Black
Swans were once thought to be sedentary, but the species is
now known to be highly nomadic. There is no set migratory
pattern, but rather opportunistic responses to either rainfall
or drought. In high rainfall years, emigration occurs from
the south west and south east into the interior, with a reverse
immigration to these heartlands in drier years. When rain
does fall in the arid central regions, black swans will migrate
to these areas to nest and raise their young. However, should
dry conditions return before the young have been raised, the
adult birds will abandon the nests and their eggs or cygnets
and return to wetter areas.
Black
Swans, like many other water fowl, lose all their flight feathers
at once when they moult after breeding, and they are unable
to fly for about a month. During this time they will usually
settle on large, open waters for safety.
Generally,
black swans nest in the wetter winter months (February to
September), occasionally in large colonies. A typical clutch
contains 4 to 8 greenish-white eggs that are incubated for
about 35–40 days. After hatching, the cygnets are tended
by the parents for about 6 months until fledging, and may
ride on their parent's back for longer trips into deeper water.
A Black Swan nest is essentially a large heap or mound of
reeds, grasses and weeds between 1 and 1.5 metres (3-4½
feet) in diameter and up to 1 metre high, in shallow water
or on islands. A nest is reused every year, restored or rebuilt
as needed. Both parents share the care of the nest. Like other
swans, the black swan is largely monogamous, pairing for life
(about 6% divorce rate).
