148 | Chapter 8
was linear, and at 2,400 m if the final approach was zigzagging. The max-
imum distance albatrosses followed odour trails was an amazing 5– 6 km.
Yet these ploys reveal far from the full story of the birds’ search for
food. There is now overwhelming evidence that birds focus their feeding
attentions on certain specific areas of the ocean. Obviously these tend
to be areas where success is more likely, where food is concentrated.
That concentration can arise in several ways. It can arise because certain
marine features, say the underwater topography, divert water flows. If
the flow is towards or at least within reach of the surface, there is a pos-
sibility of concentrating both planktonic organisms and the larger ani-
mals that eat the plankton, and are themselves seabird food. Or – and
this alternative is by no means sharply delineated from the first – local
marine conditions may promote productivity. The amount of chloro-
phyll, the energy- capturing pigment that is an indicator of plankton
presence (and hence marine productivity), may be high, increasing the
likelihood of a concentration of seabird food. It is for this reason that
many studies have correlated seabird abundance at sea with chlorophyll
concentrations. Finally seabird food may be concentrated temporarily
by the underwater activity of fishes or whales and dolphins, providing a
brief bonanza for seabirds.
Where freshwater flows into the sea, there is also an influx of nutri-
ents, creating the conditions to foster plankton growth and, ultimately,
to attract seabirds. It is for this reason that the outflow of the Columbia
River, from the United States’ north- west coast, attracts great numbers
of Common Guillemots and Sooty Shearwaters. On the other side of
the Pacific, Wedge- tailed Shearwaters, GPS- tracked by Fiona McDuie
from the Heron Island colony on the Great Barrier Reef, may visit fresh-
water plumes, especially those of the Fitzroy River. Discharging along
the Queensland coast, the plumes enhance chlorophyll and presumably
also prey abundance.
It has long been known that the input of freshwater from melting sea
ice, which is of course fresh, creates a highly productive zone along ice
margins, sometimes far from land. These zones represent hotspots – if
the term can be permitted in such an icy context – for all sorts of wild-
life from whales to seals to seabirds. Little Auks, pocket battleships of
the High Arctic, are famed for their use of such ice edges. That habit