It's a bit tricky, really. What farmers generally do often, ie late autumn, results in removing most of the bits that would have flowered next spring, so that they are hardly any berries for the birds anyway. But if you do it earlier, say late summer, you give many species time to recover and rebuild flowering buds for next spring, but you remove the berries that are present that year. Ideally, I suppose, you try and manage it so that you don't have to prune every year, or at least not heavily, and accept some loss of something the years that you do.