Factors influencing nest-site selection
Grey Crowned Cranes in and around Lake Ol’ Bolossat bred in the main
lake and small man-made wetlands surrounded by a high influx of human
activities such as water abstraction for irrigation and domestic use,
livestock grazing, and crop farming. The sizes of these man-made
wetlands (0.042 - 87 ha) are smaller compared to minimum wetland size
observed in South Africa of 162 ha (Morrison & Bothma, 1998). This
observation have the same opinion with Morrison & Bothma (1998) that
the choice of a nesting site by cranes, in general, is less dependent
upon human impact, and particularly the Grey Crowned Crane that appears
to tolerate human interference. The use of small wetlands in
human-dominated landscapes within our study area could as well be an
indication of deterioration of breeding sites elsewhere, and that cranes
are desperate for suitable breeding sites.
This study has established that there are three important variables that
Grey Crowned Cranes consider in the choice of a wetland for a nest-site
location. The result strongly suggests that this species select wetlands
with water of at least 50 cm deep, tall vegetation (60-90 cm) and place
their nests at least 100m from the edge of the wetland. However,
distance to the water edge showed a quadratic tendency perhaps because
only a small sample size of nests beyond 100m distance were accessed.
Water depth is also important as a good predictor of invertebrate
abundance (Smith & Smith, 1988), on which cranes feed their chicks.
In Mongolia, Bradter et al. (2005) reported livestock grazing as
potentially harming White-naped Cranes by degrading their breeding
habitat and disturbing breeding birds. Grazing was as well observed as a
potential direct contributor to nest loses in the Sandhill Crane (Ivey
& Dugger, 2008). Although the effect of grazing was not detected in
this study, grazing is considered as an important factor in cranes
breeding success. Possible lack of significant results was from a lack
of statistical power (e.g. grazing intensity was measured as a
categorical variable that may have had a bias). Grazing is known to
affect waterfowls in several ways such as pair numbers, nesting
densities and nesting success (Kirsch, 1969).Grazing pressure could also
result in the succession of vegetation composition and a structure that
is less suitable to Grey Crowned Crane (Morrison, 2015). The results
show that vegetation height is a key variable affecting nest-site
selection and since grazing affects vegetation height, cranes will
likely avoid areas of high grazing intensity as a nesting-site.
Findings of this study provide valuable insight into breeding ecology
and habitat requirements of Grey Crowned Crane. They can be utilised as
a basis for modification and management of crane’s breeding habitat such
as manipulation or improvement of wetlands to attract a breeding
pair(s). In degraded wetlands, for instance, a weir could be constructed
or raised to ensure that the water held back reaches at least 50 cm
in-depth in the shallow end. Low-level grazing or exclusion of large
herbivores (such as buffalos, elephants, cattle, donkey etc) would
naturally allow vegetation to regenerate and flourish on its own, and
this can be achieved by fencing small to medium-sized wetlands, natural
spring marshes or parts of extensive wetlands with a chain-link or an
electric fence in conservation areas. Besides, this information can be
adopted in legal documents such as Integrated Management Plans, National
Wetlands Policy and International Single Species Action Plans.