Factors influencing nest-site selection
This study has established that there are three principal variables
(i.e., water depth, vegetation height and offshore distance from the
nest) that Grey Crowned Cranes consider in the choice of a wetland to
locate a nest-site as discerned from the active nests accessed during
this study. Although the small sample size could be a drawback in our
study, the results provide key insights into features of the species’
nesting habitat i.e. it prefers wetlands with a standing water of at
least 50 cm deep surrounded by a vegetation of 60-90 cm tall, and place
their nests at least 30-100 m inside the wetland from the shoreline.
These results complement observations made by Urban, Fry & Keith (1986)
and Walkinshaw (1964) of a nest-site preference with knee and
should-high (1 m) of emergent vegetation and a standing water depth of
80-180 cm. Although some nests observed were in areas without an
emergent vegetation cover, these were inaccessible due to a water depth
beyond 1.5 m. In such cases, water depth was among the critical factors
considered in choosing them. Water depth is described as an important
predictor of invertebrate abundance (Smith & Smith, 1988), which forms
a larger proportion of parental adult cranes diet (Gichuki, 2000).
Inability to access all active nests due to danger of wading (muddy lake
bed and deep ditches) and presence of hippopotamus, may however have
biased our results.
Nesting cranes were observed to be somewhat sensitive to humans and dogs
approaching nesting-sites and whenever they had unfledged chicks.
However, breeding pairs were confirmed utilising smaller man-made
wetlands around the lake that are within areas dominated by an influx of
human activities such as water abstraction, grazing, and crop farming.
This use of small wetlands could be an indication of deterioration of
traditional and principal breeding site, hence cranes are desperate for
suitable breeding sites. Accessible nests were visited once to reduce
disturbances. Considering this is an endangered species, counting and
mapping of territorial pairs was done rapidly, and this may have biased
estimation of density of number of pairs occupying territories.
Although our results did not detect a significant effect of grazing in
choice of a nest-site by cranes, it is very likely that they may avoid
such sites due to disturbance arising from livestock especially the
feral dogs. Grazing from livestock can degrade crane’s breeding habitats
and contribute to nest failure (Ivey & Dugger, 2008), and is known to
affect waterfowls in several ways such as number of breeding pairs,
nesting densities and nesting success (Kirsch, 1969). Vegetation height
may also be related to concealment from terrestrial predators
(Muheebwa-Muhoozi, 2001; Olupot, 2016) and could lead to succession of
vegetation composition and a structure that is less suitable to Grey
Crowned Cranes (Morrison, 2015).
While this study identifies features of nest-sites, it is important to
note that the chemical and physical parameters of wetlands changes
rapidly over time (Shelly, Mirza & Bashir, 2011). This was corrected by
sampling nests across two breeding seasons (2017/2018, 2018/2019) which
gave us a fair distribution of wetland conditions. Census and mapping of
pairs was done in December, a time considered as the peak of this
specific breeding season. At this time, pairs were at different breeding
stages ranging from nest-building and incubation (for late breeders) to
feeding flightless chicks, and hence most territorial pairs had taken
positions. However, it is important to note that except for years with
an extended drought, cranes of Lake Ol’ Bolossat nests throughout the
year, an observation also reported in Uganda by Pomeroy (1980) and
Archibald & Meine (1996). It is thus quite challenging to determine the
exact number of territorial pairs in such an expansive site like Lake
Ol’ Bolossat.