I agree about animals being more vocal when they cannot see each other.
I also have noticed that they don't go around screeming at each other.
My experiences are much like Okanagans.
When communicating the elk are only loud enough to get their message to the intended recipient, not acting like a loud drunk, being loud for no reason.
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Did you know an elk can make all the hair on their rump stand up when alarmed? (much like pronghorn)
Not all forms of communication are vocal.
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In my experience and opinion, Roosevelt elk travel in closer proximity to each other, and merely communicate within their small area, unless they are broadcasting their location in search of other elk.
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In the area I hunt, I hear more bugles in early August than in September.
I think this time is when young bulls are actively searching for a herd that won't run them off.
Bull/cow ratios are targeted to be 12+/100, which averages out to one bull with ten cows, this seems to be consistent with my observations, small scattered herds with only one bull, yet when I see 12, or more cows, then I see 2-3 bulls.
These larger herds are rare in my area, and usually the focus of local hunters.
This could cause the elk to be a little call shy, not only from bad experiences with hunters, but also competing with older, more aggressive bulls.
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I personally get more responses from raking and chuckles than I do with location bugles.
Bugling often gets silent bulls sneaking in, unless I am right in on the herd.
I also have noticed that Roosevelt seem to come in slower than RM, often I have given up on a set-up, only to spook one of those silent bulls.