VY and FR4 comparative
migration routes – Autumn 2015
[Click for larger]
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?
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.
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 55ºN
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.)
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...
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
[Click for larger]
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
as the weather conditions improved.
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