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Must be true what they say - owls ARE wise!
As for the fishing line - what has worked for me is a 6' high fence around the chicken yard, then a criscross of 20-lb-test monofilament over the top, basically going from post to post zig-zagging and criss-crossing, mine is very irregular and has large holes but the Cooper's Hawk has been around for weeks and hasn't gone in since we put up the line. (We did lose one before that.) I think it works because the Cooper's Hawks fly very low, about eye-level, and so the monofilament net is edge-on to them and looks like a complete tangle. So the tall perimeter fence is a key part of the deal, without that the hawks will simply come in under the lines. I think it also helps that mine is at different heights, from about 6 to 8 feet up, it looks like a giant irregular cobweb.
15 or 20 pound test line is very cheap, especially 15 lb, and strong enough to stretch very tightly. I have some 10' T-posts that make a high line down the center of the yard, and I run the fishing line over that, so it's mostly above my head. Eventually I will have to replace it when it starts to break down, but it goes up pretty fast.
Whether it will really work or not depends in part on your hawks. Mine seem to avoid it and go hunt elsewhere, but if one bird comes along and figures it out, the whole thing is useless. So I don't know if it's really effective, or the hawks I have just now are not that interested in my chickens.