ZEISS Edition on Field Ornithology

For decades, the field identification of Central European Harrier species was a problem that was reflected in various handbooks and identification books. Inaccurate drawings and incorrectly labeled photographs often caused confusion, and the observations of these species stored in historical databases must be evaluated with great caution based on current knowledge.

While the identification of fully colored adult males has so far been successful without major difficulties, the species and age determination of so-called "white-rumped harriers", i.e. females and young birds, remained an unsolved problem for a long time, which manifested itself in the blatant underrepresentation of adult female Pallid Harriers in common reporting databases alone.

In response to many requests from ornithologists and birdwatchers, we have therefore analyzed this problem further and compared important differentiation criteria of these three difficult-to-identify species in photographs and drawings.

The "wing formula" for separating hen harriers from Montagu's and Pallid Harriers, which is often regarded as unproblematic and can lead to misidentification, particularly in the second half of the year due to moulting, is also examined. In addition, the identification of males moulting from youth to old age is explained and illustrated.

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The ZEISS Edition on Field Ornithology on the subject of ageing and sexing Hen Harriers, which was published by ZEISS in 2020, has provided the basis for further field studies of this rare bird of prey, which is acutely threatened with extinction in Germany. Since the behaviour of ‘female-type’ Hen Harriers could, following the publication, now be assigned to specific sexes and, to a degree, specific ages, new insights into the social behaviour of Hen Harriers in their foraging habitats have already been gained.

Studies on Hen Harriers in winter have so far been largely limited to the spectacular communal roosts. Little was known about the birds’ behaviour on their daytime feeding grounds. The clear dominance of female Hen Harriers over males of all ages, for example, is a new finding that could only be established by the distinguishing characteristics detailed in the ZEISS Edition on Field Ornithology, which has previously frequently been misrepresented in existing literature.

The recording of behavioural patterns, some of which were complex and fast moving, was primarily accomplished by meticulous observation with high-performance binoculars and spotting scopes. The outstanding dynamics of the ZEISS Harpia spotting scope, with its ability to switch between an extremely large field of view to a 70x magnification in a matter of seconds, proved decisive for success. The results of the observations were subsequently illustrated with the aid of detailed photo documentation.

In the just-published second issue of the ZEISS Edition on Field Ornithology, the Hen Harrier’s winter-territorial system, designed to conserve energy, is analysed and photographically depicted in previously unavailable detail. The bird’s territorial behaviour includes specific vocalisations, as well as communicative behaviours and a typical body posture. Weaker conspecifics are attacked and pushed out of the territories occupied by dominant females with, if necessary, their prey taken from them. Although territorial behaviour is particularly pronounced within the species, Hen Harriers, primarily females, often also defend their feeding territories from Common Buzzards, Rough-legged Buzzards, Red Kites and Kestrels.

Based on observations and photo sequences, the basic movement sequences during the flight-guided strike on the bird’s prey were also examined in more detail, and differences between the Hen Harriers and owls, which hunt in the same area at night, were established and discussed. Here, it turned out that the Hen Harrier, due to its outstanding aerodynamics is able to fly gravity-defying manoeuvres within narrowest area and in the shortest time. Directional corrections can be made until the very last moment. With the help of comprehensive photo documentation, the benefits of the harrier’s V-shaped wing posture, typical of the species during the gliding phases of the hunting flight, is also vividly explained.

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Living not far from one of the biggest Hen Harrier winter roosts in Europe at Federsee in southern Germany, I always had a somewhat close relationship with that species. In the past, no-one really tried to sex and age the “Ringtail” Harriers, when the first proper counts were carried out from 1955.

By studying the best and most popular ID books I learned that juveniles are darker above, and show a prominent pale line along the tips of the greater coverts. In the books I read I couldn´t find any contradiction of this feature, and one of the authors even gave a nice side by side comparison, so separating out the juveniles using these features seemed like a good plan. So I went to the Harrier roost which, during that winter, was located in front of a wooden watch tower, from where we could look down on the flying Harriers.

I saw many Ringtails but didn’t get very good views and then finally I found a juvenile bird with the comparatively dark upper-side and an obvious line along the tips of the greater coverts, as described in the book. While watching this bird I had the impression several times that it had yellow eyes, which I didn´t fit with the information for juveniles in many ID books. But at home I found, in the most popular raptor ID book, a note that juvenile male Harriers could be separated from their sisters by their yellow eyes. Obviously my Harrier was a juvenile male and I wanted to confirm that as soon as possible.

I arrived at the watch tower early in the afternoon, and when my bird came back to the roost in the evening I easily confirmed it had yellow eyes, making me proud about my first properly identified juvenile male Hen Harrier, at least for a couple of minutes. As the bird dropped down into the reeds I clearly saw it chase a grey 2nd winter adult male away, and when that bird took off, right beside of my bird, I recognized that my bird was much bigger and broader winged than the older male!

My hopes of having discovered the ‘Pygmy harrier’ as a new species faded soon and I was deeply unsettled, finding myself again back in the middle of the books later this evening. As my attempts in solving the problem top-down had obviously failed, I eventually decided to switch to the bottom-up method, by searching for “darker under-secondaries” and those “rufous-yellow”, “ochre-coloured” or “rusty-buff “ underparts of the body, stated being distinctive for juveniles.

I felt in need of better light too, and therefore went to the Harrier feeding areas out in the fields. It didn´t take long to find that typical bird displaying the right features in the middle of a vast meadow. I watched this individual for at least half an hour using my scope. To my surprise I saw it too had yellow eyes again, which certainly didn´t fit to most of the descriptions. But I remembered that the special raptor book stated juvenile males have yellow eyes and I ultimately identified the bird as one of these. After some minutes my bird took off and started chasing another ringtail Hen Harrier that was obviously carrying a vole. In that half of a minute of ‘harrying’, before the victim dropped its prey, it was clear that my bird was much bigger than the chased individual, and the latter was much paler, almost white, underneath.

A little weak adult female chased by a huge juvenile male didn´t make any sense at all to me, so by this time I was mentally pretty devastated and confused. I came to the conclusion that somebody had to start from zero to solve this obviously very complicated topic.

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