don't ospreys migrate together as a family unit?” It
has almost assumed the status of Frequently Asked Question, and the
answer is simple...
It's because they're ospreys...
other species of birds, for example geese and wild swans, do exactly
this: they travel from their breeding sites in family groups, with
the adults escorting their offspring to the wintering grounds,
hundreds or even thousands of miles distant. Without this guidance,
the fledglings would never know where to go. But in evolutionary
terms, the logistics of this have a very specific result: in a given
population and after only a few generations, all the birds end up
wintering in the same place! For grazing birds, this is a perfect
survival strategy. They assemble in a huge flock, food is not a
problem, (grass doesn't run away or hide) and there is safety in
numbers with many eyes watching for danger. But for ospreys and other species that hunt a limited resource, this system won't do at all. They are apex predators, not flocking geese...
thousand ospreys arriving on a single African lake would exploit the
local fish supply at an unsustainable rate. To solve this problem,
evolution has arranged matters so that young ospreys have to find
their own way in Autumn. They are born with all the navigational
instincts needed to do this, but built into those same instincts is
sufficient positional uncertainty to ensure that the young birds are
dispersed over a wide wintering area. This random end-point factor (referred to by scientists as a “stochastic element”) is one of the hidden secrets of osprey migration and is instrumental in forming many aspects of their post-juvenile behaviour.. It has evolved because a general dispersal in winter quarters gives the incoming youngsters a slightly better chance of survival as individuals.
But where does this "randomness" in the selection of a final destination come from? That's a whole other story and really needs an article all of its own.