Pallas’s fish eagle in Bangladesh 633
further away from nest sites. We confirmed a positive asso- ciation between the Pallas’s fish eagle and waterbodies (Fig. 3), and, unlike most previous studies, we quantified the distances between eagle nest sites and wetland areas (Naoroji, 2006; Sourav et al., 2011). Ours is the first analysis of the spacing ofPallas’s fish eagle
nests, demonstrating a clustered pattern (Supplementary Fig. 1). This aggregation is possibly driven by availability of food resources and suitable nest trees (Newton, 1979), which in the study area are located primarily in and around villages. This assumption is also supported by the differences in wet- land areas and density of nests at the Tanguar Haor and Derai clusters (Fig. 3). It is possible that isolated nests outside these clusters are in locations that only just fulfil the species’ requirements for food and suitable nest trees, limiting breed- ing success (Newton, 1979; Hardey, 2006). The results of our survey suggest that the majority of
breeding pairs may have used the same nesting site and nest trees for many years if not disturbed. It remains unclear what happens to the offspring as we did not find any evi- dence of offspring reoccupying the same breeding areas. It has been hypothesized that breeding sites are generally used by pairs or possibly their offspring for many years in succes- sion (BirdLife International, 2001). However, two satellite- tagged juveniles did not return to their natal territory in India; one migrated to Myanmar and the signal of the other was lost (Steele, 2017). Marking young birds at the nest with coloured leg rings or wing tags could facilitate individual monitoring, verification of site fidelity and population estimates. Satellite telemetry studies should be undertaken to examine migratory linkages between breeding and non- breeding sites.
Population status in Bangladesh
Prior to our study, it was thought that Bangladesh accounted for a small proportion of the global breeding population of Pallas’s fish eagle (BirdLife International, 2001;Steele, 2017). Sourav et al. (2011) recorded 11 nests during opportunistic sur- veys in 2009 in Sunamganj district and cited 22 nests in 1999 and 15 in 2001–2002 from previous studies. Most of these reports were concentrated around Tanguar Haor. In 2003– 2004, five nests were found in Netrokona district (Naher & Khan, 2009).Our study covered a larger area than these stud- ies and used a more comprehensive survey methodology, combining community-based interviews and field surveys of nest locations. The presence of 53 nests within the study area suggests that Sunamganj and Netrokona districts are more important, both nationally and globally, for nesting Pallas’s fish eagles than previously
thought.However, this population has declined since the early 1980s; previous estimates sug- gested that in the early 1980s, 61 villages around Tanguar Hoar had at least one nesting pair (Sourav et al., 2011). Our
community-based interviews suggested that 47 breeding pairs have disappeared from the area since the early 2000s. There are only a few recent (post-2000) breeding records
outside these two districts. These include a pair that bred in Hail Haor, Moulovibazar district, in the north-east, during 2010–2012, but the nesting tree was felled and the area was abandoned in subsequent years (SUC, pers. obs., 2019). Another pair bred in Moulovibazar city until 2015 when the nest was destroyed during a storm (S.S. Inam, pers. comm., 2020). However, a pair was found in southern Hail Haor in February 2020 (M. Foysal, pers. obs., 2020). These recent records suggest that isolated pairs may breed outside our study area in north-east Bangladesh, and further surveys elsewhere should be carried out. The most recent nesting record in Bangladesh outside the north-eastern dis- tricts was of a pair in the freshwater-dominated area in the northern fringe of the Sundarbans mangrove forest in 2006 (R. Halder, pers. comm., 2011). The lack of breeding records from outside our study area indicates that the area holds the most important, and possibly the only, permanent breeding population in the country.
Threats
The loss and degradation of wetlands, felling of nesting trees, shortage of prey, uncontrolled spread of water hya- cinth Eichhornia crassipes and the use of pesticides have been identified as principal causes of the decline of Pallas’s fish eagle (BirdLife International, 2001; Naoroji, 2006; Rahmani et al., 2012). However, the effects of inten- tional removal of nests by local people in retaliation for pre- dation on domestic poultry chicks and ducklings, and high mortality of nestlings during pre-monsoon storms, have not yet been quantified. Increased domestic fowl populations (Sarkar et al., 2017) in the agro-ecological landscapes of north-east Bangladesh may have led to increasing negative interactions between local people and Pallas’s fish eagles. Apart from isolated incidents of intentional nest damage,
people in general (63.3%, n = 830) had positive attitudes to- wards Pallas’s fish eagle. Historical notes dated from the early 19th century also indicate that people did not persecute Pallas’s fish eagles (BirdLife International, 2001), and Cripps (1878) was asked not to shoot nesting Pallas’s fish eagles in the property of a local person in north-east Bangladesh. Results of our remote sensing analysis show that the ex-
tent of permanent wetlands in the study area has declined by at least 14.7% during 2010–2020 (Fig. 3b). Similarly, other studies of land-use change reported large-scale deg- radation of wetlands in the area. During 1980–2010, c. 40% of forested and wetland areas in Tanguar Haor have been converted to agricultural land (Haque & Basak, 2017). A study of seasonal freshwater wetlands in north-east Bangladesh reported the loss of 6.9% of waterbodies during
Oryx, 2022, 56(4), 627–635 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321000314
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