Flying ants?

I saw this the other day on a different tree on the moors so I'm glad I've been able to capture it this time.  A complete swarm of house martins were feasting on something in this tree.  Given I saw a flying and yesterday, I'm wondering if that's what it is.

We had a beautiful walk on the moors from the car again.  A track I haven't taken in a long time.  It did both of us good.