Male eagle-owls advertise territory with a low disyllabic hoot, audible over a kilometre and in still conditions considerably further. Acoustic analysis shows that the fine structure of each bird's call, in frequency and timing, varies more between individuals than within them.
That has practical value. Vocal individuality allows non-invasive monitoring: a survey can record calls across a landscape and estimate how many distinct territory holders are present, and whether the same bird is present in successive seasons, without capture, handling or tagging. Similar approaches now exist for several owl species.
The white throat patch is a second, visual signal. It is exposed and inflated during calling and is far more conspicuous at night than the surrounding plumage, which suggests the display works alongside the voice. The precise information carried by the patch, and whether it reflects condition, is not settled.