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.