Potential Models for Migration of Honey-buzzard
Below three potential models are described for the migration of Honey-buzzard through Britain.
The Southwards Model is very simple consisting of a single line. In this model it is thought that Honey-buzzard have a strong southward urge in their migration, with deviations only to avoid water crossings. For juveniles the southwards thrust is thought to be an instinct. Adults in moving south also use their experience to select the best route, such as to avoid long sea crossings of the Mediterranean which consume valuable fat reserves. So there are massive concentrations of the birds in autumn at Gibraltar and the Bosporus. Physics therefore supports this model as it is energy efficient, helping the birds to survive the migration. The model is also supported by observations across the whole of continental Europe: except for detours around water, the direction is south. The model does require a source, that is sizeable populations of Honey-buzzard in northern Britain, but these have been found in Northumberland, Cumbria and Scotland where a population of 50 pairs was postulated in Birds of Scotland. The actual figure for Scotland is surely much higher. The model matches reality as Honey-buzzard populations in northern Britain were very small until the early 1990s, then increasing steadily until the present time. The extent of the movement seen each year will vary with productivity, weather and observer effort. High numbers of juveniles produced, as in 2008, increase the size of the movement. Winds against force the birds lower to the ground and slow down their exit as in 2000. Once a significant movement has started, the speed of modern communications ensures that observers are soon scanning the skies for more (as in both 2000 and 2008).
A modification of this model was developed in 2012 to bring it closer to the physics. In particular two different modes of lift were identified for Honey-buzzard – thermal and orographic – with the latter considered more important in the autumn movements as the sun is becoming weaker by mid-September. Under normal autumn conditions of fresh W winds over the Pennines the Honey-buzzard will use orographic lift to quickly navigate the hills. If suitable W winds do not occur, the birds will wait but may leave on the onset of E winds to the east coast both for orographic lift on the wind striking the coast and for navigation in the often ensuing dull conditions. The significance of orographic lift is discussed in detail elsewhere, with a map of the suggested southwards routes through the UK, one leaning to the W on westerly wings and the other to the E on easterly winds.
People who speculate that the Honey-buzzard migrants come from the continent are often assuming a migration pattern like that in the N model. So after building up fat reserves and moving S in economical soar-glide mode, the birds then move NW across the North Sea consuming 20 times more energy per kilometre in flapping motion over water than soar-glide over land and exhausting their fat reserves. Without any ado (rather like the killer picking himself up in the film Halloween!) the birds then fly S as if nothing has happened, in soar-glide motion again. This scenario is ridiculous. If a bird did manage to cross the North Sea this way, it would be exhausted and would have to replenish its fat reserves before resuming migration. So we should have seen hundreds of disoriented birds keenly feeding in woods and fields in NE England. There are no such reports. The pattern above increases strongly the likelihood of a failed migration, leading to the death of the bird. Neither adults (who know the way) nor juveniles (who have a strong instinct to go S) would allow themselves to be drifted this way.
Any model should be testable. The N model can certainly be tested on a pretty good series of data going back to the start of the 20th Century. The model has a mismatch with reality in all but 2 out of about 110 years with no significant Honey-buzzard movement in Britain being very much the rule. Northern European populations have been fairly stable during this period, even declining to some extent recently, so these are approximately a constant. In 2000 and 2008, there have been significant movements in Britain but these have been associated with the absence of source populations on the near continent, no records from North Sea oil rigs (in 2000 and 2008) and apparently fit populations on southward migration in Britain. The model also involves the birds in a very dangerous migration strategy by the laws of physics. Realising these problems a few proponents of a continental origin have produced a vaguer model in which somewhere to the east, perhaps a long way off, the birds are somehow hoovered up into the sky by some freak weather and forced to circle around for a number of days, before landing on the east coast of Britain in pristine condition. The rules of this model are that the origin of the birds is unknown, the birds cannot land anywhere between their origin and Britain and the time in the air need not be specified. This model is termed the Spiral Model. Like the N model this model has only matched reality in 2 out of the last 110 years. It could be argued that it cannot be ruled out but this is because the model is so vague that it cannot really be tested. From the physics viewpoint, this model is very unsatisfactory: how do the birds survive such a long time in the air without food? Really this model is so fanciful and weak that it is better classified as faith, not science!
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