Aln’s migration to date has prompted a number of comments on this blog and elsewhere, many expressing concern that she is perhaps weak, or uncertain. Paul has written an article which we hope will aid understanding of Aln’s strategy:-
“We left Aln on Tuesday as she turned back from the coast and was heading northwards towards the highlands of Andalucia. This region – the Sierras de Cazorla – is a designated Natural Park and the largest single protected area in Spain. It includes at its heart a number of high-altitude collection reservoirs, most of them stocked with fish, and it’s for these that Aln is heading.
The Cazorlas are a remarkable place: a remote area of steep-sided valleys and pine forest, their limestone geology means that this is a biodiversity hotspot. Alpine plants grow here that are found nowhere else in Europe. The high valleys (“navas”) of the Cazorlas are home to rare species such as mountain ibex and the almost-mythical lammergeier. Almost 70% of the remaining population of Iberian lynx roam these mountains, which are now a UNESCO biosphere zone. (A distinction shared with the R Dyfi in Wales.)
But why has Aln turned back into this area, rare and beautiful though it might be? This map shows the contrast between her route and that of her classmate Archer (Blue 9L). Both birds have flown (approximately) the same distance overall – but Archer is already at the Senegal River whereas Aln, at the same range, was flying along the arid coastline of Cabo de Gato, contemplating a sea crossing of unknown extent…
The migration instincts of young ospreys come to them out of deep evolutionary time. They are the genetic legacy of thousands of generations of ospreys before them – and ONLY those who completed those migrations successfully and returned to breed, contributed to that legacy. This long and diverse genetic history means that there is no single “best answer” to the matter of undertaking migration. It’s much more complicated than that.
Instinct equips the birds with an interlinked suite of migration strategies. These can be selected according to the conditions being experienced at the time: Archer had good weather and helpful tailwinds, so her strategy was a fast, direct passage with over-sea routings for less energy expenditure. Aln did not have those helpful winds, so HER method is very different: it is the “fly- and-forage” migration type, as used by many other species. In this strategy, shorter daily distances are achieved, and the bird is constantly on the lookout for feeding opportunities. These are logged in memory, even if not taken up at the time.
The “fly-and-forage” option has advantages and disadvantages. It is more reliable, since ospreys aren’t trying to keep to any set timetable: the goal is to arrive in Africa but WHEN they arrive is not important – except that the other birds who were able to use the fast-passage option will have got there first, and might be occupying the best wintering sites. “Fly-and-forage” has another benefit, too. It does not depend on the weight-limited fuel supply (fat reserves) that the youngster started out with, and the drastic physiological changes that enable fat conversion during long-distance flight are far less important.
So we believe that “fly-and-forage” is what Aln is doing now, and it is the real reason that she turned back at the coast. Before attempting a crossing of the Sahara Desert (where no foraging is available) she will have to do an extended stopover to gather her resources. During her flight southwards, she had already identified the reservoirs of the Sierra de Cazorlas as a possible place to do this, which is why she has made her way back there.
Aln could stay in this area for a while but she is unlikely to spend the whole winter as some have suggested. It might well be a good autumn stopover site, but Cazorlas natural park is higher than 600m above sea level, with summits at over 2000m, and it regularly snows in late winter. It will be interesting to see what she does.”