If bat poop is guano, is pigeon poop puano?
Bat poop is indeed called guano. There's probably some very good reason for that. But there's no reason that other poops would rhyme with guano. And certainly there's no logical reason why the name for bat poop would have any bearing on what the name for pigeon poop is. In the question, it seems to be looking for an alliterative significance between the P in "pigeon" and the P replacing the G in "guano." But alliteration surely has nothing to do with it, as is seen in the example of bat poop, starting with a B and guano starting with a G.
I heard bats aren't birds and whales aren't fish. You can't trust your senses.
You actually can trust your senses. But maybe not if your senses are so self-limiting, such as thinking that everything that flies is a bird and everything that swims is a fish. Scientists, who are in the know about the various species, make observations of bats, birds, whales, and fish. They have analyzed their structures, their behaviors, etc., and have classified them, as is the wont of scientists, generally given to much classifying of information. (That's one reason you should keep scientists away from graph paper, because they go bananas when given the opportunity to graph and quantify things.)
Scientists come to various conclusions about the species and so, trusting their senses, state that bats are not birds and whales are not fish.
The statement above to the contrary does speak of senses and conclusions, but draws a conclusion hastily, without exercising sense knowledge or following it through to definite knowledge.