tied to home | 105the beaks of the terns gathered overhead. Whichever was correct, the
outcome was that many terns abandoned their eggs.
Not only do short- term fluctuations influence how far seabirds need
to venture, so too do changes in feeding conditions between years.
Tracked by GPS from Coquet Island, off England’s beautiful Northum-
berland coast, Black- legged Kittiwakes embarked on markedly different
trips when feeding chicks in 2012 as compared to 2011.^25 The 2012 trips
were far quicker (averaging 2.9 h versus 5.1 h) and shorter (averaging
20 km total journey length versus 64 km), apparently reflecting differ-
ences in the availability of sand eels, their prey- of- choice.
If parents caring for chicks need to forage further from the colony
in lean periods, we might guess they would also be constrained to travel
further afield in regions where the ocean is unproductive. Depending on
offshore conditions, there can be a ten- fold difference in the distance
British Common Guillemots at different colonies fly to hunt for food
for their chicks. But a particularly nice example comes from southern
Argentinian Patagonia where Magellanic Penguin colonies are dotted
along 1,000 km of dry coast. Roughly, the further south the colony is
situated, the more productive are the waters offshore. Dee Boersma of
the University of Washington found that male penguins feeding chicks
in the south travelled a maximum distance of some 100 km from the
colony. Their northern friends travelled twice that distance.^26 Further-
more, at colonies where Magellanic Penguins in this region have to work
harder on a foraging trip, measured as trip duration, total distance cov-
ered and maximum distance from the breeding colony, the population
is least likely to be growing.^27
Quite often it will be tricky to decide whether birds from different
colonies forage at lesser or greater distances because the surrounding
seas are more or less productive or because a multitude of hungry mouths
has depleted fish stocks near the colony (the aforementioned Ashmole
halo effect). The Magellanic Penguins are an example of the former. A
study by Steffen Oppel is a probable example of the latter. Oppel studied
breeding Masked Boobies on Ascension Island in the tropical Atlantic
Ocean and on St Helena, site of Napoleon’s last exile and death, 1,200 km
to the south- east.^28 The waters surrounding both islands are equally un-
productive, the blue water ‘deserts’ of the tropics. But, including other