Chimney Swift
Some of my favorite bird sightings are ones that I see without specifically going on a trip to find them. The chimney swift I saw when I was working my regular job. I was sitting a patient and took him out to the garden for some fresh air (in 95 degree weather, but they wanted to go - not me) and I look up to see a flock of birds flying waaaay above the hospital I work at. At first I thought it was ravens or starlings, but I heard them chittering in a way I havent heard before. It almost sounded like when a steel wire is under too much weight and snaps. So I record the sound and find out its a flock of chimney swifts!
Though I originally ID'd them by their call, they also have a distinct wing pattern that I could see from hundreds of feet below. It's a very irregular wing beating (maybe when they chirp?) followed by gliding for a long while. It is hard to put into words, but it was utterly distinct. Apparently these birds don't perch but hang on cliffs (and chimneys lol) like bats! their mouth resembles a nightjar to me, I bet if you look it up theyre related somehow.
