178 | Chapter 9
If diving seabirds tend to be freewheeling on their ascents, this may
give them the extra agility that aids grabbing prey. In contrast, the hard
work of pedalling downward might hint that the descent is not the ideal
time for hunting and catching. In fact two contrasting approaches are
now yielding information on when, in the course of dives, birds actually
catch and ingest food. The first are beak- mounted sensors that detect
how widely apart the upper and lower mandibles are. Opening the bill
is obviously essential for catching prey, and the bigger the prey, the
more widely the bill must open. The second is the attachment of accel-
erometers which register the bird’s movement in three dimensions. Prey
capture is likely to be linked to sudden movements, contrasting with
the relatively steady movements of, say, the descent phase of a dive. If
information from the two channels can be combined, so much the more
informative.
Rory Wilson achieved this combination with Magellanic Penguins.
Around 90 percent of prey was captured during ascents. These were not
necessarily the principal ascents back to the surface. More often they
were small upsurges or undulations when the penguin was at depth in
the middle of a dive with an overall U- shaped profile. Moving in for the
kill from below the prey may not only give the penguin an advantage in
buoyant mobility but it may also be easier for the bird to spot its poten-
tial catch when that prey is silhouetted against the lighter background
overhead.
The level of detail that can be gleaned from accelerometer data is re-
markable. For example, Marianna Chimienti of Aberdeen University
attached the devices to Razorbills and Common Guillemots breeding
at Scottish colonies.^32 Razorbill dives were consistently V- shaped in
profile, and relatively shallow, to no more than about 10 m. Prey was
predominantly caught during ascents. This underwater behaviour was
clearly different from that of the Guillemots whose dives were of two
types. They undertook shallower dives, to 10– 30 m, where prey was
caught either at the greatest depth or during the ascent. Alternatively
the Guillemots went deeper, to around 50 m. The profiles of these dives
were clearly U- shaped. During the bottom phase of roughly constant
depth, the birds were either searching for or catching prey.