Most people don't look at bird manure as good fertilizer for several reasons. First, you have little control on how much they apply. Plus, birds aren't trained on application methods. It appears to be random. So, if the right number of birds distribute the right amount of high nitrogen, hot fertilizer, something good can happen. But, you are probably that "one in a hundred" who has just the right amount of everything.
Water clarity, chemistry and temperature plays the most important roles. Then, you know what you need and how much. Plus, you can put it out when you need it, not necessarily when a bird chooses.
I have seen a number of lakes spoiled because too many geese over-fertilized the pot.
