156 | Chapter 8
of the Equator! All these favoured areas were characterised by relatively
modest sea depths, low water temperatures (linked to the flow of ocean
currents), and brisk wind speeds, aiding low cost flight.
While the petrels are apparently choosing areas of prime habitat on
a huge, oceanic scale, there is evidence from other species that the choice
can be made on a smaller spatial scale and on a much smaller timescale;
if not on a day- to- day basis, then on a week- by- week basis. Red- footed
Boobies breeding on the island of Europa in the Mozambique Channel
between Madagascar and the continent of Africa like to spend the night
ashore. They then make daytime excursions to catch flying fish within
about 100 km of the colony. During a two- month study conducted by
Henri Weimerskirch,^16 the first month was characterised by high oce-
anic productivity, as assessed by satellite imagery measuring chlorophyll
concentrations, to north and south of Europa. Those were the directions
in which the boobies headed when going to work in the morning. In the
second month, oceanographic conditions changed and the accessible re-
gions of higher productivity were west of Europa. This now become the
boobies’ favoured departure direction. When the sea is rather barren, as
it is around Europa, it clearly benefits boobies to visit patches of tempo-
rary abundance. Perhaps they do this by following departing birds that
have been successful the previous day.
While the greatest concentrations of seabirds routinely occur where
ocean productivity is highest, it is clear that seabirds have the ability to
detect and then exploit temporary bonanzas that are available one day
- or even one hour – and gone the next in regions of lower productivity.
To make reliance on temporary feasts a viable way of life, the bird must
have the low flight costs that allow economical travel. The species with
higher travel costs are more restricted to wherever food is assured.
***
When seabirds depart the colony with a feeding area in mind, the trip
could well comprise a directed commuting phase, followed by a feeding
phase, followed by the homeward journey. Just this pattern was evident
among the Northern Gannets heading out from the Bass Rock, consis-