Pigeons.

They were too active not to be happy. Rustling, pecking and clawing as if they had some deep desire to make their way through the mock-Victorian brickwork into his study, they appeared to have twenty times the energy one would expect from two birds. He hoped it was just two birds.
The rustling he could cope with, the pecking and clawing could just about fade into the sound of passing traffic, but the other noise, a deep, droning cooing, somehow managed to pierce through every other distraction and land straight into the middle of an ever-present headache. After one put aside the pretty significant element of annoyance, the sound appeared strangely melancholic. It was as if, between all the fighting and structural vandalism, the birds were pouring their hearts out, not as a cry for help, but as a cry of helplessness. It’s like they hated fighting each other but were forced to do it by some overpowering, external force or internal drive. He had tried to make them stop. He had opened the window and looked round, hearing them but not seeing them. He had gone outside, looked up, but had seen nothing. Back upstairs the noise appeared to pause, before starting up again as he got used to its comforting absence. Outside, the air had combined into a muddled compound of grass, cigarette smoke, traffic fumes and something probably emanating from the birds, wafting around and snapping shut every passing conversation as if on purpose. Where, on other days, passing people would utter demands for more grass and fewer cigarettes, today they appeared silenced in a state of wanting to comment on the smell but not quite knowing how to put it. The birds had arrived three days after he had moved into the flat.
He would have to wait until the following year to deduce whether their appearance was seasonal.