164 | Chapter 9
the booby, head raised uncomfortably in flight, croaks. This may be an-
noyance, it may be appeasement. But the cruel truth is that the frigate-
bird will only be appeased when the booby coughs up the day’s catch.
It does and the frigatebird immediately forsakes harassment to swoop
downward and intercept the vomited fish before it falls into the sea. In
less than a minute, the booby has lost the fruits of a day’s labour. The
frigatebird is the victor.
While such piracy is conspicuous because it mostly happens close to
shore, attacks are more likely to fail than succeed and it seems only a
small proportion of the daily food of most frigatebirds is obtained by
this technique. In fact, frigatebirds secure most of their daily fare far
out to sea when they drop down from high altitude several times a day.
Thanks to frigatebirds thrice- burdened, with GPS trackers, heart- rate
monitors, and accelerometers,^1 a graphic picture of what happens during
these foraging episodes is now emerging. Occupying no more than 10
percent of the day, these episodes bring the birds to within 30 m of the
surface. Heart rate, around 200 beats/minute in soaring birds, increases
2– 3 fold when birds are at the surface. Wing beat frequency, barely mea-
surable in soaring birds, goes up to about four beats per second. Given
the importance of flying fish in the diet of frigatebirds, it seems certain
that these frantic surface forays give the frigatebirds the opportunity to
grab flying fish.
Another flying fish specialist of the tropics is the Red-footed Booby. 2
When in full foraging mode, birds are continuously landing, or diving,
at a rate of 30 landings or dives per hour, including 4.5 dives per hour
under the sea surface. But the dives are shallow: never more than 2.4 m
underwater. However sometimes accelerometers pick up dive- like move-
ments that are not linked to submersion. These probably record the
moments when the boobies are pursuing and capturing flying fish and
squid at or even above the surface.
Let us lower our focus slightly downwards into the water where a
multitude of species catches their food at the surface, or by shallow dives
from the surface, or plunge dives from a modest height. Think prions
skimming off small crustacea from the surface of the Southern Ocean,
or skimmers that fly close to the surface with their lower mandible
ploughing a watery furrow. When the bill strikes prey, it snaps shut by