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Has the juvenile plumage of Honey-buzzard evolved to mimic that of Common Buzzard? Daniel Duff 47. Common Buzzard Buteo buteo. Günter Bachmeier ABSTRACT Juvenile Honey-buzzards Pernis apivorus exhibit a striking resemblance to Common Buzzards Buteo buteo, even though the two species are not closely related. Both species, but particularly Honey-buzzards, rather frequently fall prey to Northern Goshawks Accipiter gentilis on their Western Palearctic temperate-forest breeding grounds.The author suggests that the juvenile plumage of Honey-buzzard may have evolved to resemble the plumages of the better-defended and more abundant Common Buzzard, in response to this predation pressure. Data in support of this hypothesis are presented and discussed here.These relationships are placed in the context of previously proposed mimicry relationships involving Pernis species. 118 © British Birds 99 • March 2006 • 118–128 The juvenile plumage of Honey Buzzard he similarity in appearance of Honey- their nature speculative, but this article exam- buzzards Pernis apivorus, particularly ines some arguments for this hypothesis. Fer- Tjuveniles, to Common Buzzards Buteo guson-Lees & Christie (2001) referred to buteo has often been described in the literature plumage mimicry by various honey-buzzards (more recently by, for example, Cramp & (and other raptors) in their individual species Simmons 1980, Porter et al. 1981, Gensbøl & texts, and both they and their contributor, Thiede 1997, Beaman & Madge 1998, Svensson Dr Carl Edelstam, cited a paper by Edelstam & et al. 1999 and Ferguson-Lees & Christie 2001). Ben King as ‘in preparation’, but unfortunately Forsman (1999) even called juvenile Honey- that has still not been published. These workers buzzards ‘probably the most often misidentified suggest that Honey-buzzards mimic a range of raptors in Europe’. Indeed, juvenile Honey-buz- different raptor species on their wintering zards show a close, almost uncanny resem- grounds, Common Buzzard not being men- blance to Common Buzzards in many ways, in tioned as a model species. plumage as well as to some extent in structure, Honey-buzzards and Common Buzzards are across a whole range of plumage morphs (dark of approximately similar size and shape and to pale) in both species. Human observers can share certain plumage features, most notably usually distinguish the two species on account rather uniform brown upperparts and typically of subtle differences in the manner of flight, a rather similar pattern of pale and darker areas structure and plumage, as described in the texts on the underwing, usually including obvious cited above. Nonetheless, there exists, in prin- dark carpal patches. In particular, juvenile ciple, a possibility that this similarity in appear- Honey-buzzards show a range of features which ance represents an adaptive evolutionary are different from those of adult Honey-buz- strategy, a case of ‘Batesian mimicry’ – the name zards, but which resemble corresponding fea- given to the situation where a harmless or tures of Common Buzzard. The descriptions in edible prey species evolves to resemble a toxic the above-mentioned identification literature, or otherwise harmful species, so that predators as well as the photographs in Porter et al. (1981) avoid it (Bates 1862). This, for example, is pre- and Forsman (1999), yield the following, pos- sumed to be the case with the wasp Vespula-like sibly Buteo-mimicking, features of juvenile patterning shown by many species of insect, Honey-buzzard which differ from those of such as hoverflies (Syrphidae). One alternative adult birds: possibility would be a purely coincidental i underparts often streaked rather than resemblance, perhaps resulting from the genetic broadly barred (cf. juvenile Common relationship (albeit distant) between the two Buzzard); raptor species. Raptors tend in any case to share ii underwing-tips more extensively dark; a rather limited range of typical plumage iii underside of flight-feathers more densely feature types across the different families. A barred (4–5 strong bars instead of 1–3); second explanation could be convergent evolu- iv this barring petering out in strength across tion on account of the different species outer primaries resulting in a plainer ‘responding’ similarly (but independently) to whitish area between carpal markings and shared environmental pressures. A plausible primary tips; mechanism for this latter possibility is not v generally more uniform and less promi- obvious, however. The hypothesis presented nently striped underwing-coverts; and discussed here is that this resemblance of vi tail-barring less pronounced and more even; juvenile Honey-buzzard to Common Buzzard is vii cere yellow instead of grey; a sophisticated example of Batesian mimicry, viii eye dark instead of bright yellow; and that evolution has conferred upon the rela- ix slightly shorter inner primaries and tail. tively harmless juvenile Honey-buzzard a Some of these features are, however, shared degree of protection against predation by by many juvenile raptors, making it difficult to Northern Goshawk Accipiter gentilis. These two distinguish coincidental from adaptive effects. raptors, and Common Buzzard, are sympatric Nonetheless, the sum of these features (together over a large area of Western Palearctic tem- with the other features that both juvenile and perate forest (Cramp & Simmons 1980; Hage- adult Honey-buzzards share with Common meijer & Blair 1997). Such relationships cannot Buzzards) results in a striking similarity be proven with certainty and must remain by between these two only distantly related taxa. British Birds 99 • March 2006 • 118–128 119 The juvenile plumage of Honey Buzzard 48. Rough-legged Buzzard Buteo lagopus, showing 49. Juvenile Honey-buzzard Pernis apivorus. Compare the ‘hooded eye’ – a combination of shadow and the shape of the dark feathering around the eye of slightly darker feathering – typical of Buteo species this typical-morph bird with the Buteo in plate 002; to good effect; from Heinroth & Heinroth (1926), the dark eye-mask may mimic the ‘shaded eyes’ of plate 153. Common Buzzard B. buteo and lend the young Honey- buzzard a more typically raptorial facial appearance. From Heinroth & Heinroth (1926), plate 153. 50. A young Honey-buzzard Pernis apivorus recently 51. Honey-buzzard Pernis apivorus (above) has the moulted out of juvenile plumage has largely lost the eyes set marginally further back in the head than ‘mask’ of darker feathering around the eye, and the Common Buzzard (below), which has the eye ‘pigeon/cuckoo-like’ jizz is developing. Common comparatively closer to the bill; from Heinroth & Buzzard below for comparison; from Heinroth & Heinroth (1926), plate 153. Heinroth (1926), plate 153. 120 British Birds 99 • March 2006 • 118–128 The juvenile plumage of Honey Buzzard significant resemblance to pale juvenile and adult plumages of nominate Common Buzzard. Possible reasons for mimicry-based similarity of juvenile Honey-buzzards and Common Buzzards If mimicry is indeed occurring, which raptor is imitating which? Mimicry is predicted to be most effective when a scarce species imitates a commoner one (see www.britannica.com,entry for ‘mimicry’). In the present case, Honey- buzzard is currently much the scarcer of the two species, the estimated European population (excluding Russia and Turkey) being 41,200–48,677 breeding pairs compared with 370,933–472,444 for Common Buzzard (Hage- 52. A pale juvenile Honey-buzzard Pernis apivorus meijer & Blair 1997). Data from the near- showing a striking ‘mask effect’. Does the mask have primeval habitats within BiałowieŻa forest, an additional effect in suggesting that the eye is further forward in the head than it really is? From Poland, suggest that Common Buzzard may Heinroth & Heinroth (1926), plate 153. also have been the more abundant species in prehistoric times, although not by the large One aspect which is particularly interesting margin suggested by the current total European is the presence of an area of dark feathering population estimates. Recent breeding popula- through the eye of juvenile Honey-buzzard, tion densities for Common Buzzard and which for human observers can even be used as Honey-buzzard within these habitats have been a distinguishing feature from Common recorded at around 0.5 and 0.2–0.3 breeding Buzzard. However, close inspection of pho- pairs per km2 respectively (Tomiałojć & tographs of perched birds of both species (e.g. Wesołowski 2005; W. van Manen in litt.; Heinroth & Heinroth 1926, Bijlsma 1993) L. Tomiałojć in litt.). Honey-buzzard is also a reveals that this feature resembles in form the comparatively poorly defended species, pos- combination of shadow and slightly darker sessing a relatively weak bill, and claws which feathering around the eye of Common Buzzard, are adapted primarily for digging and walking although somewhat exaggerated in juvenile rather than for subduing and killing prey (Glutz Honey-buzzard (see plates 002 and 003 in par- von Blotzheim et al. 1971); Common Buzzards ticular). The rather uniformly convex head are stronger and fiercer. This would suggest that shape and lack of projecting raptorial brows of the evolutionary pressure for the mimicry is Honey-buzzard lead otherwise to a lack of likely to be the avoidance of predation by a shadow in this area and a rather different, third species, Honey-buzzard having evolved cuckoo-like facial expression, most striking in such that its juvenile plumage mimics the adult males with their uniform

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