Survey and mapping of territorial pairs
Mapping of crane pairs occupying territories was carried out to
determine the number and positions (distribution) of most (if not all)
of the cranes utilising the lake as a nesting site. The entire 80 km
long shoreline enclosing the lake was accessed at suitable locations
that were 1-2 km apart from which pairs of cranes were comprehensively
searched for presence/absence in the marshes using a pair of 10x42
binoculars and a telescope. Since cranes occupy territories with high
vegetation cover during the breeding period, pairs are difficult to
detect in their breeding territories. Therefore, performing a regular
survey protocol (e.g., systematic sampling of equally sized plots around
the shoreline) might require a higher sampling effort. Instead, walking
or driving in motorable areas along the shoreline in a non-systematic
way ensured substantial coverage of the sections to warrant that no
pairs were undetected. Searching was made easier because of the
visibility of the cranes due to their large body size and white wing
patches. Vegetation height may however have affected detection of pairs
as this varied across the lake. When a pair was present, the location
was marked with a GPS and its relative position inside the wetland
approximated in Google Earth Pro. The narrow (120-450 m) width of the
lake, a contracted fringe of marshes between the shoreline and the open
water, and sometimes absence of emergent macrophytes, improved detection
of cranes.
According to Hinde (1956), a territory is ‘any defended area’, and is an
outcome of two distinct, independent tendencies, i.e. site attachment
and (intraspecific) hostility (Tinbergen, 1957). A crane territory was
hence considered as having been occupied by a pair if either or all of
the following observations were made: i) gathering nesting material,
building a nest or pair present in a site with a complete or incomplete
nest ii) seen lying on the nest hence contemplated as incubating), iii)
rearing a flightless chick(s), and to a lesser extent iv), pair was
recorded present more than once in a specific site and showed some
intimate courtship behaviour such as standing face to face with bills
touching, head bobbing or copulating.
Mapping and confirmation of pairs was carried within a week during the
peak of the breeding season of 2017/2018 season in December 2017 to
detect number of pairs that had occupied territories. Subsequent,
infrequent visits continued throughout the study period to monitor
breeding activities, detect new nests that were accessible for
measuring, and to collect data for other objectives of a broader study.
Since the study population was unmarked, observations across time
pertain to site re-use or its continued use by the same or different
individuals. Breeding season was contemplated as the period between
laying of the first egg and fledging of the last chick, the duration of
which was not uniform in the two seasons. Pairs taking up territories
(e.g. late breeders) after the initial one-week survey were not included
as these could have been previously located pairs changing positions as
this may have exaggerated the minimum number of territorial pairs.
Accessible nests were however measured throughout the study period to
increase the sample size and provide for seasonality.