On the subject of pilot Kenneth Arnold's June 24th, 1947 encounter with a formation of chevron-shaped silver craft zipping along at dazzling speed between the peaks of Washington State's Mount Ranier, as detailed in this Wikipedia page, I beg to differ with the suggestion that the flying objects observed by Mr. Arnold could have been a flock of high altitude Canada geese.
My divergence of opinion concerning Kenneth Arnold's airborne encounter, from whence the term "flying saucer" was coined by the press at the time, is due to Mr. Arnold's estimations that quantified the airspeed attained by the echelon formation of silver craft he watched traversing distances between known landmarks, which exceeded the maximum airspeed of all known front-line military aircraft operational during that era
Taking this off topic ... but it's a fun discussion.
One of Arnold's reasons for dismissing his initial "It's geese" decision was his 9,500 foot altitude. He was ignorant of the fact geese have been seen flying as high as 29,000 feet. 7,500 is a normal cruising altitude for them here in the states. He assumed they were 25 miles away; but they weren't showing up on radar, so he was depending on visual. An eyeball pair attached to a human head can't judge distance more than a few hundred feet, so it all depends on parallax after that.
In the air, references for parallax are the horizon and objects known to be far away, like a mountain. If the distance to the objects was wrong, a mile or two instead of 20 miles, all speed estimates are off the table. The same is true of the size and speeds he reported (note the air force at the time said his estimates were 10 times smaller than their estimates. 10x is a large error, so something was off in his math and estimates, I suspect it was distance and the objects were much closer and thus much smaller than he estimated. Angular velocities follow. And, ask yourself if you can see a 737 at 50 miles - Arnold's claim he watched the objects until they disappeared near Mt Adams, says he could.
Combine this with the bright flashes they gave off yet darker than snow, random rotations and wavering grouping, more reasons to think geese. And, feathers can iridesce in sunlight and can be pretty bright. I've personally seen birds in flight from altitude, seagulls, and they can be bright; not so bright I'd confuse them with strobes, but they flash bright enough to get your attention. A solid mirror-like disk, by the way, only reflects sunlight at a very specific angle to you. That flashing he reported wouldn't happen if the disks were moving away and randomly following terrain: they'd need to be dodecahedrons or similar to do that. Wings on the other hand are quite flexible and do warp in flight such that feathers catch light at many angles.
Even with the corroborations sited, call me a skeptic. I vote birds, not metal.