34 | Chapter 2
they begin sustained flight, indicated by ground speeds over 20 km/h
averaged across the 24- hour day.
While these examples suggest acquiring flight skills needs a few days,
other species apparently take longer. Leading a Japanese team, Ken Yoda
hand- raised a couple of hatchling Brown Boobies which were prosai-
cally named A1 and A2.^8 An acceleration data logger was attached to
each bird at fledging to allow precise quantification of the amount of
the time spent flapping, gliding and resting. For the post- fledging month
of the study, the duration of foraging trips and proportion of time spent
gliding during flight increased with the number of days since fledging,
whereas the proportion of time spent in flight decreased. It looks as if
the Brown Boobies took at least a month to acquire passable flight skills,
in particular to shift from energy- demanding flapping to more energy-
efficient gliding. This somewhat protracted shift could partly explain
why Brown Boobies continue to receive food from their parents for a
long time, 17 weeks on average, after fledging.
Without question, successful transition to full independence needs
not only competent flying ability but also decent feeding skills. To some
extent the two will develop together. A specialised aerial feeding tech-
nique is likely to require adept flying. On that basis, we might antici-
pate that young terns which specialise in snatching fish and other prey
from the water surface might metaphorically hang onto the parental
(coat)tails for a significant period. Exactly this happens, and it is by
no means unusual for juvenile terns to remain with their parents for
months. For example, after breeding in Washington state in the north-
western United States, Caspian Terns, a large species with a gigantic
orange bill resembling a carrot, head towards the warmth to winter as
far south into the tropics as Acapulco, Mexico (17°N). Parent terns car-
rying solar- powered satellite tags continue to care for the summer’s off-
spring well into the winter.*
Even more prolonged parental feeding of fledglings is the habit of
frigatebirds. Henri Weimerskirch, an urbane French research scientist,
- (^) Considering all the tern species, this is probably one of the longest periods of association be-
tween parents and their flying offspring. Another species with a notably long period of associ-
ation is the Black- naped Tern, where parents and young can remain together for up to 180 days.