I do not like leaving this without some cool way to accomplish what the OP requests: a living thing, feeding on bird feces which grows in such a way as to host a bird colony.
Living with part of the body out of the water requires vascular channels. What has those? Multicellular life: vascular plants and animals. Vascular plants that do this exist: mangroves. But let us be awesome. What about an animal that does this?
I have two dreamt-up inventions to offer.
Mollusks can get big. Giant clams are big but prehistoric bivalves got much bigger.
And the mollusk body plan is amenable to scaling up even more - giant squids are mollusks. A clam would be delighted to /feed off of bacteria that love bird feces/ as the OP states. That is the sort of thing filter feeding clams eat. Your bird island is comprised of a colony of these. They are big, with the bodies below water feeding on what dribbles down from the tall Murex-type spires of shell they extend up above the waterline for the birds.
Small clams nestle among the big ones, shell spires reaching for the surface. These small ones also feed on what comes down from the birds and grow as they do, ready to take the place of their elders when those die.
Echinoderms are durable and cool ocean creatures. They can put up with some time out of water and it is not uncommon to see a sea star making a foray into the air at low tide. Your bird host could be comprised of huge, sessile echinoderms. It is easy to picture a clump of mighty sea urchins, their ramifying spines jutting above the surface. But I like the crown of thorns starfish as the model because these things are so burly and huge. Echinoderms are very ancient, and your creature can be a survivor from the Ordovician.
Your bird host star can be mobile when young, foraging and eating as starfish do. Then as it gets large and less mobile, it seeks the company of its kind on the bird roost. It sends its spikes above the surface to host the birds and feeds on what washes down.