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.