Sunday, 7 December 2014

The Migration Merry-go-Round

Migration in Detail - (Part 3)

Way back in 1999, I knew even less about ospreys than I do now. But I did know some things about birds and I had completed my foundation course in Meteorology, so when the Rutland translocation project came to my attention, I – and many other enthusiasts – followed its progress with increasing fascination. Some of the translocated juveniles were fitted with tracking devices. In those days, the units were primitive and highly experimental: they used ARGOS doppler signalling to calculate the positional fixes (which we now know to be a less-than-ideal method.) The trackers were as unreliable and short-lived as a politician's election promises – and only marginally more accurate. But they worked.

When one of the birds, a male tagged as R03(1999), showed some unexpected course deviations over the Sahara Desert, I suggested that he was avoiding a formation of adverse weather systems (which I had detected on weather satellite images) that had developed to the south-west of his position, and that these might affect his ability to use soaring flight while over the desert. This suggestion was put to the Men Who Knew A Lot About Ospreys at the next group meeting. It was not favourably received.

Ospreys on migration use thermals very little, if at all” was the conclusion of the MWKALAO, although they didn't offer any alternative theory to explain the bird's behaviour. Being a humble and uninformed amateur, I accepted this as being the expert view...

Until now

When the published track from Finnish osprey “Helena” came in for early October 2014, it seemed to show that here again was a bird using soaring techniques while over the desert.

Unlike the examples I produced for previous articles in this series, Helena's sample is not “slope soaring” or any other kind of terrain-following flight. There are no slopes at this point, no escarpments or ridges to provide an updraught. To me, these movements looked very like direct thermal altitude modification. But it was not quite conclusive. To be absolutely certain, I needed to get my hands on the actual data files for an osprey flying over Europe (where there are lots of GSM cell towers) so that the recorded level of detail was as high as could be possible with this new technology. 

Joanna Dailey, who volunteers with the Forestry Commission osprey conservation project at
Blue VV at nest Aug 2014. Blue UV in the background
(Image: Forestry Commission England)
Kielder Forest
, arranged for me to get authorised copies of their files for three juveniles. They were carrying GSM trackers and all were operating correctly. The hunt was on.

Two of Kielder's birds followed a mainly coastal and/or over-sea route for their migration. But the third, tagged as “Blue VV”, migrated over land and through central Spain. With light winds and hot sun in late summer, this is prime territory for the formation of thermals. Each day I had been carefully saving the weather charts for this region and these told me exactly when and where to look for the evidence. And little VV did not disappoint me.

Even position fixes at 70 seconds apart does not conclusively prove that spiralling flight has taken place – but adding in the direction of travel at each data point does. During this afternoon flight, there are eight other examples of the same behaviour, roughly at even time intervals.

So... were the Men Who Knew A Lot About Ospreys wrong, fifteen years ago? Well, not really, but they may have been misled by the early technology which - given the comparatively low number of cumulative fixes - seems to show birds flying in reassuringly straight lines across the broader landscape, when in fact they do nothing of the sort. As I have tried to show in this series, we are still learning things about the minutiae of bird migration...

… and there's a lot more yet to be discovered.

In the final part of this series, we will consider the physiology and layout of raptor wings, and see how evolution has adapted different species to have the appropriate “equipment” for their varying lifestyles.

Wildlifewriter acknowledges the use of tracking data supplied by the Natural History Museum of Finland (Luonnontieteellinen Keskusmuseo)and the Finnish Osprey Foundation, and data and images by Forestry Commission England.


Rutland Water Translocation History:
Kielder Osprey Blog (Joanna Dailey):
Bird's Soaring flight (Technical):

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