how seabirds catch food | 173ing krill, there is a coincidence between catching individual krill, as re-
corded by back- mounted cameras, and side- to- side head movements as
recorded by accelerometers, exactly as one would anticipate if the birds
were spotting and then picking off the krill one by one.^20 In early 2016,
Jonathan Handley, a doctoral student at South Africa’s Nelson Mandela
Metropolitan University, attended the Student Conference in Conser-
vation Science at Cambridge University. He played video clips obtained
from cameras back- mounted on Gentoo Penguins going about their un-
derwater fishing business in the Falklands. When the penguins caught
rock cod and lobster krill, the audience burst into spontaneous ap-
plause. Everything indicated that the penguins used vision to find those
prey underwater, as was true in a remarkable instance of underwater
piracy also recorded by Handley – or more accurately by the cameras
aboard his penguins. When one penguin caught a large squid* at a depth
of about 30 m, a second and then a third penguin joined the fray before
the captor could swallow its prize. In the melee, the squid was torn
apart and shared.^21 Again it seemed overwhelmingly likely that the pi-
rates saw the squid caught by their fellow penguin, and promptly aban-
doned good manners.
Using sight to detect prey underwater obviously requires light. Yet,
even on a bright day, minimal light penetrates clear water below about
200 m, which is consequently the absolute lower limit of photosynthe-
sis. Of course the waters within 200 m of the surface are darker still by
night, and yet there is little doubt that some birds are feeding under-
water at night.
Take the case of Common Guillemots collecting food, such as spawn-
ing Capelin, for their chicks. Diving by day to 100 m off Newfoundland
they encounter light levels equivalent to moonlight, but rarely dive to
depths where the light equals starlight. Underwater cameras reveal
that, by day, the Capelin caught by the guillemots are mostly isolated
individuals, not those in denser shoals.^22 That is compatible with visual
detection.
At night the picture alters. Although the Guillemot dives are rarely
deeper than 50 m, the birds are nevertheless active in the equivalent of
- (^) The length of the squid’s mantle, roughly the body, was 15 cm.