I observed these birds in Saint Petersburg, Russia this June, mainly in the evening when the sun was setting. They are similar to seagulls and often fly (presumably hunting) at the same location as the seagulls.
Easily noticeable differences from seagulls are:
- Forked tail, like swifts have
- Narrower wings, which seem longer
- Higher-pitched short tweets, which never turn into "laughter". A sample (with car noise in background) can be heard here.
Here are some photos I could took. They are blurred and chromatically aberrated because it's hard to focus with a telephoto lens and such fast motion towards and away from me. I exposure-compensated them to make the colors visible, so the photos are a bit noisy. I hope they are useful for identification anyway.