Study area
This study was carried out at Lake Ol’ Bolossat (0°5’24.84”S, 36°25’4.77”E; elevation 2330 m above sea level, Figure 2) from December 2017 to October 2019. This lake is a 43 km2 inland lacustrine wetland located in the central highlands of Kenya in Nyandarua County. It has characteristic features of Rift Valley lakes such as being shallow and narrow (stretches for c.30 km oriented in a north-south direction). The diversity of vegetation creates a mixture of habitats and microhabitats – ranging from open water to perennial marshes. The open water occupies c.23 km2 from the central part towards the south, where the lake is wider (c.4 km) and deeper (1.5-2.5 m). The rest of the area (c.20 km2) is comprised of marshes, dominated by sedges, for example Cyperussp., Papyrus sp. (only found in the extreme northern end), andTypha sp., and this is the available habitat for waterfowl such as Grey Crowned Crane that use them for foraging, roosting, nesting and rearing chicks.
The lake basin and riparian grasslands are heavily intruded by humans for quarrying, settlements, crop farming and livestock grazing. Grazing happens throughout the year, with the number of livestock increasing during the dry season (from December to March) in most years. The study area falls within an equatorial type of climate receiving a bimodal rainfall pattern as described by Brown & Britton (1980). However, this pattern is no longer predictable due to the effects of climate change as experienced from field observations during the study period. For example, the 2017/2018 breeding season, whose commencement is marked by nest construction and egg-laying, lasted between July and February (8 months), compared to 13 months (April 2019 – April 2020). Romίrez et. al. (2018), reporting climate change as an emerging issue in waterfowl communities (and wetlands) conservation, predicts that it will affect them in different ways depending on their life-history strategies, with resident species (like Grey Crowned Crane) being affected more than seasonal visitors. There are over 120 man-made or natural wetlands around the lake ranging in size from 420m2 to 211 ha that are also used by cranes for feeding, roosting and nesting, but none was included in this study due to a few accessible nests and their ecological features variability compared to the lake.
The shoreline was divided into 17 sections to identify critical cranes breeding sites described using village names: Ziwani, Mairo-Inya, Githungucu, Kanguyo, Ngurumo, Makereka, Iria-Ini, Gakoe, Kirima, Mugathika, Mukindu, Bahati, Karandi, Fuleni, Gatumbiro, Kanguu and Kianjata. These sites were grouped into three by dividing the lake into 3 parts (north, middle and south; Figure 2) each of c.10 km long. These sections helped to identify critical areas for cranes.