Pallas’s fish eagle in Bangladesh 629 Methods Community-based interview surveys
Before conducting interviews, we obtained verbal con- sent from participants, informed them they could stop interviews at any point and explained their anonymi- ty would be preserved. We conducted all interviews in Bangla (the local language), following, semi-structured interview guidelines (Newing, 2011). We randomly selected respondents in public spaces and in households within the villages. To ensure accurate identification of the species, we showed colour photographs of adult Pallas’s fish eagles and played recordings of their breeding calls. We then asked the respondents if they knew of any current nest locations and whether the eagle had nested in their vicinity within the pre- vious 20 years. We subsequently visited all locations where nests had been reported by respondents, and recorded nests as occupied only when we found an incubating adult or nestlings. In some cases, we found remnants of old nests, which we recorded as unoccupied. We also asked respon- dents if the population of Pallas’s fish eagle in their area had increased or decreased in the previous 20 years, and what they considered to be the main threats to the eagle. To ensure systematic spatial sampling, we used a standard grid-based sampling approach with a uniform grid cell size (Harris et al., 2013). We conducted a pilot study to de- termine the most appropriate grid cell size in a 435 km2 area (10.5% of the study area; Fig. 1b) in which we knew the loca- tions of nine active Pallas’s fish eagle nests (Sourav et al., 2011).Within this smaller areawe undertook semi-structured interviews with the occupants of 135 households in 2017. During this pilot study, the householdswere in randomdirec- tions and at distances of up to 5 km from the known Pallas’s fish eagle nests. Almost all (95.1%) of the 92 respondents who lived within a 3-km radius of a nest (mean distance to nest 1,320 ± SD 800 m) were able to provide accurate information on nest locations (χ2 = 51.5,df = 1,P,0.0001), whereas only 63.8% of the total of 135 respondents within a 5-km radius (mean distance from nest 2,204 ± SD 1,427 m) were aware of the nearest nest location (χ2 = 6.27,df = 1, P=0.01; Fig. 2). Thus, we chose 3 × 3 km as the grid cell size. This resulted in 525 cells across the wider survey area, of which 23 were excluded as they comprised only open water without any villages or households. Detailed interview surveyswere conducted in the remaining 502 grid cells during the breeding season of Pallas’s fish eagle (November–March) over a 4-year period (190 interviews in 2017, 120 in 2018, 98 in 2019 and 94 in 2020), following the same protocol as in the pilot study.Within each grid cellwe conducted 1–3 interviews (Fig. 1), during a total of 12 visits and 63
days.To avoid double counting any breeding pairs thatmay have relocated to a dif- ferent nest site, we revisited each nest every year throughout the survey period.
FIG. 2 (a) Boxplots indicating knowledge of respondents who lived within or outside a 3-kmradius froma nest regarding the presence or absence of a Pallas’s fish eagle nest. (b) Logistic regression curve showing that as distance to the nest increases, the probability of nest detection through interview data decreases.
Nest site characteristics
At each occupied Pallas’s fish eagle nest we recorded the geographical location with a GPS, the species, height, cir- cumference at breast height and crown density of the nest tree, and the distance to the nearest human settlement, road, wetland and river. To determine differences between nesting trees and other available trees at the nest location, we collected the same information from an equal number of randomly chosen trees, which were at a fixed distance (200 m) but in a random direction from the nest tree (Hardey, 2006). We identified tree species according to Ahmed et al. (2009).We determined tree height using a clin- ometer and calculated distances remotely, using coordinates determined with a GPS, in QGIS 2.18 (QGIS Development Team, 2016). We visually estimated the crown density of the nest trees as the amount of light blocked by branches and foliage, in four categories: open (0–25%), partially open (26–50%), partially dense (51–75%), and dense (76–100%), followingMiron & Chowdhury (2019).
Identifying priority areas for conservation
Satellite tracking of three juvenile Pallas’s fish eagles has shown that the mean home range in winter is 38.9 ± SD 43.8 km2 but juveniles appear to spend most of the first win- ter within 1 km of the nest location (Steele, 2017). The home range size of large raptors is closely related to food availability, especially during the breeding season when more resources are required to feed the young (Newton, 1979;Hardey, 2006). Here, we defined a circle with a 3-km radius, with the nest location at the centre, as the primary nest habitat. Many nest sites used by different pairs were close together. When the 3-km radius around four or more nest sites
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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