how seabirds catch food | 169supremely able to catch prey on the darkest nights. Very probably the
fact that the principal prey, hatchetfish and lanternfish, are biolumines-
cent assists the petrels’ hunt.
Nonetheless a follow- up study checked how at- sea activity during in-
cubation varied with phase of the moon. When the moon rose during
the night, as happens with a waning moon, the amount of time in flight
increased by about ten percentage points. The increase coincided with
the moon’s rise, and not time of night. The reverse occurred during a
waxing moon; when the moon set in the middle of the night, flight ac-
tivity dropped.^9 Barau’s Petrel is another species more active at night
when the moon is showing.^10 It is, however, difficult to tell whether the
increased activity of the birds under moonlight is because it is easier for
them to catch prey, or because the prey takes precautions and is more
wary about approaching the surface when the moon is visible, obliging
the birds to work harder.
***
It is now time to venture further underwater. Quite a lot further. Of
course, the maximum depth reached by seabirds varies according spe-
cies, so I shall initially aim to sketch the depths attained – or plumbed –
by the more adept divers among the seabird community. These depths
also raise questions about how the birds cope with underwater physio-
logical stresses that would tax or indeed kill any human diver. Finally I
shall investigate when, during the dive, the prey is caught.
Despite sometimes hitting the water at up to 100 km/h, and lacking
external nostrils (in order to avert the risk of water going up their noses),
gannets and boobies are not especially deep divers. For example Blue-
footed Booby dives average about four metres with females reaching
slightly greater depths than males. That sexual difference is repeated
in Northern Gannets where the females’ mean depth is 4.7 m and the
males’ mean 3.2 m. The maxima are 18 and 11 m respectively.^11 In both
species, the underwater profile of the dive can be either V- shaped, where
the bird dives to maximum depth and immediately returns to the sur-
face or U- shaped, where the bird flaps its wings and probably chases prey
at the maximum depth.