adUlt migrations | 77daylight is spent sitting on the water.* In other words, the birds quit the
South Pacific to spend their non- breeding period sitting out North Pa-
cific winter storms and enduring 16- hour nights. Natural selection does
indeed propel animals along curious routes.
While Clay and I failed to identify when Murphy’s Petrels moulted
during their North Pacific sojourn, this knowledge gap has been ad-
dressed in another recent study. A group from the French Centre Na-
tional de la Recherche Scientifique carried out a project on three small
petrels nesting in the sub- Antarctic Kerguelen archipelago.^34 The three
species were Antarctic and Thin- billed Prions and Blue Petrels. All
spend the non- breeding period, about 8 months, in the Southern Ocean.
During that time they moult, a process that lasts two and a half months
in Thin- billed Prions and Blue Petrels, a month longer in Antarctic
Prions. But how does the researcher assess when that shedding and re-
growing of the feathers is happening in birds weighing barely 200 g
fluttering among the white caps of the Roaring Forties?
For each species, the immersion sensors on the birds’ legs show a dis-
tinct period, corresponding to the moult, when the daily time with wet
feet climbs from two hours to almost ten. That is when the main flight
feathers moult. Unexpectedly this moulting period occurs in the early
part of the non- breeding period for Thin- billed Prions and Blue Petrels
and at the end for Antarctic Prions. As sketched earlier in the chapter,
the Thin- billed Prions have headed west from Iles Kerguelen to an area
south of the Cape of Good Hope and also south of the Antarctic Polar
Front. This is where they moult, as do the Blue Petrels. In stark contrast
- and this was confirmed by the stable isotope picture (Carbon - 13) – the
Antarctic Prions moulted about 15 degrees of latitude further north,
many of them east of Kerguelen towards Australia and New Zealand.
Besides pinpointing when and where the moult takes place, a task com-
pletely beyond the capacity of seabird biologists 20 years ago, the study
raises many questions. For example, why do such species as Thin- billed
and Antarctic Prions, similar species that are frustratingly difficult to
tell apart, organize their years in such different ways?
- (^) The interaction between flying, sitting on the water, and feeding will explored in Chapter 9.