tied to home | 91colonies closer to, rather than further from, prime feeding areas – if this
is feasible. Philip Ashmole is a seabird biologist who has thought and
written influentially about these issues.
It is satisfying to have one’s name commemorated in the name of an
animal, even if it is a mere bug. It must be even more satisfying to be
linked to an important idea that influences the trajectory of scientific
thought and research. But, oh, the saintly joy of being linked to a halo!
That beatific joy has befallen Ashmole. As a member of the British Or-
nithologists’ Union Centenary Expedition to Ascension Island in the
early 1960s, he studied seabirds under the tropical Atlantic sun. His
thoughts turned to how the numbers of tropical oceanic species might
be regulated.^7 Predation and disease were having little obvious impact,
and space for extra breeding pairs remained available. Outside the breed-
ing season immense numbers of birds could spread over such an expanse
of ocean that food might not then be in short supply. But the situation
was possibly different when the birds were assembled at the colony while
breeding. Then, as numbers at the breeding site grew, so the birds would
deplete food in an expanding ‘halo’ surrounding the colony, obliging
them to travel further from the colony and/or spend more time feeding.
Eventually this would result in reduced breeding success. Growth of the
birds’ population would halt, especially if the increasing size of the col-
ony led to ever- longer deferment of the start of breeding.
Ashmole’s halo is a useful prism through which to observe many of
the results that have emerged as birds’ forays from colonies during in-
cubation and chick- rearing have been documented, particularly by GPS
tracking. Not only will I investigate the extent of those forays but also
how they are affected by the size of the colony and the proximity of
neighbouring colonies.
Once the clutch is laid, parental co- ordination is a vital element of
successful incubation. And the stakes are high. Seabird eggs tend to be
large for the size of bird, and represent a substantial physiological in-
vestment. Nowhere is this more relevant than in the smaller species that
lay but a single egg, for example the storm petrels. The smallest storm
petrels lay eggs that may weigh 29% of the female’s weight. That is roughly
equivalent to a 20 kg human baby. Whilst women readers will surely
shudder at the thought of giving birth to a 20 kg baby, they would have