how seabirds catch food | 171to complete hundreds of dives per day when absent from their colonies.
To pluck a couple of examples, the maximum depths reached by Manx
and Sooty Shearwaters are 55 and 70 m, although the average depths are
far less, respectively 10 and 16 m.^16 In general, the heavier a shearwater is,*
the longer it can remain underwater and the deeper it can dive. Pro-
vided the comparison is made between similar creatures, this is true of
other seabirds, and indeed more widely of marine mammals. The reason
is that the larger animals have greater oxygen storage capacity while
their metabolic rate per unit weight is lower – which allows the oxygen
taken underwater at submergence to last that much longer.
Within that framework, the pattern shown by the auks, all of which
are divers, broadly follows expectation. The Little Auk or Dovekie,
weighing roughly the same as the diving petrels, reaches a maximum
depth of about 25 m and remains underwater for a maximum of 1.5 min-
utes. The largest extant auks, Common and Brünnich’s Guillemots, weigh-
ing about one kilogram, can dive to 80– 90 m^17 or even 170 m,^18 and re-
main underwater for 2–3 minutes. We can be confident that those values
would be comfortably exceeded by the extinct Great Auk!
The underwater feats of shearwaters and auks pale in comparison to
those of the penguins where the relationship between diving ability and
size is again apparent. The smaller penguin species may be able to re-
main underwater for around five minutes and reach a depth of 100 m. It
is the two largest species that amaze. Early hints of their abilities came
from pioneer work in the 1970s and 1980s by Gerald Kooyman of Cali-
fornia’s Scripps Institution of Oceanography. Now we know King Pen-
guins can reach over 300 m, but they must concede pride of place to
Emperor Penguins. In 2007 Barbara Wienecke and colleagues from the
Australian Antarctic Division published a paper with a title ‘Extreme
dives by free- ranging emperor penguins’. It is a title that smacks of an
extreme fishing programme on TV, and yet it is a sober report of science
on a near- industrial scale. I can only quote from the paper’s abstract.
“We examined the incidence of extreme diving in a 3- year overwintering
study of emperor penguins Aptenodytes forsteri in East Antarctica. We
defined extreme dives as very deep (greater than 400 m) and/or very
- (^) Sooty Shearwaters (800 g) weigh about twice as much as Manx Shearwaters (420 g).