It is a bird drinker - I've seen exactly the same ones for sale in the bird section of the pet shop. My first 2 Luxes didn''t come with them so I didn't even realise that there was a gap/hole for them to fit into...oops!....luckily, its too small for the inhabitants to escape, and I cover it up by using the spring in front of it. But, my second two Luxes (bought a week later - but from a different shop) had the bird drinker....its a real pain because if you don't fit it, it leaves a hole. I have young robos in one of my cages so I've ended up fitting the bird drinker to stop them escaping but I don't use it for water - I use a normal water bottle on the other side of the cage instead.