Those look like the black rubber feed bowls that are quite common here. In the picture I posted a couple of posts up you can see the underside of a very large one. I have them on top of the tree stumps I use for me to sit on so they remain poop free.I think you would be right when they have muddy feet which is likely to be the case here in winter. I'm making a few changes regarding feeding and watering. The hard plastic washing up bowl it seems I bought only a few months ago has a split in it.
I'm going to try these. One for feed and one for water. 5 litres for water should be plenty for five to ten chickens given I change the water daily. The washing up bowl holds far too much and is too tall.
https://www.tanks-direct.co.uk/5-litre-black-gorilla-tub/p5759
The small version - about the size of what you showed - is much more useful for actually feeding and watering the chickens!