[lochaber]

yeah, I don't think it would work unless you had a really huge tank, and just one or two tiny birds.

Birds have a high metabolism and eat, and thus excrete quite a bit.

As far as using animal manure for fertilizing crops, chicken manure has to be composted/'matured' before it can be applied to crops, or it will burn them.

I imagine it's similar with most birds - you'd be putting a lot of really potent waste into that tank.