108 | Chapter 5
Where the species under comparison are very different, the question
of feeding segregation is meaningless. South Polar Skuas patrolling an
Antarctic Petrel colony to grab petrel eggs and chicks are of course for-
aging in a totally different way to the krill- eating petrels. The question
gains interest when two species breeding alongside each other are more
similar. Consider Macaroni and Rockhopper Penguins. The former is
almost twice the mass of the latter but both are crustacean specialists of
the open ocean, the Southern Ocean, that happen to be adorned with
colourful head crests and frequently share breeding stations. One such
is Marion Island, four days steaming south- east of Cape Town, where
the penguins were studied, via GPS tracking and time-depth recorders,
by Otto Whitehead of Cape Town University’s FitzPatrick In stitute of
African Ornithology.^33 Both during chick feeding and in the subsequent
build- up to moult the larger Macaroni Penguins dive deeper and for
longer than the Rockhoppers. They also tend to venture further from
Marion.*
Favoured for studies of this ilk are the larger auks. This might be
anticipated. In the North Atlantic Razorbills, Common Guillemots,
and Brünnich’s Guillemots nest in clamorous colonies, often together
on guano- smeared cliffs. Each species appears to bring the single chick
rather similar prey. How, if at all, do they divide up the available re-
sources? Consistently, studies report that Razorbills dive to lesser depths
than do the Guillemots (e.g. to 12 m as compared to 30– 35 m for the
Guillemots of both species breeding off south- west Greenland^34 ). This is
almost certainly related to the fact that Razorbills possess larger wings
in relation to body mass; good for flying but almost a hindrance under-
water. This has two further consequences. Razorbills may fly further
from the colony, at least when feeding chicks at a Baltic Sea colony, and
they appear to undertake V- shaped dives, catching fish on the ascent,
while Guillemots use their greater diving skills to search for and catch
prey at the bottom of their U- shaped dives. These contrasting dive pro-
files are considered in more detail in Chapter 9.
- (^) During the month- long pre- moult excursion, maximum foraging ranges of Macaroni Pen-
guins averaged 903 km, greater than the Rockhopper average of 696 km.