176 | Chapter 9
examination of the plumage showed that each cormorant body feather
has a loose, instantaneously wet, outer section and a highly waterproof
central portion.^27
There is another cormorant species we have already met, the Imperial
Shag that is confined to cool waters of the Southern Hemisphere. It,
too, has a waterproof inner plumage that traps air and hence heat. The
trapped air creates less buoyancy and is therefore less troublesome on
deeper, as opposed to shallower dives. That is probably the reason why
Imperial Shags are deeper divers than most other cormorant species of
lower latitudes.^28
Because of air trapped in the plumage, plus that in the lungs and air-
sacs, seabirds are positively buoyant when first submerging. Their natu-
ral tendency is to float back upwards. Therefore they have to work hard
to reach greater depths. As the birds reach those greater depths, so the
air compresses and eventually a depth is reached where they are neu-
trally buoyant. This depth depends on the quantity of air in the lungs
and airsacs at submergence; the more air to be compressed, the greater
the depth of neutral buoyancy. In fact penguins anticipating diving
deeper take in more air before submerging which then allows a longer
dive and increases the depth at which they are neutrally buoyant. There-
fore, rather neatly, there is a close correlation between diving depth and
duration, as was evident from the stand- out numbers earlier in this
chapter.
Having, in Rory Wilson’s evocative descriptions, pedalled down-
wards, the bird faces an ascent that is relatively easy free- wheeling. This
is simply because the compressed air expands at shallower depths, in-
creasing positive buoyancy and serving to thrust the bird upward at
an ever- faster speed. For example, Magellanic Penguins beginning their
ascent from 50 metres initially ascend at a little more than half a metre
per second. When they have reached 20 metres, the ascent rate has
topped a metre per second.^29
This rapid freewheeling ascent has two consequences. The first is that
the bird often emerges rapidly onto the surface, like the proverbial cork
popping out of the bottle. This is of no particular consequence if the
bird is in mid- ocean. It is far more significant when the penguin emerges
onto ice – and familiar indeed are TV sequences of penguins apparently