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.