Journal of Threatened Taxa |
www.threatenedtaxa.org | 26 July 2023 | 15(7): 23514–23520
ISSN 0974-7907
(Online) | ISSN 0974-7893 (Print)
https://doi.org/10.11609/jott.8415.15.7.23514-23520
#8415 | Received 20 February 2023 | Final received 25 April 2023 |
Finally accepted 16 July 2023
An assessment of the diet of
Brown Fish-Owl Ketupa zeylonensis
(J.F. Gmelin, 1788) (Aves: Strigiformes:
Strigidae) from two localities in the
foothills of the Western Ghats of Goa, India
Stephen Jonah Dias 1 & Atul Sinai Borker
2
1,2 Planet Life Foundation, C8/G4,
Maple Grove Apartments, Belloy, Nuvem,
Salcete, Goa 403601, India
2 Luta Innovation, 887/13, Kamat Nagar, Porvorim, Socorro, Bardez, Goa 403501, India
1 stephendias46@gmail.com
(corresponding author), 2 borker.atul@gmail.com
Abstract: The Brown Fish-Owl Ketupa zeylonensis is
a large nocturnal bird of prey that has a vast distribution range. However,
there is a significant literature gap on the ecology of this species in the
Western Ghats ecoregion, particularly in regard to its food spectrum. In the
present study, we assessed the diet composition of this species in the
foothills of the Western Ghats of Goa, India. The diet was evaluated by analysing the undigested prey remains in regurgitated
pellets obtained from the banks of forest streams and roosting sites. A total
of 104 pellets were collected from two localities that exhibited similar
landscape characteristics. Our analysis indicated that crabs contributed to a
significant proportion of the diet of the species (75.47%), followed by
amphibians (frogs, 8.02%), fishes (7.08%), reptiles (snakes, 2.83%), birds
(2.36%), scorpions (1.89%), and insects (Odonata, 0.47%). Additionally, 1.89%
(n = 4) of the prey items could not be identified due to their disintegrated
nature. Furthermore, an assessment of Food Niche Breadth (FNB) indicated that K.
zeylonensis exhibited a high degree of
specialization in terms of its diet in the study areas.
Keywords: Diet analysis, feeding ecology,
food niche breadth, food spectrum, forest streams, owl pellets, prey
composition, relative frequency of occurrence.
Abbreviations: CITES—Convention on International
Trade in Endangered Species of Wild Fauna and Flora | FNB—Food Niche Breadth |
IUCN—International Union for Conservation of Nature | NP—National Park |
RFO%—Relative Frequency of Occurrence | UNESCO—United Nations Educational,
Scientific and Cultural Organization | WS—Wildlife Sanctuary.
Editor: Prachi Mehta, Wildlife Research and
Conservation Society (WRCS), Pune, India. Date
of publication: 26 July 2023 (online & print)
Citation: Dias, S.J.
& A.S. Borker (2023). An assessment
of the diet of Brown Fish-Owl Ketupa zeylonensis (J.F. Gmelin,
1788) (Aves: Strigiformes: Strigidae)
from two localities in the foothills of the Western Ghats of Goa, India. Journal of Threatened Taxa 15(7):
23514–23520. https://doi.org/10.11609/jott.8415.15.7.23514-23520
Copyright: © Dias & Borker 2023. Creative Commons Attribution 4.0
International License. JoTT allows unrestricted use, reproduction, and
distribution of this article in any medium by providing adequate credit to the
author(s) and the source of publication.
Funding: No funding was acquired to carry out the study.
Competing interests: The authors declare no competing interests.
Author details: Stephen Jonah Dias holds a master’s degree in wildlife biology and conservation. He served as research associate at Planet Life Foundation. He has previously worked on otters in human-dominated landscapes of Goa, India. Presently, he is working on several research projects pertaining to the ecology of mammals and birds in Goa. Atul Arun Sinai Borker has served as director of Planet Life Foundation. Currently, he is working on wildlife research and conservation projects in the Western Ghats.
Author contributions: SJD & ASB: Conceptualized the study and carried out the field work; SJD: Performed the data analysis; SJD & ASB: Prepared the draft of the manuscript.
Acknowledgements: We are grateful to the trustees
of Planet Life Foundation and the staff of Nature’s Nest Nature Resort for
their support during the project. Our gratitude to the Goa State Biodiversity
Board for granting the necessary approvals for the project. Special thanks to Swanand Patil of Arcane
Conservancy for his assistance during the initial stages of the project and for
providing us with the photograph of Brown Fish-Owl from his personal
collection. We would also like to thank Rohan Gaonkar
and Gautam Gad for extending their support. We are thankful to Sophia Dias, Anselia Da Costa, and Dr. Claret Shastry for proofreading the manuscript. Lastly, we would
like to appreciate the efforts of the reviewers for their constructive comments
and suggestions on the manuscript.
Introduction
Birds of prey occupy the apex
position in food web assemblages. Therefore, they are considered to be
important bioindicators of the environments in which they persist
(González-Rubio et al. 2021). The taxonomic order Strigiformes
is represented by 250 extant species of owls distributed across the world (Gill
et al. 2023). This order is divided into two families: (i)
Tytonidae, which includes barn owls, bay owls, and
grass owls, and (ii) Strigidae, which includes true
(or typical) owls (Sieradzki
2023). India is home to 32 species of owls, 13 of which are found in the state
of Goa (Baidya & Bhagat 2018; BirdLife
International 2020). The Brown Fish-Owl Ketupa
zeylonensis is a nocturnal bird of prey that is
distributed across southern and southeastern Asia with isolated populations
occurring in Turkey and Iran, and vagrant populations occurring in Seychelles
(Birdlife International 2016). It is a large bird (approx. 56 cm) having bright
yellow eyes and outward-facing ear tufts. It exhibits rufous-brown upper parts
with heavy streaking, and pale underparts with dark streaks (Ali 2002; Kazmierczak & Perlo 2012;
Grewal et al. 2016). The species is classified as ‘Least Concern’ in the IUCN
Red List of Threatened Species. Although global populations of this species
have not been evaluated, it is suspected to be in decline due to habitat
destruction (Birdlife International 2016). In addition, the species is listed
under ‘Schedule I’ of the Indian Wild Life (Protection) Amendment Act, 2022 and
under Appendix II of CITES (Ministry of Law and Justice 2022; CITES 2023). In
India, this species faces threats from the illegal wildlife trade, persecution
by fishermen, and its use in witchcraft (Ahmed 2010).
The Brown Fish-Owl inhabits
deciduous, semi-deciduous and evergreen woodland ecosystems and is found in
close proximity to water bodies. Its diet is reported to constitute crabs,
fish, frogs, reptiles, birds, mammals, and carrion (Ali 2002; Bindu &
Balakrishnan 2015; Grewal et al. 2016).
Owls are highly specialized
hunters that regurgitate undigested prey remains such as bones, feathers, hair,
scales, and other exoskeletal structures of their prey in the form of compact
pellets. The analysis of regurgitated pellets has proven to be a robust
technique to assess the food spectrum of owls and understand the diversity and
population structure of prey species (Meek et al. 2012; Andrade et al. 2016).
In an Indian context, published literature on the diet composition of the Brown
Fish-Owl is sparse. Vyas et al. (2013) reported the food spectrum of K. zeylonensis from Jambughoda
WS in Gujarat. However, there is a significant literature gap in the diet
composition of the species from the Western Ghats ecoregion, particularly in
the context to the Indian state of Goa. This study was carried out to
understand the diet composition of the species in two sites located in the
foothills of the Western Ghats of Goa.
Materials
and Methods
Study Area
Goa is located on the western
coast of India (15.492°N, 73.826°E) (Figure 1). The Western Ghats is a 1,600 km
long mountain range that runs parallel to the western coast of the Indian peninsula
and extends through the Indian states of Gujarat, Maharashtra, Goa, Karnataka,
Tamil Nadu, and Kerala. These mountains are recognized as one of the world’s
eight ‘hottest hotspots’ for biological diversity and endemism (Molur et al. 2011; UNESCO 2023). In Goa, these mountains
pass through the eastern regions of the state where a significant section of
the range is protected through four protected areas: Mhadei
WS, Bhagwan Mahavir WS & NP, Netravali WS, and Cotigao WS. The vegetation type of the Western Ghats of Goa
is varied and includes tropical evergreen, semi-evergreen, and moist mixed
deciduous forests (Goa Forest Department 2023). This study was conducted along
forest streams that originate from the Western Ghats. The sections of the
streams surveyed for this study were located outside the boundaries of
protected areas. Study Area 1 was located near Mhadei
WS and Study Area 2 was located near Bhagwan Mahavir
WS & NP. The streams that were considered for the study were of the
perennial and intermittent type. The general vegetation type of the study areas
is dominated by tropical evergreen, semi-evergreen, and moist mixed deciduous
forests. In addition, both study areas were located in close proximity to
plantations and human settlements. The streams considered for this study are
part of a larger catchment system that empties into the Mahadayi
River of Goa (see Figure 1). The aerial distance between the two study areas
was approximately 16.7 km.
Data Collection
This study was conducted from 20
October 2022 to 5 February 2023. Prior to this study, Brown Fish-Owl activity
in Study Area 1 was established by conducting field surveys. In addition, the
feeding and breeding activity of this species in Study Area 2 was recorded for
over two years with the help of camera traps and direct observations
respectively. This was part of a larger nocturnal wildlife monitoring effort by
Planet Life Foundation, Belloy, Nuvem,
Goa and Nature’s Nest Nature Resort, Surla, Sancordem, Goa. Brown Fish-Owl pellets were collected from
the study areas once a week. The pellets were usually found deposited along
stream banks and in close proximity to roosting sites (Image 1). A total of
four roosting sites were identified across our study areas based on repetitive
pellet deposition observed during our surveys. The entire pellet was collected
and temporarily stored in plastic zip-lock bags. Prior to analysis, we manually
removed all conspicuous debris from the pellet by hand. Following this, the
pellets were soaked in 70% ethyl alcohol for 24 h to kill all microorganisms.
The pellets were then air-dried for 24 h to remove moisture. During analysis,
the dry weight of each pellet was recorded using a weighing balance with 0.001
g accuracy. The prey items in the pellets were then sorted into eight
categories: crabs, insects, scorpions, fishes, amphibians, reptiles, birds, and
unidentified prey. These prey categories were established based on literature
review and field observations. As we did not have access to reference
specimens, the items in the pellets could not be identified at the species
level. Identification of the prey items was carried out using reference books
and taxonomic keys (Verma 2014; Ganguly
et al. 2015; Saxena & Saxena 2019; Mehta et al. 2020; Mishra et al. 2021).
As most of the items in the pellets were conspicuous, identification and
sorting were possible by the naked eye. However, we used a compound microscope
(ESAW SM-02, ESAW India, Ambala Cantt, Haryana, India) set at 10x magnification
to identify the inconspicuous prey remains. Arthropods were identified
primarily from structures such as mouthparts, chelipeds, pereiopods, abdomen,
and carapace (in the case of crabs); wings (in the case of insects); pedipalps,
cephalothorax shield, mesosoma, metasoma, walking legs, and telson (in the case
of scorpions). Scorpion identification was also aided by shining an ultraviolet
light at 395 nm and observing fluorescence (Gaffin et
al. 2012) (Image 1). Chordates were identified from endoskeletal
and exoskeletal structures such as bones and scales (in the case of fishes),
bones and mouthparts (in the case of amphibians i.e., frogs), vertebrae, ribs,
and skin (in the case of reptiles i.e., snakes), and bones and feathers (in the
case of birds). The prey items that could not be identified were sorted into
the ‘unidentified’ category. For each pellet, we estimated the number of
individuals for each prey category (Table 1). The data for both study sites was
pooled and subsequently analyzed.
Data Analysis
We estimated the Relative
Frequency of Occurrence (RFO%) for each prey group by dividing the number of
occurrences of each prey category by the total number of occurrences of all
prey categories multiplied by 100 (Mehta et al. 2020). To assess the diversity
of prey in the owl diet, we estimated the Food Niche Breadth (FNB) by employing
the standardized Levin’s Index (BA) formula (Levins
1968; Colwell & Futuyma 1971; Mehta et al. 2018)
as follows:
x 
Where Pi is the
proportion of ith prey category and
n is the number of prey categories recorded in the diet of the Brown Fish-Owl.
This standardized index computes a value that can range from 0–1. Values closer
to 0 indicate a specialist diet whereas values closer to 1 indicate a generalist
diet (Mehta et al. 2018).
Results
A total of 104 Brown Fish-Owl
pellets were collected during the present study (50 pellets from Study Area 1
and 54 pellets from Study Area 2). The average dry weight of the pellets was
estimated to be 4.053 g (SD = ± 2.627; Range = 0.590–12.953). The total number
of prey individuals recorded was 212. The average number of prey individuals
per pellet was estimated to be 2.029 (SD ± 1.074; Range = 1–5). The diet of the
Brown Fish-Owl was dominated by crabs followed by amphibians (frogs), fishes,
reptiles (snakes), birds, scorpions, and insects (Odonata). The unidentified
prey individuals constituted a minor portion of the diet (n = 4, Table 2).
Although we were unable to positively identify the type of prey items in the
‘unidentified’ category due to their disintegrated nature, we were able to
identify the remnants as vertebrates. In such cases, all the unidentified
remains having similar characteristics were assumed to originate from a single
individual. The number of occurrences of prey categories was largely comparable
across the two study areas. However, insects were only present in the pellets
collected from Study Area 1 (Odonata, n = 1) and scorpions were only present in
pellets collected from Study Area 2 (n = 4) (Figure 2). Lastly, the Food Niche
Breadth (FNB) value was estimated to be 0.1, indicating that the Brown Fish-Owl
exhibits a high degree of specialization in terms of its diet in the study
areas. The diet composition of the species in the present study has been
detailed in Table 2.
Discussion
The Brown
Fish-Owl is a nocturnal predator that is known to feed on a wide variety of
prey, such as fish, frogs, crabs, small mammals, birds, and reptiles. It is
also reported to occasionally feed on carrion (Ali 2002). Published literature
on the diet composition of K. zeylonensis in
India is sparse. A study conducted by Vyas et al. (2013) on the breeding behaviour of K. zeylonensis
in Jambughoda WS and surrounding areas in Gujarat,
India reported fishes, crabs, insects, and prawns in the pellets of the
species. However, the authors identified several other prey groups such as
amphibians, reptiles, and birds from direct feeding observations and analysis
of discarded prey items at the nests. This indicates that pellet analysis when
supplemented with other observational protocols can significantly aid in the
understanding of the food spectrum of the species. The diet composition of K.
zeylonensis in Jambughoda
WS was very similar to our observations in the Western Ghats of Goa with minor
differences (Figure 3). In addition, the study in Jambughoda
Wildlife Sanctuary was conducted during the pre-monsoon season (March–April) as
compared to the present study that was conducted during the post-monsoon and
winter seasons (October–February). Furthermore, fish owls are specialist birds
of prey that have preferences for certain prey groups (Sieradzki
2023). Our data analysis supports this fact as the food niche breadth
assessment indicated that K. zeylonensis is a
specialist predator that feeds mainly on crabs whilst supplementing its diet
with other invertebrate and vertebrate prey groups (Figure 2; Table 2).
Pellet analysis is considered to
be a robust indicator of the food spectrum of owls. In addition, such analysis
can shed light on the richness, evenness, and abundance of prey groups
constituting owl diet in the foraging environments (Heisler et al. 2015;
Andrade et al. 2016). The present study was conducted due to the gap in
knowledge in regards to the diet composition of K. zeylonensis
in the Western Ghats ecoregion, particularly in the state of Goa. However, it
is imperative to note that pellet collection in the present study was conducted
for a relatively short period of time (post-monsoon and winter seasons), and the
diet composition of owls is reported to change based on seasonal variations in
prey availability (Kafkaletou-Diez et al. 2008; Santhanakrishnan et al. 2010). This may be an important
factor to consider in landscapes such as the Western Ghats that undergo changes
in hydrology across seasons. Organisms in such aquatic ecosystems may exhibit
population changes on a seasonal scale that may influence the diet composition
of the Brown Fish-Owl. Therefore, further assessments are required to
understand the trends in the diet composition of the species across a seasonal
gradient in the Western Ghats landscape.
Table 1. Details of key body
parts examined for the identification of the number of prey individuals in each
pellet.
|
Prey Category |
Key body parts used for
assessing the number of individuals |
Details of analysis |
|
Crabs |
Mouthparts, chelipeds,
carapace, abdomen |
The number of duplicates of
exoskeletal structures (either whole parts or fragments) was used to estimate
the number of individuals in each pellet. |
|
Insects |
Wings |
|
|
Scorpions |
Pedipalps, cephalothorax
shields, and telson |
|
|
Amphibians (Frogs) |
Mouthparts, vertebrae (e.g., urostyle), pelvic girdle, humerus,
radio-ulna, femur, tibio-fibula, and
astragalus-calcaneum. |
|
|
Fishes |
Parts of the axial skeleton
(skull, vertebrae, and ribs), and scales. |
Microscopic examinations of the
morphological patterns on fish scales were conducted based on the principle
that the patterns serve as useful taxonomic identifiers of fish species (Bräger & Moritz 2016). This was further supported by
observations of the bones from the axial skeleton. As it was difficult to
determine the number of individuals of the same species, all endoskeletal remains of similar size were assumed to be
derived from a single individual unless morphological examinations of the
scales indicated more than one species in the pellet. |
|
Reptiles (Snakes) |
Vertebrae, ribs, and skin |
Identifying the number of
individual snakes was straightforward in instances where the vertebral column
was found to be relatively intact in the pellets. However, in instances where
the vertebral column was found to be in a dismantled state, we used the
general shape and size of the vertebrae and ribs to estimate the number of
individuals. This was further supplemented by the remnants of snake skin
present in the pellets. |
|
Birds |
Parts of the endoskeleton and
feathers. |
The number of duplicate endoskeletal remains was utilized to estimate the number
of individuals. In cases where only feathers were present, feathers having
similar morphological characteristics were assumed to originate from a single
individual. |
Table 2. Diet composition of the
Brown Fish-Owl Ketupa zeylonensis
in the foothills of the Western Ghats of Goa.
|
Phylum |
Prey category |
n |
RFO % |
FNB |
|
Arthropoda |
Crabs |
160 |
75.47 |
0.1 |
|
Insects |
1 |
0.47 |
||
|
Scorpions |
4 |
1.89 |
||
|
Chordata |
Fishes |
15 |
7.08 |
|
|
Amphibians |
17 |
8.02 |
||
|
Reptiles |
6 |
2.83 |
||
|
Birds |
5 |
2.36 |
||
|
Unidentified |
4 |
1.89 |
||
|
Total |
|
212 |
100 |
n—Number of individuals in each
prey category | RFO %—Relative frequency of occurrence | FNB—Food niche
breadth.
For figures
& image - - click here for full PDF
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