Terns? In Arvada on Tuesday

[Viki Lawrence]

In the early evening on Tuesday in Arvada I saw a flock (30 or so) of what I perceived to be terns flying around apparently catching insects. They were mostly over the golf course. White below, gray above with black wing tips but there was variety among the flock. Adults and immatures I assume. I could not see any markings on their heads. Can anybody guess as to what they were? I did no see forked tails and clearly saw one fan out briefly.

Thanks,
Viki