|
The most significant characteristic of the Great Bustard
is its large size. Males (left) may weigh up to 15 kg,
while females (right) rarely exceed 4-5 kg and have a
more slender silhouette and more cryptic plumage.
Great Bustards have one of the highest sexual size dimorphism
among Vertebrates.
|
|
|
During most parts of the year both sexes live in separate
flocks. This is just one of the consequences of the
strong sexual size dimorphism, which determines marked
sex differences not only in food requirements, but
also in their breeding, dispersal and migratory behaviour.
Males (left) develop in spring a colourful breeding
plumage that they exhibit during sexual display in
front of other flock-mates and females. Tufts of modified
feathers, the so-called moustachial feathers, grow
at each side of the beak. Each year these feathers
grow longer, reaching over 20 cm in old males.
|
|
Females
(right) tend to flock with genetically related individuals.
They are more gregarious and philopatric than males,
remaining frequently at their natal areas throughout
their entire lives.
As
could be expected from the strong sexual size dimorphism,
the sex ratio is naturally skewed toward females, so
that the latter can easily double males in number. The
sexual selection pressures acting upon males are the
cause of their higher natural mortality, as well as
their higher vulnerability to human-induced threats.
|
|
During
display, males turn up their wings and tail to show their
pure white ventral parts. This is how they try to attract
females for mating.
|
Sexual
dimorphism in size is also related to the strong sexual
selection and polygyny that are characteristic of
this species. All males of the breeding group gather
at traditional display arenas, termed leks, where
they fight in winter and early spring for a higher
position in the flock hierarchy. In spring, all adult
males of the flock display at the lek site to attract
females. Only the best males of the group will succeed
in mating, and some of them will copulate with several
females. Females visit the display arena and choose
their mates among displaying males. Incubation and
chick rearing is exclusively performed by females.
|
Nests are shallow scrapes in the ground partly covered
with some dry vegetation. Females lay one to three greenish,
brown spotted eggs. |
|
|
The
chicks hatch from late May onwards, after four weeks
incubation.
Although most females older than two years attempt
to breed every year, on average only 10-12% will rear
a chick per season. The offspring have a prolonged
maternal care period of six to over twelve months.
|
|
The
plumage of the chicks is cryptic to facilitate hiding
in the vegetation when a predator approaches.
|
|
|
