144 | Chapter 8
minimum when it is in a prime feeding region. That minimum displace-
ment rate can identify feeding areas.
While movement patterns gives powerful clues to likely feeding,
other devices give more direct evidence. Depth gauges – of which more
in Chapter 9 – are telling. Very evidently, a species that feeds by diving,
say a Common Guillemot or Sooty Shearwater, is far more likely to be
feeding when making repeated dives than when floating on the surface
or flying. Combine the depth recorder with a position, and it is possible
to make fair inferences about where the diving bird is feeding. If the
basic depth information can be combined with accelerometer informa-
tion, providing a picture of the twists and turns during an underwater
chase, then the chance of identifying actual feeding episodes improves.
For species that are predominantly aerial, say albatrosses, the num-
ber of landings per hour and total time on the water, as detected by tiny
leg- mounted saltwater sensors, can reveal where a bird feeds. The prin-
ciple is straightforward, but the complications are considerable. For in-
stance, it may not be obvious whether a night spent on the sea is a night
of feasting or of snoozing.
Finally, as mentioned in Chapter 1, there are devices which record
when a bird opens its mouth and by how far, and devices which record
a drop in stomach temperature. This drop will occur when food, roughly
at sea temperature and thus cooler than the internal body temperature
of a warm- blooded bird, is swallowed.
***
It could be that a seabird’s approach to food- finding is supremely crude,
to traverse the ocean in the hope of encountering, or perhaps I should
say blundering into, food. The evidence that certain areas are consis-
tently used would argue against this. So too would observations of birds
streaming away from colonies in formations that speak of order. Half-
way between Norway’s North Cape and Svalbard, I remember passing
Bjørnøya. The island was barely visible in the raw Arctic mist. That poor
visibility was no deterrent at all to the Guillemots, both Common and
Brünnich’s, that streamed past our ship in organised lines or chevron
formation as they left the colony. Surely these birds knew where they