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Nests
The
simplest nest of course is no nest at all. Quite a few birds do not
build any nest at all, though they do make a choice of where to lay.
Beyond this is a simple scrape in the ground. Nightjars (Caprimulgidae)
do not make even a scrape. They lay their two eggs directly onto the
ground. While Short-eared Owls (Asio flammeus) lay simply on
some trampled vegetation. Many waders, such as Plovers, lay in a simple
scrape on bare ground, relying on the cryptic colouration of their eggs
to protect them from predation.
Some
of the more unusual examples of no nest laying include the Potoos (Nyctibius
spp.) which lay their single egg on top of a broken off tree stump.
The bird then sits on top of the stump with its head pointing to the
sky. In this position it looks like an extension of the dead timber.
It generally chooses stumps of similar diameter to itself. Even trickier
are the 'nests' of Fairy Terns (Sterna nereis), another species
which lays only one egg. The egg is laid on a branch on a rock face,
generally on an area too small to support 2 eggs. Laying the egg in
a position where it won't roll off is tricky and young birds often lose
their first attempts before they get the hang of choosing a safe spot.
Fairy Terns have two adaptations to help them survive this seemingly
precarious nesting habit - the adults have evolved to be very careful
when settling on and getting off the egg - both sexes incubate the egg.
The second adaptation is that young Fairy Terns have disproportionally
large feet and very sharp claws which help them hang on.
The
master of no nest nesting, however, has to be the Emperor Penguin (Aptenodytes
forsteri). These birds nest in the Antarctic during winter when
the temperature can fall to less than -30 C. They nest well inland and
when the female has laid the single egg she leaves for the coast to
feed. Meanwhile the male has immediately taken over care for the egg
by moving it up onto the top of his feet. He has special folds of skin
on his belly which enfold the egg keeping it safe against all the severity
of the elements. Here he stays, a living nest with an egg on his feed
for the next 60 days without a meal or much movement until his mate
returns and the egg hatches. During this time he has maintained the
internal temperature of the egg at 40 C, as much as 80 C higher than
the surrounding ambient temperature.
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