I mostly agree with Bob's comments about bird manure. Lots of waterfowl manure will pretty quickly lead to more rapid eutrophication of your lake. This will lead to degraded water quality where you have fewer and fewer weeks of clearer water. In the begining stages, a little "greener water" will appear good and beneficial.
However, as eutrophication escalates you will soon be noticing more frequent occurrence of greenish algae films on the surface especially during the warmer months of June, July, August when you will be wanting to use the lake most often for recreational purposes and then algae will most abundant and water quality the worst on a seasonal basis.
I would not encourage the accumulation and buildup of waterfowl. I would discourage then from using the pond as much as it is practical for you. You will get plenty of bird manure from just the migrating waterfowl. Believe me, when the pond balance tips to the eutrophic side of nutrient standards and the lake starts down the path of degraded water quality, you will long for the good old days when you had clear water and fewer problems. Sometimes aesthetics and recreational water quality is better than having extra poundages of fish.
