44 | Chapter 3
substantially delayed start to breeding, over half of all individuals alive
may be in their pre- breeding years.
In the previous chapter, I recounted how Black- legged Kittiwakes
fledging from Svalbard variously went, in their early years, to the north-
ern regions of Baffin Bay to the west and to Novaya Zemlya to the east.
They may even have visited other colonies during those early wander-
ings. Børge Moe’s study hinged on the recovery of geolocators up to four
years after deployment. Such long- term deployment and eventual re-
trieval is currently exceptional. Probably this will change as battery lives
are extended.
Instead of following birds from fledging, an alternative means of
checking behaviour during the years of immaturity is to attach devices
to young birds as soon as they (dare) set foot in a colony. This may be
less than ideal; the birds have been at sea, their whereabouts unknown,
for several gap years. Nonetheless, it potentially allows the checking of a
period in their lives when, following the arguments sketched above, they
may still be behaving like teenagers, exploring, perhaps visiting other
colonies, and improving their feeding skills.
There are three principal means of identifying these young birds at a
colony. The most conclusive is the presence of a metal ring applied to
the leg when they fledged. This has the great advantage that it means the
age of the bird is precisely known. Another possibility, available only
for a minority of species whose feather patterns change with age, is to
use the plumage pattern as a tolerably accurate signal of age. For exam-
ple, this works for Northern Gannets and Wandering Albatrosses and
other great albatrosses in the genus Diomedea. Finally the behaviour of
immatures is different from those birds actively breeding. The former
may spend time in groups, so- called ‘clubs’, while the latter scurry like
responsible employees between home and hearth and their workplace,
the sea. The risk here is that birds of breeding age, taking a year off for
whatever reason, may mistakenly be included in the sample of putative
immatures.
While most of the information I am about to describe comes from
electronic devices, there is another important, slightly indirect, route
to knowledge. Stable isotopes [see Box], extracted from minute samples
of a bird’s blood, muscles, bones or feathers, can provide supplementary
information on its diet and travels.