146 | Chapter 8
presumably because shoals of the main prey, Peruvian anchovy, were
extremely ephemeral. But the birds were not left without information
about where to search. When setting forth, they briefly joined a large
offshore raft of Cormorants swimming on the Pacific. This raft, a per-
petually changing cluster of around 300 birds, was not aligned to the
wind, but it was remarkably well aligned to the unbroken columns of
Cormorants returning from successfully feeding on large prey patches.
Thus the alignment of the raft gave departing Cormorants useful infor-
mation about the direction of the currently- active feeding hotspot. And
the information was used; the departing birds flew off in the indicated
direction.
Once out at sea, an obvious clue to food is the presence of other feed-
ing birds. Showing off his blue feet in a dance of self- important self-
advertisement, a male Blue- footed Booby beside the tourist path on the
Galápagos hopes to attract a female. However, attraction, if not love, is
guaranteed if he starts to dive offshore. A booby slanting into Galápa-
gos coastal waters that often seem so shallow as to render the dive fool-
hardy will assuredly attract other boobies hoping to share the fish shoal.
This is a commonplace observation. Just as vultures keep a watch for
other vultures as a guide to the presence of carcases, so seabirds certainly
watch the activity of other seabirds (and fishing vessels: Chapter 10) as
a shortcut to finding food. The fact the plumage of so many seabirds is
substantially white, and conspicuous against the sea’s surface, certainly
increases the chance that feeding birds will be spotted from afar.
In the tropics shoals of tuna drive smaller fish to the surface. The
water boils as the frantic prey fish try to escape their underwater nem-
esis, but there is of course a catch. Small prey fish at or near the surface
can be caught by birds, ever on the look out for tell- tale signs of shim-
mering fish scales. It seems very probable that the birds use sight to spot
the opportunity.
What is intriguing is how the species of birds that gather above tuna
may differ according to marine productivity.* For example in the east-
- (^) This can be assessed by the density of chlorophyll in the water and also the depth of the ther-
mocline, that sharp temperature discontinuity between warm surface water floating on top of
denser cooler water below. A deeper thermocline is indicative of little mixing of the surface
waters. Consequently the surface waters become stripped of key nutrients and correspondingly
unproductive.