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different species consistently and predictably use different tracts of the
ocean. At the extremes, the answers are trivial. Species that feed under-
water by working hard and chasing the prey at depth, for example pen-
guins and auks, need high prey densities. Otherwise their modus ope-
randi is untenable. Underwater prey density is low in the blue water
‘deserts’* of the Pacific, and there is no possibility of encountering a
penguin in the immense emptiness of the central South Pacific. At the
other extreme, I once travelled aboard the passenger ferry near Rum in
Scotland. The ferry closed on a mob of gulls and Manx Shearwaters, and
the skipper slowed the vessel as a Minke whale swam beneath the hull.
Leaning over the deck rail, I could look vertically down through the
clear water into the whale’s blowhole. There was clearly food aplenty to
attract shearwaters and gulls and whale. Where there is abundance, spe-
cies can amicably share feeding areas.
But between those extremes lies the norm, where species that are
modestly or trivially different in size or structure clearly use different
areas. Sometimes the differences in structure do yield a clue as to why
the species choose different sea areas, a pattern reminiscent of the dif-
ferent species gathering above tuna shoals in Pacific waters of different
productivity.
Take the case of Masked and Red- footed Boobies which often nest
alongside one another on tropical islands. The smaller, lighter Red-
footed nests in shrubs, the larger Masked on the ground. When studied
on Palmyra Atoll in the equatorial Pacific, Red- footed Boobies travelled
further afield (GPS data), perhaps because of their lower wing- loading
and lesser flight costs, and took more oceanic food (stable isotope data)
than the Masked Boobies.^21 Thus the structural difference between booby
species was plausibly associated with the use of different feeding areas
but the size difference between males and females was not reflected in
any comparable difference in the feeding habits of the two sexes.
Perhaps it is frustrating or perhaps it is intriguing to think of species
pairs where the two species clearly have different habits at sea – for no
very obvious reason. The list of potential species pairs is long. Here are
- (^) Called deserts because the surface waters are very low in nutrients, such as nitrates. Conse-
quently marine productivity is low, and bird food correspondingly scarce.