By Jigme Dorji
As usual, many bird enthusiasts in Bhutan went around searching for White-bellied Heron nests from February 2013. The first nest was found in Bertichu, a tributary of Mangdechu basin in Zhemgang and the second nest in Burichu, a tributary of Punatsangchu basin (see map). While the former nest was used in the previous year, the latter was a new nest. With two nests found in two major river basins in Bhutan, the researchers were hopeful to see at least one chick fledging in Bhutan but the reality turned otherwise.
In April 2013, the nest in Bertichu was found destroyed, the cause of which was not known to our local bird watcher. However, with less human disturbance seen in the area, the destruction of the nest could probably be attributed to natural predation. The cases of chicks of the white-bellied heron falling prey to predators like the Serpent Eagle, Pallas Fish Eagle, Osprey, Yellow Throated Martin and to some small cats was reported in the past.
In the second nest at Burichu the chicks could not be hatched despite the breeding pairs sitting on the nest till the end of June 2013. On the first week of July, it was seen that the breeding pairs had already abandoned their nest leaving their infertile egg. Though we have seen so much commitment from breeding pairs to incubate their lone egg, it was unusual to see such a long sitting on the nest. The reports in the past have indicated that the chicks are normally hatched in April and by July they fledge out of nest. The reasons for the unsuccessful hatching were not known. Since the nest was just at the proximity to Wangdue-Tsirang Highway (200 meters crow flight distance), the disturbance by vehicular noise might have caused stress to bird sitting on the nest thereby not being able to maintain constant temperature required for incubation of the egg. The other reason might be disturbance by humans, mostly by conservationist and visitors who kept visiting the area on regular basis to take photographs and monitor the nesting site. The causes might vary in opinions of different people but the immediate concern would be to see why such natural process failed after 10 years of its successful breeding in Bhutan. The Royal Society for Protection of Nature (RSPN) started monitoring this bird since 2003.
Referring to the literature, Bhutan is known for potential breeding of the White-bellied Heron in its entire home range. The first nest was found at Zawa, a remote village under Ada gewog in Wangduephodrang in 2003. Subsequent nests were seen in many other places like Basochu, Kisonachu, Ada, Nangsina and Bertichu with 4.5 mean chicks hatch per season at standard deviation of 2.67 (RSPN, 2011). The report also indicate that out of 5.62 mean egg hatched (S.D = ± 3.88), the mean chicks fledged was 3 (S.D = ± 2.44). From this, it is evident that the mortality of egg and chicks together accounts to roughly 50% of the eggs produced by breeding female. The mortality of egg (x ̅ = 1.12, S.D = 1.8) and chicks (x ̅ = 1.25, S.D = 1.83) are major causes for slow growth of the heron population in Bhutan (RSPN, 2011).
Towards the end of April this year, researchers from the Royal Manas National Park, Jigme Singye Wangchuck National Park, Royal Society for Protection of Nature and local bird watchers conducted a day long population count in the entire habitat known for occurrence of the White-bellied Heron. The survey started from 6.00 am to 6.00 pm with two or more surveyors stationed at each location. The 12 hours effort of 16 researchers sighted only 14 individuals of which two are found double counted given the proximity of sighting location and time variation within 12 hours. However, local residents had reported 24 individuals sighted in the month of January and February. While this bird is known for post breeding dispersal, April is still the breeding period and therefore the 50% variation of reported population and actual count remained highly contradictory (See graph). This difference can be true if there are 12 breeding pairs in Bhutan so that the assumption could be the rest 12 individuals were sitting on the nest during counting. Keeping numbers counted as minimum and number reported as maximum range, the White-bellied heron population in Bhutan can be within 12-24 individuals (x ̅ =18, S.D = 8.48). This means that the bird population has dropped from 26 in 2010 to 18 in 2013, with average annual loss of 2 individuals.
The report published by RSPN shows that the number of individuals have increased from 14 in 2003 to 30 in 2009. Literally, this period can be considered as the pre-Punatsangchhu hydropower construction phase, as the major construction came only in 2010. While the direct impacts of the hydropower on this bird in particular will be hard to assess, we can at least see the population trend after the hydropower construction in Punatsangchhu basin. The hydropower construction may not be the single cause for the population decline but can partly be attributed to them, as this bird is highly vulnerable to disturbance (tolerance distance to human disturbance is estimated to 200 metres). The Punatsangchhu basin is the largest habitat for te White-bellied Heron in Bhutan and therefore, the survival of this bird will depend on how safe this basin is for them to feed, roost and breed. In fact, if this bird exhibits a high survival rate during the project construction phase, it will indicate the low level of impacts caused by project activities on the surrounding environment. Thus we believe that monitoring population trend of White-bellied Heron in this basin can be indicative of many others impacts caused by hydropower construction on biodiversity and the environment.
This report is submitted for the wider interest of the conservationist, environmentalist, policy makers, developmental agencies and general public alike. This work is a continuation of the author’s dissertation research submitted to the College of Natural Resources, Royal University of Bhutan in 2012 with financial assistance from the Royal Society for Protection of Nature (RSPN) and Rufford Small Grant Foundation.
Dorji is a Heron Researcher and member of IUCN-SSC Heron Specialist Group