What it turned out she was really doing was drinking from it! I watched for a couple of minutes as she got herself a good long drink from her own little fountain. Another hen came and did the same. I often run the hose past their tree so now I'm wondering how long they've been doing this.
Now I think I'll give them a go with a nipple waterer and see how they take to it.
What sort of odd or unexpected behaviors do you see with your birds? Post pictures!