Friday, 18 September 2015

"Side By Side" - FR4 and VY on migration

VY and FR4 comparative migration routes – Autumn 2015

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Of the entire European population of ospreys, only a tiny proportion are
satellite-tracked every season. Of these few, it's always found that the juvenile birds follow very different migration paths – for ospreys are solo travellers and each individual chooses its own direction. But HOW do they choose these routes?

This year (2015), we are tracking Blue VY from nest site #1 at Kielder Forest. She left “home” on 30th August and made steady progress southwards, for conditions were favourable at the time. Consistent breeze from the north and good visibility are all that a young osprey requires at this time of year.

By coincidence, another (and unrelated) juvenile started its migration at almost the same time: Blue FR4 from the Scottish Wildlife Trust nest at Loch of the Lowes passed through 55ÂșN about one day behind VY. The latter remained “in the lead” as the birds made their way southwards – but FR4 gradually caught up, helped in no small measure by VY's evident reluctance to make early starts! By the time both were in southern Spain, their tracks had converged both in distance and time. At roost on the 8th September, the pair were on identical courses and only 18 km apart. (Even so, they remained totally unaware of each other's existence, and would never meet.)

This unusual situation provides some insights into the factors that might influence a juvenile's choices during migration, and the most obvious of these is weather – or in this particular case, the absence of it...

With such good conditions prevailing, both ospreys were able to follow their instincts and take the most efficient and direct routes southwards. This included the “inland” route over the western Sahara desert, which is much shorter than following the African coast as many previous tracked birds had done. However, such a “routine” migration track did not cast much light on the behaviour of individuals – until, that is, the youngsters made it into southern Mauritania...
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On the afternoon of the 14th, the young ospreys encountered their first real “problem” on this migration. The rainy season in sub-Saharan Africa was well under way – and that means variable monsoon winds out of the west and slow-moving low pressure systems that track along the desert margins. One such system was over the Senegal River valley as VY and FR4 arrived, and they found that the helpful northerly winds which had borne them all the way from Britain were gone.

As a result, our ospreys had to adopt a different flight strategy and it was the same in both cases – a course alteration SE to avoid the low pressure with its rain and poor visibility. Once across the river, they were able to turn south again, then back towards the SW on the 16th as the weather conditions improved.

It's important to understand that the birds were not being “blown off course” at any stage. They were simply adapting their route and direction to take account of the winds and other conditions at the time – just at all other times.

And this is why birds that migrate independently can sometimes follow the same path. 

Images: Google Earth
VY Data: Forestry Commission England