Honey-buzzard Strategies for Autumn Migration across Northern Britain: the Significance of Orographic Lift

Newton (1979, p.195-196) gives the basic situation: “the second group includes the broad-winged raptors, such as eagles, vultures and buteonine hawks. They perhaps expend less energy on migration but, being more dependent on updrafts or thermals, they are obliged to make as much of their journey as possible overland, on what is often an indirect route taking advantage of land bridges or short sea-crossings. In the absence of mountain updrafts, they are also restricted to the middle part of the day, when thermals are best developed”.

Alerstam (1990, p.261) considers in more detail the conditions for soaring flight: “By soaring the birds make the most of the free energy in upwinds. These they can find at hills and mountain slopes where the wind is deflected upwards, as well as in 'lee waves' which are formed beyond the lee sides of mountains. Birds which soar and glide over longer distances, however, as a rule rely not simply on upwinds at hillsides and in lee waves but benefit primarily from the rising air of thermals”.

Honey-buzzard arrive on migration in northern Europe typically in May and early June. At this time the sun is relatively hot and the days long, permitting long-distance movements through soaring on thermals (thermal lift). The breeding season is one of the latest for any species in Europe but in many areas is still complete by mid-August, enabling at least many adults to depart in the same way as they arrived, through thermal-lift. In the extreme north and north-west of their range, fledging may not occur until late August or early September and the birds may not depart until mid-September. By this time thermals are becoming much weaker and are restricted to perhaps only a few hours in the middle of each sunny day. Sunny days themselves may be scarce and the limited time available each day will delay the exit of the birds. To facilitate their exit, the birds may use other forms of available energy such as orographic lift, the meteorological term for updraft caused by the wind blowing into a hill or a mountain. Orographic strictly refers to mountains but any ridge or obstruction, such as a cliff, can be used beneficially by the birds; hence equivalent terms are ridge lift and obstruction lift. This energy is available through the whole day thus giving the birds a much longer flight period.

Alerstam (1990, p. 271) describes the best known raptor migration site in the world where orographic lift is employed: “Raptor watching is a popular pastime in North America. The most important observation points are Hawk Cliff, ..., and Hawk Mountain in the north Appalachians, where the raptors follow the mountain slopes that have the best upwinds”. Hawk Mountain is of most concern here, representing one point along the Appalachian range which is aligned SW-NE from Alabama (USA) to New Brunswick (Canada). In some cases migrating raptors soar and glide for hundreds of kilometres along terrain features that produce updrafts, for example, along the parallel ridges of the Appalachian Mountains (Mandel et al, 2011). Work in eastern USA (Bohrer et al, 2011) showed that Turkey Vulture nearly exclusively used thermal lift, whereas Golden Eagle primarily used orographic lift during migration. The same study showed that the strongest orographic uplift is associated with strongly negative thermal uplift, because the strong winds required for orographic uplift produce shear that tends to break apart thermal formation. Golden Eagle were often observed in the study area migrating in late autumn on days with high winds and heavy overcast conditions that immediately follow the passage of regional cold fronts. In Europe most attention has been paid to thermal lift, with many studies around the Mediterranean where the sun is relatively strong and the summer season long. However, with the rapid development of wind farms, it is necessary to develop models of the risks associated with birds colliding with the blades. In such models it is necessary to include both thermal and orographic lift.

Migratory large raptors breeding in northern Britain number just two: Osprey and Honey-buzzard. The former is not a typical broad-winged raptor migration-wise being capable of long sea crossings with much flapping flight; the Osprey perhaps should not be classified anyway as a broad-winged raptor as it has narrow wings with only 4 protruding primary tips visible in flight. The Honey-buzzard is a typical broad-winged raptor migration-wise, avoiding long sea-crossings and concentrating at places such as Gibraltar, Sicily and Malta to minimise each leg over the sea. In Northumberland the Honey-buzzard breeding season is late with juveniles leaving the nest at mid-August in lowland sites and through into early September at upland sites. More limited observations in Scotland suggest a similar lateness there.

While a few adult Honey-buzzard do leave Northumberland at the end of August or in early September, many birds hang on to mid-September to look after the young of the year before departing. By this time as observed above thermals are getting weaker and the duration of strong sunshine in the middle of the day very short. The birds therefore may well turn to orographic lift to facilitate their escape. The obvious source of orographic energy is the Pennines, which conveniently runs N-S in the desired direction for the exit. The SW Northumberland study area for Honey-buzzard lies just N of the North Pennines, with some of the valleys in the study area (Devil's Water, Allen, upper South Tyne) also oriented N-S. The prevailing wind in the area is westerly (SW in warm front, NW in cold front) in September and the wind is often strong at this time, with breezes of 20-35 kph commonplace. Hence there is a strong updraught on the E side of the valleys in the Pennines, which might support the birds' flight for the 120 miles of the length of the Pennines, from just south of Haltwhistle/Hexham/Bywell in the north to the Peak District in Derbyshire in the south. If the wind was steady enough the birds could actually move onto the extreme W of the Pennines and ride the ridge lift on the steep slopes of the dramatic Pennines escarpment. In W/NW winds following rain, the visibility is normally very good, encouraging the birds to be more adventurous. These ideal conditions occur after the passage of a cold front as is well known to raptor watchers at Hawk Mountain in the Appalachians. The heavy overcast conditions cited above for the Appalachians will only occur very briefly after the passage of a cold front in northern England.

The birds will wait for good conditions of a clearance in the weather on W winds but what if these are not forthcoming. It is believed that 2000 and 2008 were years in which the birds decided they could wait no longer to get away. As the weather cleared to some extent the winds were easterly. It could be argued that the birds could go to the E side of the Pennines for orographic lift but there are a number of reasons why this is not possible. Visibility is often very poor in the Pennines on E winds making navigation difficult. And E winds often do not have the body of W winds making reliance on them for orographic lift more hazardous. Even during normal hunting the birds are very reluctant to get up in E winds and ride the ridges in the same way as they do in W winds. Further there is no clear escarpment on the eastern side of the Pennines: the moors tend to merge more gently into the lowland areas than on the western side. So the birds move towards the coast where in Durham and Yorkshire they can get orographic (obstruction) lift from the E winds flowing over the the sea cliffs. Further south the cliffs are replaced by sand dunes through Lincolnshire but there will still be some lift from the wind striking the slightly elevated shore, or even buildings. In Norfolk and Suffolk the birds can either carry on following the shore or cut across the land: energy-wise there is probably little difference. The lift the birds get here is exactly that obtained by gulls such as Herring Gull, Lesser Black-backed Gull and Great Black-backed Gull which enables them to glide along the coast effortlessly at right-angles to the E wind. Indeed confirmation of this idea came to me in a recent visit to Teignmouth, Devon, in November 2011, when I saw the large gulls effortlessly gliding past at right-angles to a strong SE breeze. There is also a navigational bonus to be obtained from following the line of the coast: the birds will not get disoriented in poor visibility.

The relief map shows the suggested migration routes across the UK from major Honey-buzzard breeding concentrations. These routes are a modification of the Southwards Migration Model to reflect the fact that while the movement is basically south, it may lean to the west or east depending on the wind direction. Perhaps counter-intuitively the birds move to the W in westerly winds and to the E in easterly winds, because of the goal of maximising orographic lift. The routes are broad-brush: false precision has not been attempted. Route W shows the ideal western route using orographic lift along the W side of the Pennines and ending up in the Isle of Wight area before crossing the Channel to Normandy and the Channel Islands. This is a very favourable route from the energy point of view and also safe as it avoids the perils of the SW peninsula which slopes away into the Atlantic, potentially encouraging birds into a hazardous long ocean crossing to France (or even Spain). Route E shows the northern part of the eastern route adopted in 2000 and 2008, making use of the orographic lift from sea-cliffs and to a lesser extent from the shore. Route E splits into E1, that adopted in 2000 and going through London and Sussex to Normandy, and into E2, that adopted in 2008 and going through Norfolk and Suffolk to Benelux. Route S is plotted from the Tay Valley, a very significant area nationally for Honey-buzzard, across the Firth of Forth at Queensferry and then splitting into at least two routes, one (S1) going almost due S coming into the Haltwhistle/Alston area of the North Pennines and the other (S2) going further E coming into the Tyne Valley E of Hexham. Both S1 and S2 are safe; a more westerly route such as that observed for birds satellite-tracked from Inverness is too close to the Atlantic for comfort.

Nick Rossiter

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