Honey-buzzard Jizz
Wing Loading
So what does the Honey-buzzard flight look like in its breeding areas? Physically the Honey-buzzard has a 15% lower wing loading, the ratio of weight to wing area, than the Common Buzzard (Ferguson-Lees & Christie (2001, p.65)) because of, on average, its slightly longer wings and slightly lower body weight. Ferguson-Lees & Christie (2001, p.341, p.693) give wing measurements as 370-441mm for male Honey-buzzard and 372-447mm for female Honey-buzzard, compared to 350-418mm and 374-432mm for male and female Common Buzzard respectively. On average, male Honey-buzzard appear to have a 5-7% longer wingspan than male Common Buzzard and female Honey-buzzard a 0-3% longer wingspan than female Common Buzzard. Detailed measurements by Cieślak & Dul (2006) indicate that the Honey-buzzard is very similar to the Common Buzzard on primary feather lengths for P1-P8 but the Honey-buzzard has P10 10% longer than in Common Buzzard and P9 7% longer.
Weights vary enormously. The same source gives weights for male Honey-buzzard as 440-943g and female Honey-buzzard as 450-1,050g and weights for male Common Buzzard as 427-1,180g and female Common Buzzard as 486-1,360g. The largest Common Buzzard appear to be significantly heavier (25-30%) than the largest Honey-buzzard while there is little difference for the smallest (or leanest) of each species.
This lower wing loading (weight/wing area) enables the Honey-buzzard to fly earlier in the morning than other broad-winged raptors on migration. It also enables it to soar in less favourable thermal conditions than Common Buzzard so they can be active earlier in the morning, often an hour or two before Common Buzzard soar. Another measure used for flight classification is the aspect ratio: wing-span2/wing-area. Panuccio et al (2010) quote the Honey-buzzard as showing an intermediate morphology between those of the large raptors with a low aspect ratio, such as buzzards, vultures and eagles, and those of raptors with a high aspect ratio, such as kites, harriers and the osprey.
There is one complication in this simple analysis. During the breeding season Honey-buzzard increase in weight with Cramp (1980, p. 22) giving weights for males as average 632g (range 440-770) in June increasing to 836g (790-943) in August and for females as average 620g (450-885) in June rising to 962g (790-1050) in August. Ferguson-Lees & Christie (2001, p.341) note that adults are 20-80% heavier in August than in June. The very low weights of 440-450g at the bottom of the ranges in the post-spring migration period are striking. Honey-buzzard feed very little on migration, particularly across the Sahara Desert. A Honey-buzzard in May, crossing the Mediterranean, may have an empty crop and look rather emaciated. Honey-buzzard in their breeding areas, once they have recovered from the spring migration, are therefore likely to look solider than those on migration. With fuller crops they will also look significantly thicker around the base of the neck although the head and upper neck will remain clearly thin. The extent to which the increase in weight actually affects identification perhaps needs further investigation. Panuccio et al (2006) show a Honey-buzzard in Italy with a full crop which has a much thicker neck than Honey-buzzard depicted in field guides. The same authors point out that most Honey-buzzard are thought to fast on migration to improve soaring efficiency through having a lower weight. But some birds seen closely did have fuller crops and must have fed recently. This work by Panuccio et al (2006) is important as it shows the variation in crop size that occurs and the effect this will have on the appearance of the front of the bird.
Looking at identification criteria used in France where the Honey-buzzard Bondrée apivore is a familiar breeding species, it is clear that the thin neck is downplayed. For instance on oiseaux-libre net the first photograph shows a Honey-buzzard with a very heavy body and a long neck thick at the base but rapidly narrowing to accommodate a small protruding head. At oiseaux net some photographs taken of birds in flight show very thin necks and are presumably of migratory birds. Photographs of perched birds show thick necks and these are presumably of breeding birds. The text says “Diffère des buses au vol par les ailes plus étroites à la base, la queue plus longue et une tête plus petite sur un cou plus long”, that is: differs from buzzards in flight by the wings that are narrower at the base, the longer tail and a smaller head on a longer neck. A sketch in Ou Voir les Oiseaux en France on p.189 (coordinator Philippe J Dubois, publ: Nathan, Paris, 1989) shows a long tail, small head and a neck that is long but quite thick.
Besides the invalidation of the rule, heavy build = Common Buzzard, light build = Honey-buzzard, it is likely that the heavier birds will have a slightly different jizz. In particular it is likely that, as observed in this study, the heavier Honey-buzzard will raise their wings to some extent at low altitude to increase their lift. Either the front of the wing will be raised slightly (to increase the angle of attack) or the tip will be raised slightly (to reduce turbulence). The raising of the wings at very low altitude may also be a deterrent against Goshawk Accipiter gentilis by the mimicking of a Common Buzzard at a vulnerable time in the flight.
Even more intriguing is the evidence that Common Buzzard lose weight in the breeding season with Ferguson-Lees & Christie (2001, p.687) reporting that all individuals are about 20% lighter in weight at the end of the breeding season than in winter. Cramp (1980, p. 190) gives weights for nominate buteo as: April-May male average 732g (range 552-846), female 881g (486-1197); June-July male 692g (600-813), female 865g (727-946); August-September male 706g (427-850), female 911g (800-988); October-November male 828g (620-985), female 1052g (710-1327). The heaviest weights are in October-November, the lowest in June-July.
The surprising conclusion, nowhere mentioned in identification guides, is that Honey-buzzard are on average actually heavier than Common Buzzard for part of the breeding season. In June male and female Common Buzzard are 9% and 40%, respectively, heavier than male and female Honey-buzzard. In August it is the other way round: male and female Honey-buzzard are on average 18% and 6%, respectively, heavier than male and female Common Buzzard. As mentioned above though, there will of course be enormous individual variation, depending in part perhaps on the richness of the habitat. In rich feeding areas Honey-buzzard appear to put on weight earlier, during late May and June.
Juvenile Common Buzzard have the following weights in August-September: male 680g (495-796), female 823 (569-1008). In the study at Inverness at least one of the juvenile Honey-buzzard tracked, the one that made the epic journey in 2002 (1010g), was over 1000g when 'fitted-up'.
Reversed Sexual Dimorphism
There is little information in the literature to show that female Honey-buzzard are significantly heavier than males. However, such weight differences show up very clearly when a pair is in display in the spring or escorting young in the autumn. The male appears lighter, more harrier-like perhaps, while the female is heavier, more buteo-like. The male has the characteristics expected of a Honey-buzzard from many identification guides with long narrow tail, long wings, small head on long neck and buoyant flight. The female appears more cumbersome with the preceding features less obvious though in active flight she does perform as well as the male. Such reversed sexual dimorphism is found in many birds of prey and some data to support its existence in Honey-buzzard would be very useful. Newton (1979 pp.19-31) discusses reversed sexual dimorphism in birds of prey and showed (at p.23) that such dimorphism is most marked in species, such as the Accipiter, that hunt very active prey such as birds and least marked in species, such as the Pernis presumably, that hunt more sluggish prey such as insects. Newton used wing-lengths as the measure of size but does mention that weights vary much more with a 20% increase in wing span leading to a 100% increase in weight. For Honey-buzzard the wingspans reported above are very similar for male and female. So the difference between the sexes in weight is not a consequence of wingspan differences as in the Accipiter or Circus.
In the harriers Circus (Cramp 1980, p.115, 126) Marsh Harrier females have wings 5% longer than males and Hen Harrier females have wings 11% longer than males. Weights as expected show more difference with Marsh Harrier females 34% heavier than males and Hen Harrier females 52% heavier than males. My feeling with Honey-buzzard is that the situation is similar to that in Marsh Harrier with the wingspan difference being slight but the weight difference being more obvious to such an extent that it is clearly discernible in the field, particularly when the male and female are seen together. In Honey-buzzard the reversed sexual dimorphism is therefore in weight rather than in wingspan. Why does it exist? Newton (1979, pp.26-27) thought that this problem is rather intractable; he did however think that the idea that the female's larger size might assist in nest defence had more to commend it than others.
Interestingly the Oriental Honey-buzzard orientalis does show a considerable difference in reported weights with males in the range 750-1,280g and females 950-1,490g (Ferguson-Lees & Christie (2001, p.346)). Means are not quoted.
Soaring
When a Honey-buzzard launches itself into the air from trees, it is always an impressive sight at such low altitude. The large size and broad inner wing can, to the inexperienced raptor watcher, cause confusion with larger raptors at this stage such as Osprey Pandion haliaetus. Flapping is kept to a very strict minimum as the bird floats searching for a thermal with wings level or with the tips slightly raised. The tail can be flexed to some extent as the bird steadies itself. Forsman (1999) notes the slight raising of the tips at times. It is only slight and the wings are always smoothly curved, never kinked at the elbow as in Common Buzzard. Honey-buzzard gain height amazingly quickly even when thermals are not strong. They also do this in a very majestic way with every sinew strained to give maximum wing area. Roberts (2003) noted the steadiness of Honey-buzzard in ascent compared to the unstable Common Buzzard:
“If you look at Common Buzzard all day, as we do with hundreds in the area and the population through the roof, you try and pick out a Honey-buzzard from a bunch straight-away as being rock stable while the Common Buzzard is rocking about. Little things like this are important”.
In the soar the wings quickly become level and they are often pushed forward. This latter feature is noted by Forsman (1999) but not by Ferguson-Lees & Christie (2001). The wingtip looks fuller and more rounded in Honey-buzzard than in Common Buzzard because of its longer P9 and particularly longer P10. The inner wing also looks broader in Honey-buzzard than in Common Buzzard because of its bulging secondaries. Closer to the body though the Honey-buzzard wing base is narrower giving a pinched-in appearance in this area. The effect is of an uneven width to the Honey-buzzard wing while the Common Buzzard tends to have wings of uniform width. The long tail (as long as the wing width or slightly longer) is a prominent feature and it is usually slightly fanned during the soar. The neck is often retracted during soaring so the well-known small head and extended neck are not that obvious. It is relevant that Roberts (2003) does not emphasise this as an identification feature while the tail is rated as of high importance. He said:
"Obviously length of tail is important. [shows photo of Common Buzzard, then one of Honey-buzzard]. You cannot see much markings on them, the difference is obvious immediately without showing any markings. The difference in structure is obvious, forget plumage. If someone starts describing plumage to you, stop listening. Someone should be telling you about the shape of the bird. Obviously the tail is important. The one characteristic that does stick out is the tail. This [photo] is a typical Honey-buzzard with long full tail. Tail is always bulging a bit with a notch at the end. You can see the long tail on a Honey-buzzard a long way off”.
The tail shape probably varies more than Roberts indicates. As the Honey-buzzard is a ground-feeder, the tail shape can be affected by wear and tear (Ferguson-Lees & Christie, 2001, p.338).
Some experts have queried whether Honey-buzzard ever retract their necks. Examples of many Honey-buzzard in soaring mode with retracted necks are found on Plate 23 in Forsman (1999) and in a photograph on p.32 of Birdwatch, no 156 (2005). A group from Forsman is shown here. In nearly every other mode of flight the extended neck and small head is an excellent identification feature for Honey-buzzard.
Active Flight in Soar
A very good field identification point is the mode of active flight. Unfortunately Honey-buzzard generally avoid this as much as possible in their breeding areas. They can float without any active flaps for five to ten minutes. In active flight the Honey-buzzard shows much more movement in the wings than Common Buzzard, which tend to give a series of stiff shallow wingbeats (unless they are carrying prey). The Honey-buzzard has slower and deeper wing beats than the Common Buzzard and there is a rippling action along the wing in each beat. The wingtip generally goes much lower than in Common Buzzard on the downstroke and the wings are distinctly arched at this point. Indeed much Honey-buzzard wing activity is below the horizontal while much Common Buzzard wing activity is above the horizontal.
Top of Soar/Stalled Mode
Honey-buzzard often soar to a great height and then level off and either float slowly around in reconnaissance or drift off towards a distant feeding area. At this point when peering around they can show the classical extended neck and small head. They can also show a characteristic 'vulture' pose with wings pushed very well forward, tail fully fanned (and therefore looking shorter) and extended neck and protruding small head. They do not quite hover but, in slow circling, repeatedly come to a brief almost complete standstill (or stall).
Descent
Descent is normally done in a dive. Sometimes just one dive is done but more typically a series of dives are performed, each one taking the bird lower until a final swift drop back into the trees with legs well out-stretched.
Active Flight at Low Altitude
When flying out of woods at low altitude over fields or through the tops of the trees, Honey-buzzard can look like large Carrion Crow Corvus corone. Compared to Common Buzzard they show longer necks, smaller heads (often held raised above the body level), longer tails and a more steady, regular, swinging wing action. The barring on the underside of the wing is relatively conspicuous in such flights on the downstroke perhaps because they are being viewed from the side. The tail bands though remain invisible.
Display in Spring
There are two periods of activity: display and post-fledging. Display takes place in Northumberland in May and early June. On first arrival in late April or the first half of May the birds are rather lazy perhaps feeding and resting after the long migration. The lazy flight with long periods of effortless floating, interspersed with the very occasional stall or spell of active flapping, occurs over their breeding areas at this time. Display is brief and in poor springs may not be too conspicuous, though it does occur even in drizzle if few decent days occur.
In better weather the display is obvious. The male can appear already up in the air and may call with a characteristic thin tri-syllabic (somewhat like a Grey Plover Pluvialis squatarola) whistle or a di-syllabic whistle (somewhat like juvenile Common Buzzard). See calls for further details. The female then comes out of the trees in a normal soar. The display can be spectacular with one bird, usually the male, performing a smooth long dive before suddenly rearing up steeply and pausing at the top of the climb. At this point the classical butterfly display can be seen as described by Porter et al (1981):
"After a long glide it rises steeply and, slightly hovering, 'shakes' wings above back in rapid succession, usually six or seven times. This performance [is] repeated several times during display flight".
But the shaking often does not occur and the bird after pausing simply resumes with another dive, giving a roller-coaster progress across the sky. Another display feature is the follow-me pattern of flight by a pair. In slow action one bird follows the other, faithfully copying every turn and move. This occurs particularly at high altitude in fine weather but can also occur at low altitude if the birds come out of the trees together. The birds can alternate in taking turns as to who is to lead. The commonest form of display is mutual circling where the birds soar together keeping the same height as each other and on opposite ends of the circle they are carving out of the sky. Display can occur at great height with the birds having a tendency to disappear into the bases of dark grey clouds where the thermals are strongest.
Fast Gliding
This is most noted in display flights, with birds moving at some pace across the sky. The pose is perhaps more like a harrier or a kite with obvious long thin tail, carpal well forward, long hand trailing behind, neck and head protruding well and tips of wings slightly depressed so that the shape of the bird head-on is smoothly arched. The body looks small. While the pose of Honey-buzzard in this mode of flight is supposed to be very distinctive compared to Common Buzzard, it is worth adding the cautionary note that Common Buzzard also appear to have long tails in this mode. Sometimes in this mode, Honey-buzzard accelerate with a flap-glide action rather like Goshawk Accipiter gentilis with a few quick shallow beats followed by relatively long pauses. This flight action may also be used occasionally in spring when patrolling their territory.
Summer Secrecy
After the spring display, Honey-buzzard then become very difficult to detect. As Steve Roberts (2003) says: "Anybody who has done any work with Honey-buzzards knows that once they start incubating they don’t fly out of the woods, they dig tunnels and walk out". In Northumberland the period between mid-June and early August is not a good time to search for Honey-buzzard unless you are very experienced with their appearance. Glimpses may be brief as the birds fly low over the trees or along glades into or out of their nest sites. However, non-breeding birds can be more obvious into July as they continue soaring in fine weather while nesting birds sit tight. In addition even breeding birds will soar occasionally, some using the tactic of commuting between feeding areas and nest sites by vertically soaring to a great height, planing at high speed and finally diving into their destination from some height.
Fledging in Autumn
Honey-buzzard become more active as fledging approaches with some anxious patrolling mid-August. After fledging (from mid-August-early September in Northumberland) the family parties engage in considerable soaring activity. These family parties typically comprise three birds in Northumberland (two adults, one juvenile). The first evidence that a flight is about to begin is sometimes provided by a series of thin calls -- from the female in the trees or the male in the air or even from the juvenile on the ground or in low trees somewhere. The female leaves the trees before the juvenile which in its first flights may be quite hesitant, reluctant to get much above the trees and floating in an unsteady manner. Some juveniles in late August show outer primaries that are still growing. Once in the air the female often escorts the juvenile which when it gains height may be ambushed by the male. The flight can become quite vigorous as the juvenile rapidly becomes more experienced with considerable diving at each other. Some of the autumn flights are made at great height as the adults prepare for departure.
Post-fledging
Once the adults leave, which at least for the male, is quite soon after fledging, the activity drops markedly, particularly for isolated sites. A single juvenile may engage in some practice flights but is generally well out of sight feeding. More recently the juveniles from adjacent nests have formed groups after the adults have left and these groups engage in strenuous practice flights on the edge of the moors. These groups may leave together or in a trickle, typically from15th-25th September.
Kite-like or Buzzard-like?
Experienced Honey-buzzard watchers appear to rely on detecting the kite-like (milvus) features in the otherwise general buzzard-like feel (buteo) to the bird. Honey-buzzard thus have a long tail expertly twisted in flight (allowing for impressive manoeuverability) and wings long and more sharply pointed than in buzzard, held slightly arched downwards in flight and with deep and loose wing-beats in active flight (Couzens 2003, p. 60,70). However, some kite-like features are often missing in flight in Honey-buzzard such as projecting carpal joints in the soar and head and long tail held in slight droop. It should be emphasised that there are differences of opinion as to whether the wings of Honey-buzzard are broad (as in buteo) or narrow (as in milvus). In truth the apparent width of the wings is very variable, depending much on the flight action: in the soar the wings are broad at the wingtip and across the secondaries, in the glide narrow throughout. Indeed Honey-buzzard look more buzzard-like when soaring and more kite-like when gliding and in active flight.
© Copyright Nick Rossiter 2003-2012
