets/000/002/848/KingfisherPoster_original.pdf