where seabirds find food | 147ern Tropical Pacific, when productivity is high, flocks above tuna are
dominated by boobies, with petrels scarce.^3 As productivity falls, the
flocks are composed mostly of Juan Fernández Petrels and Wedge- tailed
Shearwaters. At the lowest productivity, a species with very low flight
costs, the Sooty Tern, forms the core of the flocks, which then include
few petrels. This pattern, to which we shall return, makes sense if, at low
productivity, the only species able to exist are those with the low flight
costs that enable cheap transit between rare and scattered food patches.
At higher productivity it becomes possible for larger species to balance
their energy budgets, and their presence excludes the smaller species. This
exclusion likely happens in two ways, either the boobies jostle aside the
smaller petrels and terns by their sheer physical size, or the boobies dive
to catch prey at depths out of reach of the more aerial species.
In addition to vision, there is another sense that helps seabirds find
food. It is the sense of smell, and it is especially useful for petrels and
albatrosses, birds where that part of the brain involved in smell, the ol-
factory bulb, is notably large. Exploiting this sensitivity, offshore bird-
watching trips routinely use chumming to lure rarely- seen petrels into
view. They approach the vessel, and the delighted birders aboard, by
flying upwind into the odour trail. Not only can petrels pick up the
fishy smell of chum (and, incidentally, the smell of their own species),
but they can also detect the chemical dimethyl sulphide, which is pro-
duced by phytoplankton in response to grazing by zooplankton and po-
tentially provides petrels with a clue to the whereabouts of prey.
If birds are using smell to find food items, they might fly across the
wind in the hope of detecting an odour plume drifting downwind from
the food. Having detected the plume, they would then turn into the wind
and follow the odour trail to source, possibly zigzagging as they ap-
proached. When Wandering Albatrosses were fitted with GPS- loggers to
record their precise position and stomach temperature sensors to record
the moment when food at Southern Ocean sea temperature entered the
stomach, it transpired that about half the food items reached by flight
(as opposed to by swimming) were approached directly.^4 The other half
were reached by an abrupt turn that was followed by a direct or a zigzag-
ging approach to the item, exactly as would be expected if smell was a clue
to the possibility of lunch. On average the turn towards the point of even-
tual prey capture occurred at a distance of 1,300 m if the final approach