Purple Heron night flight calls
Separating Grey Heron and Purple Heron NFCs is much more difficult than separating the bitterns and night heron. Bandwidth is equally wide and pitch in both Grey and Purple is similar, though Purple tends to be a little higher-pitched. In my experience, most variants of Purple can be readily separated from Grey by ear, and the difficulty centres on a small part of the variation. When the two species become higher-pitched and more strident (probably due to agitation), they tend to sound more similar. I remember a situation where an Ardea heron was approaching us in Bulgaria with strident calls and we first tried to identify it aurally. After we agreed on the identification as Grey we raised our binoculars and, frankly, had to admit it was in fact a Purple. In general, however, the sound of Purple’s krrrk is croakier, more rasping and drier. Lower-pitched calls may even resemble calls of male Garganey Anas querquedula in timbre.
Looking at sonagrams, you will usually see a wide block of a call in either species. In Purple Herons the different harmonics should be merged beyond recognition, whereas in Grey Herons at least part of the call will usually have clearly separated harmonics. As for the croakiness of Purple, sometimes this shows as prominent vertical lines, similar to the ones in a male Garganey’s call. This appears to come from a broader spacing of the particular elements that together make the krrrk call; logically this should sound more rasping.
All in all, the differences are much clearer as soon as you listen to these calls. In fact, some differences that I can hear are not yet visible for me in sonagrams. So it is always safest to listen to a recording thoroughly, likewise with the other species above.
Purple Heron Ardea purpurea, Besh Barmag, Siazan, Azerbaijan, 18:30, 10 October 2017 (Jonas Buddemeier). A rather quick succession of krrrk calls in night flight.
