Question: why start off south-west?

Graham Ramsay emailed as follows: There appears to be a strong western component to the initial movement of the translocated birds. Why should this be? If they were still in Scotland this would take them over into Ireland. Are there many sightings of migrating Ospreys in Ireland? Several birds do appear to go SW from Rutland Water (as shown in the map on the right.) One theory is that they initially follow the valleyof the Welland River . There is thought to be a cross-country fly-way running from the Wash to the Severn estuary that is used particularly by waders. This route roughly follows the course of two rivers, the Avon and the Welland. Perhaps some of the Ospreys also use this route? We now have some limited evidence that Scottish juveniles may also fly SW initially (see the maps of S08 and S11) and there are certainly some reports of migrating Ospreys passing through Ireland. Notice that at the heart of Graham's comments is the question as to whether migration routes are inherited as opposed to being determined by geographical factors. Duncan Halley of Norsk institutt for naturforskning in Trondheim, Norway, took up this point as follows:

 

Much experimental evidence (mostly on passerines) suggests that migration orientation is under rather strong genetic control. In this connection, it is worth noting that the Scottish population was founded, and quite recently, by immigrants from Sweden and Norway. Assuming that birds tend to retrace their route, roughly speaking, on their return, this raises the possibility that the founders of the Scottish population were dis-proportionately birds with a strong (unusually strong?) tendency to move SW on the first leg of migration, and so be likely to hit Scotland on their return migration in spring. Their descendants would have inherited this propensity, although now it is probably selected against, given it may involve long flights over the sea with no resting place.

It would be interesting in this context to know what the pattern of initial movements in the source Scandinavian population is. A recent study of two satellite tagged adults from Eastern Sweden showed an initial SE movement, in one case to the Volga, followed by a turn to 190 degrees, so that one bird arrived at wintering grounds on the Ivory Coast and the other (Volga) bird at Cabora Bassa dam in Mozambique. The Scottish birds also seem to pursue a roughly 190 heading later in their migration.

See also a summary of two papers on Swedish migration.