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.