116 | Chapter 6
the tropical day, they make 12- hour excursions that take them up to
150 km from the colony. The birds were variously asked to carry GPS
recorders, accelerometers, time depth recorders, activity recorders, and
altimeters, fortunately not all at once! When travelling between forag-
ing spots, the birds flapped for about one- third of the time, in 1.7 second
bursts, and glided for the remainder in 3.8 second bouts.^5 This is likely
to be a highly efficient means of progression, judging by data from the
booby’s near- relative, the Cape Gannet. Monitored from South African
colonies via electrocardiograms, the gannets’ average heart beat during
flapping was 250 beats/min. This dropped almost instantaneously by
13 percent, to 217 beats/min, when the birds entered a gliding phase.
And 217 beats/min is barely faster than the heart rate of birds resting
on the water, around 210.^6 Like albatrosses, these birds can travel the
seas very economically, even when flapping.
In general sustained flapping is more problematical for larger birds than
for smaller. Vultures and eagles are not enthusiastic flappers. Instead
they routinely soar in updrafts or thermals to gain height and then glide
downward while progressing across country horizontally. Remarkably –
and this has only been fully appreciated over the past 15 years – frigate-
birds adopt similar tactics over the open ocean.^7 Truly bizarre birds with
sharply angled wings, frigatebirds cannot safely land on the sea. Their
plumage lacks waterproofing and their long wings obstruct take- off. This
combination of features is resolved because, at sea, frigatebirds exploit
thermals under cumulus clouds in the tradewind belt north and south
of the Equator.* Having used these thermals to ascend up to 2,500 m –
and very occasionally as high as 4,000 m where the temperature falls
below freezing – the frigatebirds can then glide downward to around
90 m before ascending once more. This cycle continues day and night
for as long as the frigatebird is at sea. Only sporadically, perhaps half a
dozen times in the 24 hours and most probably by day, does a frigate-
bird come lower, to the sea surface, where there is the prospect of food.
The capacity of frigatebirds to spend days at sea raises one very ob-
vious question. Do they “sleep through all the night with open eye” as
- (^) Along the Equator lie the Doldrums, the zone of light winds so frustrating for sailors and so
inimical to frigatebirds.