The diet of Indian Eagle Owl Bubo bengalensis and its agronomic significance
Satish
Pande 1 & Neelesh Dahanukar 2
1 Ela
Foundation, C-9, Bhosale Park, Sahakarnagar-2, Pune, Maharashtra 411009, India
2 Indian
Institute of Science Education and Research, Sai Trinity, Sutarwadi Road,
Pashan, Pune, Maharashtra 411021, India
Email: 1pande.satish@gmail.com, 2 n.dahanukar@iiserpune.ac.in(corresponding author)
Date of
publication (online): 26 August 2011
Date of
publication (print): 26 August 2011
ISSN
0974-7907 (online) | 0974-7893 (print)
Editor: Reuven Yosef
Manuscript details:
Ms # o2536
Received 04 August 2010
Final received 16 March 2011
Finally accepted 29 July 2011
Citation: Pande, S.
& N. Dahanukar (2011). The diet of Indian Eagle Owl Bubo
bengalensis and its
agronomic significance. Journal
of Threatened Taxa 3(8): 2011–2017.
Copyright: © Satish
Pande & Neelesh Dahanukar 2011. Creative Commons Attribution 3.0 Unported
License. JoTT allows unrestricted use of this article in any medium for
non-profit purposes, reproduction and distribution by providing adequate credit
to the authors and the source of publication.
Author
Detail: Satish Pandeis a Fellow of Maharashtra Academy of Sciences. He is an Interventional
Vascular Radiologist and Assoc. Professor of Radiology at B.J. Medical College,
Pune. He works in ecology and field ornithology and has made several video
films on raptor ecology, marine ecosystem and conservation. He has published
more than 40 papers and has authored several field guides and popular books on
ornithology, nature education, orchids and other subjects for popularization of
science and to promote conservation.
Neelesh Dahanukar works in ecology and evolutionary biology with an
emphasis on mathematical and statistical analysis.
Author Contribution: SP laid the
foundations of the work and collected field data. ND performed statistical
analysis. Both SP and ND wrote the paper.
Acknowledgements: The study
was supported by Ela Foundation. We are thankful to Amit Pawashe, Dr. M.N.
Mahajan, Kumar Pawar, Prashant Deshpande and Shivkumar Pednekar for their
assistance in the field work. We are also grateful to Dr. M.S. Pradhan, Dr.
D.B. Bastawade, Dr. S.S. Talmale, Dr. R. Sharma and Dr. Anil Mahabal from
Zoological Survey of India, W.R.S. Akurdi, Maharashtra, for their help in
identifying the prey species from pellet analysis and prey remains. SP thanks
Dr. Hemant Ghate, Head, Department of Zoology, Modern College, Pune and the
Head, Department of Environmental Sciences, University of Pune. We thank three
anonymous referees for constructive comments on the earlier drafts of the
manuscript.
Abstract: If the
importance of wildlife in agricultural pest control through predation can be
conveyed, it can play an important role in the conservation of wildlife. However, such a strategy needs to be
backed with convincing data. We
studied the habitat preference, diet and reproductive behavior of the Indian
Eagle Owl (IEO) Bubo bengalensis in order
to understand its role in agricultural pest control. The Owls preferred landscapes with a higher percentage of
agriculture and fed on rodents, birds, reptiles, arachnids, insects and other
prey species. Despite being a
generalist feeder, its diet was dominated by agricultural pests, which
contributed 88% of the total prey biomass. Out of the13 rodent prey species, which comprised a major
part of the diet, seven were identified as major agricultural pests and were
98% of the total rodent biomass in the diet of the IEO. The dependence of the IEO on rodent
pests was further reflected by positive correlation between rodent biomass
consumed and the breeding success of the owl. The IEO, therefore, plays a positive role in the biological
control of crop pests. However, owls
spent a longer duration of time in agricultural habitats, where they also had
higher productivity. Thus IEO may
be subjected to anthropogenic activities, human contact and interference. Since this owl is still hunted due to
superstitious beliefs, scientific evidence elucidating the importance of the
IEO in agricultural pest control can be used for its conservation by educating
the farming community.
Keywords: Agronomic
significance, Bubo bengalensis, crop
pests, Indian Eagle Owl, rodent control.
For figures, tables -- click here
INTRODUCTION
Agriculture
is a major source of livelihood in India. Indian agriculture is rapidly shifting from natural subsistence type
farming to a managed intensive agricultural practice. Despite the developments in the infrastructure for production
and storage of agricultural produce, it is estimated that rodents damage
between 2–15 % of the crops annually throughout the country; while severe
damage can escalate to 100% (Parshad 1999). As a result, agricultural pest control is a major concern.
Chemical control using pesticides and biological control through predators and
pathogens have been suggested for pest control (Howard 1976; Parshad
1999). However, chemical
pesticides and control of pests using pathogens often affect the environment
and human health adversely (Hearn 1973; Wodzicki 1973; Kaukeinen 1982; Littrell
1990; Gillies & Pierce 1999). Hence, utilization of natural predators is an environment friendly
solution to pest control (Wodzicki 1973; Singleton 1994; Johnson et al. 1996).
A
number of natural predators of the agricultural pests have been identified for
their use in pest control. While
some of them, such as cats, can be domesticated (Wodzicki 1973), even wildlife
can be considered as natural enemies of crop pests (Johnson et al. 1996). If the importance of wildlife in pest
control can be backed up with convincing data it will serve two purposes. First, predation by wildlife can be
promoted as an environment friendly pest control method and second, if the
importance of wildlife in pest control is conveyed to the farmers, it can be
used as a step towards conflict resolution and conservation of wildlife. This is especially true for predators
like owls, which are often killed as they are considered as bad omens and also
for their demand for use in black magic (Kasambe et al. 2004; Devkar 2009).
Owls
are important for controlling agricultural pests as their diet is dominated
with rodents (Neelanarayanan et al. 1999, 2007; Pande et al. 2004, 2007). Even though the potential of owls in agricultural
pest management has been suggested earlier (Wodzicki 1973) and the possible
importance of natural predators in integrated pest management programs is also
recognized (Jain et al. 1993) a strong argument supported by relevant data is
still missing.
In
this study we elucidate the importance of the Indian Eagle Owl (IEO) Bubo bengalensis as a potential predator
of agricultural pests by studying its reproductive output in relation to diet
and habitat selection. We propose
that such scientific findings based on first hand field data could be used to
promote conservation awareness and the eradication of superstition about these
biological controllers of agricultural pests.
METHODS
Study
Area
The
study was conducted around Pune (18032’N & 72051’E)
on the Deccan Plateau, and around Alibag (18028’52”N & 73014’52”E)
and Chiplun (1700’2”N & 73018’57”E) in the coastal
region of Maharashtra State, India. The average annual precipitation in the study area, which is derived
from the southwest monsoon, ranges from 250–1250 mm in the Deccan Plateau
and 1500–3500 mm in the coastal region. The temperature ranges between 60C and 400C
during winter and summer respectively. Agricultural cropland consists of seasonal Triticum aestivum, Zea mays, Sorghum vulgarae, Panium miliaceum, Oryza
sativa, and open type of cultivation of lentils,
pods, leafy vegetables, fruit orchards of figs (Ficus sp.), pomegranate (Punica granatum), custard apple (Annona reticulata) and guava (Psidium sp.) (an open type of cropland with
better visibility). The perennial
grassland community in the study area is Sehima-Dichanthium type arrested in
sub-climactic seral stage of succession due to grazing, grass cutting and
burning (Roychoudhary 1966; Murthy & Sanjappa 2001). The grasses are Aristida setacea, Aristida adscenscionis and Heteropogon contortus with a presence of bushXanthium strumeriumthat has spiny seeds.
Data
collection
During
the breeding season (October–March) of 2004–05 and 2005–06,
we identified 44 occupied nest sites. The Eurasian Eagle Owls are known to nest near their preferred hunting
areas (Frey 1973; Olsson 1979; Leditznig 1992) and their breeding success
depends on the distance between the nest and foraging area. Hence, we selected an area of 1000m
radius centered around the nest in order to analyze the landscape features in
all of the nest territories. Following Donázar (1987),
Penteriani et al. (2001) and Pande et al. (2007) we categorized each circular
plot into six habitat categories: (a) agriculture, (b) scrub, (c) grassland,
(d) water body (perennial or seasonal), (e) hills, and (f) rural habitat around
human habitation, using ‘look down’ visual surveys conducted from high vantage
points and estimated the percentage occurrences of each category (Bibby et al.
1998).
At
least five visits were made to each of the nest sites each year during the
breeding seasons. Owl pellets and
prey remains were collected from all nest sites and were separately analyzed
for each nest for every breeding season. Pellets were dried in an oven, dissected and all identifiable prey
remains were scrutinized (Penteriani et al. 2002). To avoid duplication, items found in pellets were used only
when not found as prey remains in the same visit (Penteriani 1997). We did not encounter separate prey
remains that were not found in pellets. Prey in pellets were identified to orders, families, genera or species by
using published literature (Tikader & Bastawade 1983; Tikader & Sharma
1992; Daniels 2002; Ramanujam 2004) or by comparing with specimens in the
collections of the Zoological Survey of India, Pune. Pellet contents were grouped into six categories, namely
insects, reptiles, birds, rodents, bats and other prey species. The number of individuals in each diet
category was considered as the abundance for that category. The fresh masses of prey species were
estimated by weighing specimens in the field using Pesola scales (least count
0.1g) or by using published data (Spillet 1966; Khajuria 1968; Ranade 1989,
1992; Kanakasabai et al. 1998; Pande et al. 2004, 2007). This was used for calculating the
biomass of each prey type in the diet of the owl. Percent biomass for each diet category was calculated by
estimating the relative percent contribution of each category to the overall
diet.
Breeding
time and nest site occupation were monitored to record the dates of egg-laying,
to monitor the number of hatchlings and count the number of fledglings (Frank
& Lutz 1997; Penteriani et al. 2002). We considered the date of laying of the first egg as the date for the
onset of the breeding season. The
incubation period or duration of breeding was calculated as first egg laid till
last egg hatched based on the observation that the hatching is asynchronous in
IEO (Ramanujam & Murugavel 2009). We determined breeding success or productivity as the number of
fledglings per nest.
Statistical
analysis
We
performed the Kruskal-Wallis test and multiple pairwise comparisons using the
Mann-Witney U test with Bonferroni correction (Bonferroni corrected
significance level used was a =
0.0033) to see if the preference for different habitats was different. Associations between prey items,
habitat preference and breeding success were analyzed using Redundancy
Analysis (RDA) assuming that these are interdependent variables. RDA was performed in the freeware
Biplot 1.1 (Smith & Lipkovich 2002). To see if the dependent variables were linearly dependent on the
independent variables we performed permutation tests with a null hypothesis
that dependent and independent variables are not linearly related to each other
(Legendre & Legendre 1998).
RESULTS
AND DISCUSSION
IEO
builts terrestrial nests on hill slopes, earth cuttings, rocky outcrops and
under bushes, where the surrounding areas, which are its hunting grounds,
consisted of agriculture, scrub, grassland, water body, hills and rural
habitats. IEO preferred to nest in
landscapes with a high percentage of agriculture followed by grassland and
scrubs (Fig. 1). Preference for
different habitats was significantly different (Kruskal-Wallis K = 141.199, p
< 0.0001), with a preference for agriculture dominated habitats than the
second most dominant grassland habitat (Mann-Witney U = 5075.000, p <
0.0001).
The
IEO showed high versatility in the choice of food depicting its feeding habit
as a dietary generalist (Table 1). It fed on rodents, birds, reptiles, arachnids, insects and other prey
species. Rodent prey included
Lesser Bandicoot Rat (Bandicota
bengalensis), Large Bandicoot Rat (B. indica), Indian Bush Rat (Golunda ellioti), Soft-furred Field Rat
(Millardia meltada),
House Mouse (Mus musculus),
Field Mouse (M. booduga),
Elliot’s Spiny Mouse (M. saxicola),
House Rat (Rattus rattus),
Indian Gerbil (Tatera indica),
Long-tailed Tree Mouse (Vandelura
olivacea), Common House Shrew (Suncus murinus), Pigmy Shrew (S. etruscus) and Anderson’s Shrew (S. stolizcanus). Bat prey included
Indian Fulvus Fruit Bat (Rousettus
lesheanaulti) and Lesser Dog-faced Bat (Cynopterus sphinx). Bird prey included Ashy-crowned
Sparrow-Lark (Eremopterix grisea),
Rufous-tailed Sparrow-Lark (Ammomanes
phoenicura), Blue Rock Pigeon (Columba livia), Common Myna (Acridotheres tristis), Jungle Myna (A. fuscus), Egret species (Egretta sp.), Asian Koel (Eudynamys scolopacea), Large Grey Babbler (Turdoides malcolmi), Painted Francolin (Francolinus pictus), Quail species (Coturnix sp.), Eurasian Collared Dove (Streptopelia decaocto), Common Kingfisher (Alcedo atthis), Little Green Bee-eater (Merops orientalis), House Sparrow (Passer domesticus), Sunbird species and
House Crow (Corvus splendens). Reptiles included lizards (Calotes sp.), geckos, skinks and snake (Coelognathus helena). Arachnids included Mesobuthus tamulus, Heterometrus xanthopus, Heterometrus granulomanus, Galeodus orientalis and Galeodus indica. Insect prey included Rhinoceros Beetle
(Oryctes rhinoceros),
Long-horned Beetle (Batocera rufomaculata),
Stag Beetle (Lucanus cervus)
and Grasshoppers and Mantids. Other unidentified prey items included juveniles
of Fellidae and Leporidae (Lepus nigricolis)
and amphibians.
Even
though our analysis of the diet suggests that the Indian Eagle Owl is a dietary
generalist, which concurs with published literature (Ali & Ripley 1969;
Ramanujam 2006), the abundance and total biomass of different groups of prey in
the diet showed that rodents were the most important prey followed by birds and
bats (Table 1). Abundance of
insect prey was also very high but the biomass of insect diet was minute. Of all prey items, 73% of relative
abundance and 81% of prey biomass was of pests of agricultural significance
(Table 2). Thus, the IEO is an
important predator of agricultural crop pests, particularly rodents. Owls also feed on a variety of other
agricultural pests like insects and bats and venomous organisms like snakes and
scorpions (Table 2).
Of
13 species of rodent prey, which formed the major part of the diet of the IEO
(55% relative abundance and 85% total biomass), seven were agriculturally
important pests (Jain et al. 1993; Parshad 1999). Agriculturally important rodent pests contributed 88% of the
abundance and 98% of the biomass of the total rodent in the owl diet. To understand whether the rodents
actually came from the agriculture dominated habitats we performed Redundancy
Analysis (RDA) with relative abundance of different prey items as dependent
variables and percent habitat types as the independent variables (Fig. 2). There was a significant relationship
between dependent and independent variables (permutation test pseudo-F = 0.380,
p < 0.0001). The relative
abundance of rodents in the diet was significantly correlated with increase in
the agricultural habitat (correlation coefficient r = 0.3996, p = 0.0001). Although bats and birds were also
positively correlated with the agricultural habitat, they were more strongly
correlated with the increase in rural and scrub habitats respectively (Fig. 2).
We
also performed RDA to understand how habitats and relative abundance and
percent biomass of different prey types affected the productivity and duration
of breeding in the IEO (Fig. 3). Our analysis suggests that both productivity and duration of the
breeding of IEO was high in agriculture and scrub dominated habitats (Fig. 3a,
permutation test pseudo-F = 0.166, p = 0.006). Productivity was strongly
correlated to the high relative abundance (Fig. 3b, permutation test pseudo-F =
0.309, p < 0.0001) and percentage biomass (Fig. 3c, permutation test
pseudo-F = 0.302, p < 0.0001) of rodents and birds followed by bats.
Our
findings of RDA point to two important outcomes. First, owls have a high productivity in the agriculture
habitat (Fig. 3a), which could be attributed to the increased access to rodents
(Fig. 2) which alleviates their productivity (Fig. 3b and 3c). As a result, owls are not just the
predators of rodents, important agricultural pests, but are in turn dependent
on them to increases their productivity. Therefore, there appears to be a delicate interdependence between owls
and rodent populations. However,
this interdependence points to another alarming threat to the owls. Chemical pesticides are used for rodent
pest control which can affect the non-targeted wildlife (Kaukeinen 1982;
Littrell 1990; Gillies & Pierce 1999; Newton & Wyllie 2002). Because the IEO has shown a dependence
on the rodents, use of these rodent pesticides could affect IEO populations
because of secondary poisoning. Second, since the productivity of owls is
higher in the agricultural lands, the duration of breeding in the agricultural
land is greater (Fig. 3a). As a result, owls may be prone to detection and
anthropogenic activities including persecution and interference.
Unfortunately,
the IEO is often subject to indiscriminate hunting, out of superstition or fear
(Pande et al. 2005) or trapping for use in black magic (Kasambe et al. 2004;
Devkar 2009). If we can promote
the importance of owls in the control of agricultural pests, especially
rodents, then such a strategy will help reduce human persecution of the
owls. We believe that interactive
educational programs based on scientific data, like this study, can be used to
remove superstitions and further owl conservation.
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