Misleading my readers

A few days ago, I tweeted that Autumn was here, as there was mist in the valley, and the swallows had left.

I accompanied it with a rather beautiful picture of the valley filled with mist: and not a swallow in sight.

However, since then, I have noticed that, although there are no swallows or house martins dipping around the house, as they have been doing all summer, there are large numbers of them just up the hill on the lane to the common.

Taken this morning, about 50 metres higher than our house.

They are gathering on the phone wires, clearly chatting about the onset of September, and the need to fly South soon. But they haven't gone.

Taken this morning, another few metres higher

Which raises interesting questions in my mind. Is that just chance?  Is it that our swallows have moved up the hill, and if so is that preparatory to flying south?  Will it give them a better start?  Or is it that our swallows have gone, but their hardier cousins, who have been living at a (slightly) higher altitude all summer, are still around?

Are there any ornithologists out there able to enlighten me?

(I see there is published research on this, but my curiosity doesn't extend to the £30 price tag to read it...)