pigeon

Type "Pigeon Guano" into Google and there is a lot of information that supports it as an extremely good fertilizer. This was taken from one search result.

"Pigeon guano, for instance, has been prized in Europe as a super-manure since the Middle Ages when folks kept dovecotes and pigeon lofts atop their houses, growing the squabs for food and using the manure to fertilize gardens and fields. Pigeon manure rates higher than other fowl at 4.2% nitrogen, 3% phosphorus, and 1.4% potassium. It is harder to find and gather than other manures, and is best if composted thoroughly before using."

Source: Taunton's Complete Guide to Growing Vegetables and Herbs - online version

That said there are also the same number of results pointing out concerns about health, and the diseases that wild pigeons deposit in their faeces.

Personally if I kept Pigeons I would use their faeces, as one way or another it would eventually return to the ground. Just handle it wisely making sure you wash after and maybe wear a breathing mask if it is dry and dusty. If you do, do report back as it would be interesting to see/read your results.

Edit to attribute quote.

I have racing pigeons and always used their manure. In fact, I have to fight off my mother who'd gladly take most of it for her blueberries and other plants 

Like any manure, the dry stuff is dusty so a cheap mask is a good idea. I mostly direct spread ours in the fall so it can break down over winter.