the meandering years of immatUrity | 51take him south to the icebergs, around 60°S. A direct consequence of
this age- and gender- related effect is that the stable isotopes measured
in the two sexes increasingly diverge as they age, from no differences in
immature birds to major differences in old (>30) birds, reflecting the
males’ use of colder seas. Exactly why the males move southward with
age remains a puzzle.^10 A dismal explanation is that they are forced south
by competition with more vigorous males still in their twenties. A more
positive explanation is that they benefit from the stronger winds, reduc-
ing travel costs in those higher latitudes.
Many of the details of birds’ activities in those early years before they
re- appear at a breeding colony remain sketchy. The omissions will doubt-
less be somewhat rectified as batteries for geolocators become longer
lasting, and the use of long- lasting solar- powered PTTs becomes more
widespread. But it will certainly remain challenging to document the
daily routine of an animal that has no reason whatsoever to come within
100 kilometres of an eager researcher. Only when the bird needs to make
landfall to breed does the advantage shift towards the researchers. They
will be waiting – and the findings already secured are astounding.