174 | Chapter 9
starlight, conditions they would eschew by day.^23 Whether the birds
have the visual abilities to detect prey from afar in such conditions is
uncertain. The alternative possibility is that the Capelin are sometimes
packed in such tight shoals that random encounters are sufficiently fre-
quent to make the guillemots’ dives worthwhile. But I do admit that the
idea that a guillemot might dive at night in the hope of bumping into a
fish sounds bizarre. Possibly the idea becomes less bizarre when one re-
members that the water temperature at 50 m off Newfoundland is close
to freezing, chilly enough to render Capelin somewhat sluggish.
While any tendency by seabirds to dive less deeply when it is darker
could be driven by light – or its absence – another factor might be ver-
tical migration of the prey. If the prey are closer to the surface at night,
it would be unnecessary to then dive to daytime depths. And Capelin
do indeed migrate, concentrating around 20 m below the surface by
night, and sinking beneath 200 m by day.
Also migrating vertically are the lanternfish prey of King Penguins
which, like the Guillemots, dive less deeply by night. During the night,
shallow dives under 30 m are the norm for King Penguins, whereas deep
dives, typically to 100– 200 m, occur only during daylight.^24 However,
lanternfish are well scattered during the night, and so any random ap-
proach to capture would be unlikely to work for the King Penguins. But
the lanternfish, unlike Capelin, are bioluminescent, so it may be that
individual fish can be detected from afar by King Penguins.*
Diving to any sub- surface feast necessarily poses problems. An obvi-
ous problem is that, when underwater, the bird cannot breathe and
must eke out those oxygen stores with which it submerges for as long as
possible. Since this is not a physiology book, I shall only sparsely outline
the adaptations that help the diving bird overcome the difficulties.
These adaptations have been most extensively studied in Emperor Pen-
guins. Given its size, and therefore predictable rate of oxygen consump-
tion, an Emperor Penguin could remain underwater for about five min-
- (^) Lanternfish are also a main prey of Southern Elephant Seals that share many a sub- Antarctic
beach with King Penguins. The peak sensitivity of the vision of female Seals is at the very wave-
length at which lanternfish shine. http://dx.doi.org/10.1371/journal.pone.0043565 (accessed 14
June 2017).