Comparison of Feather Lengths for Honey Buzzard and Common Buzzard

HB mean mm

CBz mean mm

ratio HB/CBz

HB min mm

HB max mm

CBz min mm

CBz max mm

sample

19

51

feather

P1

242

239

1.01

P2

251

248

1.01

P3

263

262

1.00

P4

281

279

1.01

P5

309

303

1.02

P6

342

339

1.01

P7

345

338

1.02

P8

339

329

1.03

P9

311

291

1.07

275

330

275

317

P10

233

211

1.10

196

248

197

236

Cieślak p.102

Cieślak p.120

Cieślak p.102

Cieślak p.120

Abbreviations: HB Honey Buzzard Pernis apivorus; CBz Common Buzzard Buteo buteo

Feather lengths from: Cieślak, Marian, & Dul, Bolestaw, Feathers: Identification for Bird Conservation, Natura, Warsaw (2006).

Ratios calculated by NR

Comments: Cieślak & Dul (2006) make available feather measurements from significant sample sizes and with all feathers measured using the same procedure. The length of the feathers P1-P7 for Honey Buzzard and Common Buzzard is very similar with the Honey Buzzard's feathers ranging from 0-2% longer on average. P8 is 3% longer for Honey Buzzard than for Common Buzzard. The differences become more significant with the two outer feathers: P9 is 7% longer for Honey Buzzard and P10 10% longer. The effect is to give Honey Buzzard a fuller and rounder wing tip without the pinched-in effect that many Common Buzzard show on the leading edge of the primaries.

It's an average of course so not an infallible guide. However, for a 'buzzard' with two obvious structural pointers to Honey Buzzard -- a long tail (equal to wing width) and a full rounded wingtip – the chances of it being Honey Buzzard rather than Common Buzzard are very high.

Forsman (1999) shows a number of plates (15-35) of Honey Buzzard in flight of which all but one, on the fully extended wing, show P9 appearing almost as long as P8 and the tip of P10 appearing beyond the tip of P5. The exception is plate 34, showing a female resembling a juvenile, which has a more Common Buzzard-like wing formula. The Honey Buzzard formula is fairly close to that of Red Kite Milvus milvus. Forsman (1990) shows a number of Common Buzzard plates (316-329), not admittedly the clearest of plates in his book, but the relative shortness of P10 is clear on those in flight, the tip of P10 appearing to generally not reach as far as the tip of P5. P9 also appears to be clearly shorter than P8 on the spread wing. Of course if Forsman used the wing formulae shown here in selecting his plates for publication, then his plates are not a fair sample on which to test the hypothesis.

It is not entirely clear whether the formulae apply to juveniles of both species as Cieślak & Dul do not give information on the age of the birds supplying the feathers. However, tentative indications are that they do. Forsman's plates show Honey Buzzard juveniles on migration with the same formula as adults. Also in this study, the stills show juvenile Honey Buzzard with relatively long P9 and P10 and the effect is even exaggerated on a bird which was presumed to be making its maiden flight. In a personal communication from Marian Cieślak (February 2008), it was reported that c60% of each of the specimens for Common Buzzard and Honey Buzzard were juveniles. They do intend to provide separate data for adults and juveniles of both species in due course.

The minimum and maximum figures above show there is some overlap so this feature should not be used on its own but in combination with other assessments. Forsman (1990, p.32) notes the more rounded tip of the Honey Buzzard compared to buzzards. Perhaps even more informally the paddle-winged shape of Honey Buzzards is another reflection of the wing formula. If Cieślak & Dul (2006) were to make available the feather lengths for all the individuals involved in the samples, then more refined statistics could be derived.