Since the article didn’t mention what the commonality between all the distress calls was (or I overlooked it), I’m going to assume it’s a sudden uptick to maximum call frequency in the shortest possible bursts, and as loud as possible.
Yeah, they didn’t describe the calls in that article, but I their spectrograms are in the original paper. It seems that they tested two different foreign calls from the same species. They compared these two calls to local alert calls and a couple non-alert call controls. These two foreign calls were actually a bit different, one increased in pitch as you predicted. But surprisingly, the other one started fairly high and decreased in pitch(although this one did seem to have a bit more power). Neither of these 2 were nearly as high pitched as the local alert calls, but both were quite a bit higher than the controls.
Edit: I tried to put the spectrograms here, but I think it’s not working. They are in the paper though.
