Abstract:
The Sri Lanka frogmouth (Batrachostomus moniliger) is a small-sized nocturnal bird largely restricted to
tropical lowland forests with thick undergrowth. The motionless roosting posture as well as their plumage
coloration and color patterns (which resemble that of tree stems and branches) enable them to camouflage
and thereby avoid predation. Through opportunistic field surveys over a 20-year period (January 1998-
February 2018), we surveyed different bioclimatic regions of Sri Lanka covering 500 sites, and
documented presence of Sri Lankan frogmouth in 83 of sites. In these 83 sites, we recorded 136 birds
including seven nesting pairs and chicks. Our survey confirmed frogmouth presence in four floristic
regions of Sri Lanka, particularly from lower elevations (11-767 m). Most sightings were made in
lowland rainforests while savannah woodlands had the least number of records; no frogmouths were
recorded inside anthropocentric land-cover types such as commercial-scale farmlands or plantations.
According to the habitat suitability model we constructed (MaxEnt-based), much of the lowland wet
zone, particularly the southwestern corner, was predicted as the most suitable areas for Sri Lankan
Frogmouth while the northeastern coastal plains, and the mid-western and northwestern parts of Sri Lanka
seemed least suitable. According to The Maxent model’s internal jackknife test of variable importance,
temperature seasonality is the most important predictor of frogmouth’s distribution. Sri Lanka frogmouths
have a high fidelity for their roosting sites as they remained in the same tree at least for a week; preferred
roosting trees were medium-sized Dicot species with a dense canopy cover and variable canopy heights.
These roosting sites are relatively cool, humid with little exposure to direct sunlight. Both roosting and
nesting trees were relatively isolated from neighboring canopy trees, thus, the understory surrounding the
roosting tree was dense. Six of the nesting sites observed were located in the lowland wet zone rainforests
while the other was in savannah woodlands of the intermediate zone. These nests were positioned
approximately 66% of maximum canopy height of the host tree. Nests were constructed on relatively thin
branches that formed an acute angle against the main stem. These nests are shallow, circular-shaped pads.
The nest interior was cushioned with cotton, parts of fishtail palm, and down feathers while the nest
exterior contained pieces of lichens and tree bark. Sri Lanka Frogmouth’s home ranges appeared to be
very small in nesting season, a maximum of 60 m radius area around the roosting site. Both male and
female birds alternate nest-guarding duties through most of the night-time. The major threat for the
frogmouth in Sri Lanka includes habitat loss due to expansion of commercial-scale agriculture and
monoculture plantations, illicit forest encroachments, and clear-cutting.
