I could make do easily with a P900, a friend has one and I have played with it. I wouldnt mind having the extra reach to get closer to those birds.Common sense would indicate that the P900 goes far enough. But I think the P1000 is calling me.
But September is so far away.
Maybe look for a used P900 and run with it for a couple months and decide if I want to pay an extra $500 for a small sensor UBERzoom
On the matter of the RX10 models, the price of the P1000 is getting up there, and to be absolutely honest, I'd rather have the larger sensor, and arent the RX models environmentally sealed?