Distribution pattern and density of breeding pairs
The varying breeding pair densities across the lake suggest variance in availability and abundance of food resources. Even though Brown & Britton (1980) describe food supply in perennial wetlands as probably stable, they disagree that it is not the overriding ultimate factor, and could therefore potentially affect breeding densities. In the current study, it has been shown that other factors are determining this density, the critical one being the suitability of a nest-site. The density observed is possible from the vastness of Lake Ol’ Bolossat marshes across a 30 km stretch, supported by a rich diversity of widespread satellite wetlands (over 120 in the study area), providing a favourable breeding environment sites for occupation by breeding Grey Crowned Cranes. Other than being an indication of the suitability of a wetland as a preferred choice for a breeding site, a high breeding density might moreover be a suggestion of degradation of breeding habitats, increased poaching, disturbances and threats in other breeding sites forcing cranes to seek and congregate in a few sites that are still relatively safe or offering desirable conditions.
Considering only the marsh area (and not the entire wetland area), an average breeding density of 6.50±0.84 (SE) pairs/km2demonstrate the importance of Lake Ol’ Bolossat as a key breeding site for the Endangered Grey Crowned Crane. As observed on the ground, absence of disturbances from livestock and humans would very likely lead to the occupation of more breeding territories in the extensive marshes since the minimum distance between neighbouring breeding pairs in this study was observed to be c.50 m. However, as the wetland assumes management following its gazettement as a Protected Wetland Area in July 2018 and the drafting of an integrated management plan, breeding density is expected to rise in the future.
As pointed out earlier, information on the features of a Grey Crowned Crown breeding habitat has been reported as extremely lacking, and therefore an essential and high priority knowledge gap (Morrison, 2015). The breeding pair density reported in this study contributes to this knowledge gap in terms of breeding habitat requirements, number of breeding pairs and an accurate estimate of the amount of suitable breeding sites available at Lake Ol’ Bolossat. The results of this study compare reasonably with that of greater sandhill cranes (Grus canadensis tabida ) that had an average density of 5.25±0.36 nests/km2and an average distance between nests at 222±70 m (Barzen, Su, Lacy, Gossens & Moore, 2016)
The breeding pairs occur all around the lake except along a 4 km stretch between Bahati and Karandi villages on the south-western shores (see Figure 4). This section is characterised by a steep shoreline of open water with violent waves, a dense 1 m tall tussock grass consociation ofPennisetum spacelata thus presenting a high-risk foraging area for cranes and their chicks. The riparian grassland here too is dominated by heavily grazed fields in some parts in addition to a complete lack of marshes along the shore that breeding pair of cranes require.
In the study area, Gatumbiro and Kianjata breeding sites had the largest area of marshes and were hence expected to have the highest density of breeding pairs but surprisingly had the least densities. This could be attributed to the fact that Gatumbiro is largely a floating marsh that changes position from time to time besides a high traffic of fishermen on canoes or plastic drum rafters and a busy quarry in the heavily encroached and settled riparian grassland creating a disturbance. On the other hand, Kianjata marshes are spread out and stable in terms of vegetation (nest substrates), security and shelter from the continuous cover of sedges exceeding 1 m. However, the marshes are frequented by a large number of livestock, fishermen, and poachers and were recently subdivided into smaller parcels that now present an unfavourable landscape dominated by homes and barbed wire fences.
Breeding densities at Lake Ol’ Bolossat in different breeding seasons deserves to be determined to establish the lake’s carrying capacity. Breeding densities in other wetlands across the species range in Africa would also require to be established for identification and protection of key breeding sites. These sites will be of great magnitude in the long-term population recovery plans. In eastern Africa, where the population is still described as rapidly declining, it is paramount that efforts are made to understand the activities surrounding all known and/or potential breeding sites and address the challenges, if any, at the local/site level. Information on breeding sites, challenges and breeding densities would ultimately need to be captured in conservation plans for the species.
Given its prominent significance as a key breeding site for an Endangered species alongside the presence of other globally-threatened bird species (e.g. the Endangered Sharpe’s Longclaw Macronyx sharpei ), rare plants and an inadequately studied biodiversity, we propose that Lake Ol’ Bolossat be accorded a higher protection status i.e. be re-gazetted as a National Park. This will ensure sufficient control of undesirable anthropogenic activities especially poaching and control on the degradation of the wetland and riparian land conditions.