A former co-worker had a small pond in his back yard... subdivision, house on one side, 6' fence on three sides. Green heron landed in his maple tree, cleaned out his pond in 3-4 days. The GBH is not the only bird you have to look out for. I lost MANY fish to kingfishers before I figured out why they (the fish) kept disappearing. I've seen an osprey flying just east of here with a bright red fish in its talons, at least a foot long. And just this past summer, a customer came to me with a horror story of seagulls cleaning out his pond, again in a small back yard. If you LIKE feeding wild birds, get a hundred feeder fish and keep the birds happy, but if you value your finny friends (as most here do) , then NET YOUR POND!
John