INTRODUCTION
Knowledge of environmental features that determine habitat quality is critical to developing effective strategies for preserving and restoring natural areas (Hobbs, 2003). Nest-site selection is an integrative behavioural process that evolved to maximize reproductive success (Catlin et. al., 2019). Therefore, understanding what key variables influence nest-site selection in a species is the first step towards its effective management and conservation (Hobbs, 2003; Swaisgood et. al., 2017; van de Loock, 2019).
Nest-site selection involves the specific choice of a site to build a nest from all possible sites (Burger, 1985). This choice must: avoid or reduce risks of predation (Holway, 1991; Götmark, Blomqvist, Johansson & Bergkvistv, 1995), or increase offspring survival (Kolbe & Janzen, 2002). Nest-sites are hence an important part of an animal’s niche (Gould, 2008), and a major determinant of reproductive success for many organisms since the quality of a nest-site is often linked to a species breeding success (Hatchwell, Russell, Fowlie & Ross, 1999). It is also a fundamental factor in the survival of a species through generations and is often a poorly understood component of many organisms’ reproductive investment (Baden, 2018). The choice of a nest-site determines the available resources and threats that the animal and its offspring encounter, making it an important fitness-related decision (Tolvanen et. al., 2018).
While information on the breeding behaviour of most of the 15 extant species of cranes is available, it is cited as lacking for Grey Crowned Crane Balearica regulorum . Walkinshaw (1964) wrote on taxonomy of this species where he identified one genus (Balearica ), two species (B. regulorum, B. pavonina ) and four races, two in each species. Standing at 100-110 cm, adults appear similar in plumage but the male is larger (Figure 1; Walkinshaw, 1964; Archibald & Meine, 1996). The species occupies a mixture of wetlands and open grasslands or savanna, and human-modified habitats such as cropland and irrigated fields (Archibald & Meine, 1996; Austin, Morrison, & Harris, 2018). They are omnivores feeding on seeds, fresh tips of grasses, insects and small vertebrates (Pomeroy, 1980; Archibald & Meine, 1996). The cranes population at Lake Ol’ Bolossat share the marshes with several avian groups such as herons, ducks, geese and ibises.
Grey Crowned Crane is a solitary bird (living as a pair or a family unit) during the breeding season whereas the non-breeders congregate in foraging fields forming flocks. In eastern Africa, cranes breed all year round (Archibald & Meine, 1996). Mating for life, the nesting behaviour starts with courtship dance with the male as the aggressor (Walkinshaw, 1964). They nest in moist habitats preferring those with a standing water of 80-180 cm deep with knee- and shoulder-high sedges where the nest platform is usually constructed over a mound of heaped aquatic vegetation collected around the nest-site by both sexes (Walkinshaw, 1964). A clutch of 1-4 bluish-white eggs that turn brownish through the 28-30 days incubation period are laid, with an average of 2.53 and a mode of 3 eggs (this study). In Kenya, breeding cranes have been reported from across where they occur. At Lake Ol’ Bolossat, the breeding season variably start between April and August into the dry season in March-February, and may last over 12 months in years with good precipitation.
Although still Africa’s most abundant crane species, its population has been described as declining over the years. Urban (1988) estimated a population of 100,000 individuals, and a decade later, at 85,000-95,000 (Urban, 1996). Other estimates have been 50,000-64,000 (Beilfuss, Dodman, & Urban, 2007), and 26,500-33,500 (Morrison, 2015). The Kenyan population too has declined from 35,000 (Urban, Fry & Keith, 1986), 10,000-12,500 (Morrison, 2015) and 8,000-10,000 (Wamiti et al., 2020). As such, this species is listed as Endangered on the IUCN Red List of Threatened Species. This is as a result of continued population declines attributed to habitat loss and fragmentation, and illegal removal of birds and eggs from the wild for food, traditional use, domestication and international illegal trade market (BirdLife International, 2020).
On 4th July 2018, an area of 147 km2enclosing the lake and a riparian zone was gazetted as a Protected Wetland Area under Section 42(2) of the Environmental Management and Coordination Act (EMCA), No. 8 of 1999 [Revised 2012]. Prior to this, operative management of the lake’s diverse resources has been largely absent. This has been the main cause of habitats degradation that affects many species including Grey Crowned Crane. This declaration provides for, among others, development of a comprehensive management plan (process near conclusion) and regulation on sustainable use of resources. The lake was designated as an Important Bird Area in March 2008 following confirmation of globally-threatened birds (Wamiti et al., 2008). Despite Morrison (2015) having not listed it as one of Kenya’s fundamental sites for the Grey Crowned Crane, recent research has given away that the lake is indeed a population stronghold, ranking second in the country (Wamiti et al., 2020). Except for a few studied breeding sites (e.g. Gichuki, 1993), information on the number of breeding pairs in different parts of Kenya is currently unavailable. This study makes a substantial attempt to contribute to this knowledge gap.
A literature exploration on Grey Crowned Crane’s published material in Google Scholar and WorldWideScience implied that bulk of this has been on conservation (e.g., Meine & Archibald, 1996; Beilfuss, Dodman, & Urban, 2007) and population and ecology (e.g., Gichuki, 1993; Amulike, Fuller, Houlihan Griffin, 2020) while that on breeding densities is largely lacking. Morrison (2015), describing the Grey Crowned Crane as an icon of Africa’s wetlands and grasslands, has listed factors influencing breeding productivity as an essential knowledge gap that hinder effective implementation of the species management and conservation actions. The purpose of this study was, therefore, to contribute to this knowledge gap.
Specifically, we investigated the critical factors influencing nest-site selection of the Grey Crowned Crane at Lake Ol’ Bolossat. We measured variables at the nests that are known to have compelling biological influences on the choice of a nest-site location based on i) food and nesting materials availability, and ii) nest concealment and susceptibility to predation. We hence hypothesized that cranes select sections of the lake with higher water depth (as a proxy of the availability of macro invertebrates and small aquatic vertebrates – e.g. Baumgärtner, Mörtl & Rothhaupt, 2008), while avoiding flooding areas close to the shore. They also choose nest-sites that are inaccessible and concealed from terrestrial predators (e.g. Jobin & Picman, 1997; Jedlikowski, Brambilla & Suska-Malawska, 2014). Therefore, the preference for a nest-site is predicted to be positively related to both water depth and offshore distance from the nest. Moreover, factors that reduce the complexity of vegetation structure around the nest such as grazing might impede cranes to hide their nests from predators. Therefore, nest-site selection is predicted to be negatively affected by grazing intensity and positively affected by vegetation height. The number and distribution pattern pairs occupying territories were also estimated. This study’s information will be important for the conservation and management of key breeding sites of this endangered species in Kenya and other range States across Africa.