Prey selection by the Barn Owl Tyto alba (Scopoli, 1769) in captivity
V. Vanitha1 &
R. Kanakasabai 2
1,2 Department of Zoology, A.V.C.
College, Mannampandal, Mayiladuthurai,
Tamil Nadu 609305, India
Present address: 2 Principal, Shree RaghavendraArts & Science College Keezhamoongiladi, Chidamparam, Tamil Nadu 608102, India
Email:
1 vanithabaskar@rediffmail.com,
2 ragha_gc@sancharnet.in
Date of online publication 26
July 2009
ISSN 0974-7907 (online) |
0974-7893 (print)
Editor: Rajah Jayapal
Manuscript details:
Ms # o1926
Received 21 January 2008
Final received 09 May 2009
Finally accepted 18 June 2009
Citation: Vanitha, V. & R. Kanakasabai (2009).
Prey selection by the Barn Owl Tyto alba (Scopoli, 1769) in
captivity. Journal of Threatened Taxa 1(7):
361-365.
Copyright: © V. Vanitha & R. Kanakasabai2009. Creative Commons Attribution 3.0 UnportedLicense. 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 Details: V. Vanitha was a Lecturer in
Zoology at the A.V.C. College (Autonomous), Mayiladuthurai.
She obtained her PhD. on ‘Status and management of captive Asian elephants (Elephas maximus) in
Tamil Nadu, Southern India from BharathidasanUniversity, Tiruchirapalli,
India in 2008. Her other research areas include behaviouralecology and management of captive Asian elephants.
Dr. R. Kanakasabai was the Head of the Department Zoology at the
A.V.C. College (Autonomous), Mayiladuthurai.
Presently he is serving as the Principal in Shree RaghavendraArts & Science College Chidamparam, Tamil Nadu.
Author Contribution: The
first author designed and conducted the study as a partial fulfillment of her
Master of Philosophy degree, while the second author supervised her research
work with technical inputs as a Research Adviser.
Acknowledgement: We are thankful
to Prof. Rajamohan, Principal, A.V.C. College for
providing all the facilities to the present study, Dr. K. Manimozhi,
Dept. of Zoology, A.V.C. College for his valuable suggestions on the
bio-chemical aspects analysis and Dr. K. Thiyagesan,
Dept. of Zoology, A.V.C. College Mayiladuthurai and
Dr. P. Neelanarayanan, Nehru Memorial College Tiruchirapalli for their technical advice in the study. We
also acknowledge the valuable comments to this manuscript by Dr. G. Leonardi INFS, Bologna, Italy and Dr. R. Nagarajan, Lecturer Dept. of Zoology A.V.C. College, Mayiladuthurai.
Abstract:We investigated prey selection of the Barn Owl Tyto alba under
captive conditions where birds were allowed to choose among individuals of
varying size from four field rodent species: Bandicota bengalensis, Millardia meltada, Tatera indica and Mus booduga. Owls
showed little species preference and a tendency to favourthe medium weight class in all prey species except M. booduga. Preference for body parts consumed varied
according to prey size, ranging from the head alone in the large weight class
to the entire body in the small weight class. Biochemical measurements showed that protein, carbohydrate and lipid
levels were higher respectively in the brain, liver and muscles of all three
species and weight classes studied. The preference for medium weight prey
despite a lower nutrient content compared to large weight prey is attributed to
a greater ease of capture.
Keywords: Barn
Owl, nutritional value, prey species, prey size preference.
The Barn Owl Tyto alba (Scopoli, 1769) is one of the most
widely-distributed predators of rodents (Taylor 1994; Leonardi& Dell’ Arte 2006; Neelanarayanan 2007a, b). These owls feed on a wide variety of prey and
their preference varies from place to place. In the Cauvery Delta of southern India the Barn Owl mainly feeds on the
Indian Mole Rat Bandicota bengalensis, Soft-furred Field Rat Millardia meltada and the House Rat Rattus rattus (Neelanarayanan2007b). They are known to shift their
prey preference between the pre-nesting and nesting seasons (Santhanakrishnan 1987). Opinions about Barn Owl feeding behaviourvaries widely. Taylor (1994) stated that
Barn Owls are specialists and depend mostly upon small mammals, but show
considerable flexibility. On the other hand, Mikkola(1976) stated that the Barn Owl is often portrayed as an opportunistic i.e.,
non-selective predator. Predator prey
selection may depend on the expected nutritive value of the prey, as well as
the time and energy cost of searching, subduing and consuming it (Davies 1977;
Griffith 1980). Studies on the feeding behaviour of Barn Owls showed that prey preference depends
on the availability of species, size, sex and nutrient contents of the prey (Schoener 1969; Fast & Ambrose 1976; Bellocq& Kravetz 1994). According to Ille (1991) body size and
energetic (nutrient) values of the prey species play a significant role in Barn
Owl diet selection. A review of
literature from the Indian subcontinent indicated that there is no study
available on nutritive values of prey and its influence on prey selection of
Barn Owls. Hence, this study was carried
out under captive conditions to assess prey selection in terms of prey species,
prey size and body parts of prey eaten by Barn Owls using four common field
rodent species viz., Indian Mole Rat Bandicota bengalensis, Soft-furred Field Rat Millardia meltada,Indian Field Mouse Mus boodugaand Indian Gerbil Tatera indica.
Materials and Methods
r-p
D
= ---------------
r
+ p -2rp
Where r is the proportion of a
particular species in the diet and p is the proportion of that prey in the
population.
Two-way ANOVA was used to assess
difference in the three nutrient levels of the organs and weight classes of
three prey species. There were
significant differences in nutrient levels among the three weight classes and
organs. Therefore, Least Significant Difference (LSD), a post-hoc multiple
comparison test was used to examine the difference among a pair of means
independent of the number and magnitude of the means (Sokal& Rohlf 1981).
Observations and Results
Prey species preference: The Barn Owl seemed to have more preference for B. bengalensis (29%) and M. meltada(27.8%), while the T. indica and the field
mouse, M. booduga were the least preferred
diets as their relative preference index values were respectively 24% and 19.2%
only, which was confirmed by Jacobs’ Preference Index (Table 2).
Prey size preference:When B. bengalensis and T. indica were offered, the Barn Owls preferred mostly the
medium weight class, followed by smaller weight class and rarely large weight
class (Table 3). In the case of M. meltada, the Barn Owl preferred mostly medium weight
class followed by large weight class and smaller weight class.
It is apparent from the results that rodents which grew
up to 200g (B. bengalensis, and T. indica) the order of preference by Barn Owl was medium
weight class > small weight class > large weight class. On the other hand, in the case of M. meltada that grew up to 100g, the preferential order
was medium weight class > large weight class > small weight class. It is
obvious from the results that the prey in the 50 to 150g size is the preferred
size for Barn Owls (Table 3). There is
less preference for prey below 50g as seen in the case of M. booduga and also for prey above the 150g class as seen
in the other two species.
Preference of body parts of prey: Barn Owl’s prey parts selection varied according to prey
size class (Table 4). Owls swallowed
mostly entire body in all the three species when the prey size was in small
weight class (B. bengalensis 79%, M. meltada 100%, T. indica 69%). On
the other hand, in the medium weight class prey, the owls fed mostly on head
and body portions and excluded tail (B. bengalensis54%, M. meltada 65% and T. indica 49%). In
the case of larger prey weight class, owl preferred mostly head portion of B.bengalensis 39% and T. indica 38% and as an exception the head and body parts for M. meltada (33%). In general, Barn Owl consumed mostly the
entire prey in the smaller size class, head and body in the medium weight class
and head alone in the larger weight class prey.
Biochemical estimation
i. Quantitative estimation of protein: The
brain had higher protein content than the liver and muscle in all weight
classes of different rodent species (Table 5). Among the three species of rodents, protein content of brain, liver and
muscle in the three size classes was highest in B. bengalensis (except liver of small weight class) followed by T. indica and M. meltada. In the case of M. booduga, also a similar trend in protein content was
observed (Table 6).
ii. Quantitative estimation of
carbohydrate: Generally, carbohydrate content was greater in the
liver than in the brain and muscle of all weight classes and prey species
(Table 5). Among the three prey species, carbohydrate level was highest in
liver, brain and muscle of B. bengalensis (except in liver of small weight class compared to T. indica) followed by T. indica and M. meltada. Liver had the highest content of carbohydrate even in the case of M. booduga (Table 6).
iii. Quantitative estimation of
lipid: Unlike protein and carbohydrate, lipid level was highest in the muscles
relative to the brain and liver of all weight classes and rodent species studied. A comparison of lipid content among the three
rodent species showed that M. meltada had higher lipid content for most classes (except in brain of large
weight class) compared to B. bengalensis and T. indica (Table
5).
The three nutrients viz., protein, carbohydrate and
lipid showed significant differences between different weight class and also
between the different organs studied (Two-way Anova p < 0.05) (Table 5). A
pair-wise comparison (by LSD) of the three nutrients among the three weight classes
showed that the brain, liver and muscle of large weight classes had
significantly higher protein than that of the other two classes in all the
three species except for the muscle of B. bengalensis (Table 5). There were
significant variations in carbohydrate levels, with reference to size classes
(the large weight class had the highest values) in all the three species except
for liver in B. bengalensis (Table
5). Similarly, the lipid level was also
significantly higher in the large weight class compared to the other two
classes for all the three organs in all the species (Table 5).
Discussion
The Jacobs’ preference index indicated
that the barn owl’s preference among the four prey species were B. bengalensis > M. meltada> T. indica > M. booduga,
which is similar to the pattern of diet selection in natural habitat by Barn
Owls (Neelanarayanan et al. 1995; Neelanarayanan2007b). Although Barn Owls in the present experiment consumed all four species,
a relatively higher consumption of B. bengalensisand M. meltada among the four species tested
is in accordance with the findings of Neelanarayanan(2007b) in the natural habitats of Cauvery Delta, who attributed such
preference to abundance of these species in crop fields. However, in the present study all the four
prey species were supplied in equal number (one individual from each species in
multiple choice test), the owls still consumed more B. bengalensisand M. meltada, suggesting prey abundance may
not be the only major factor influencing the diet selection as reported
elsewhere on barn owls (Colvin 1984; Yom-Tov &
Wool 1997; Leonardi & Dell’ Arte 2006). Other factors that could influence the prey
selection by Barn Owls are habitat type (Colvin et al. 1984; Santhanakrishnan 1987), reproductive status of owl (Santhanakrishnan 1987), body size and energetic (nutrient)
values of the prey species (Ille 1991). However, since the present study was
conducted under captive conditions, factors such as habitat type and
reproductive status are controlled and the choices are restricted only to
species, size-class and nutrient value.
In the prey choice test, there was
strong preference for the medium size class of B. bengalensis,
and T. indica (100-150g) and M. meltada (50-75g). The findings of the present investigation corroborate with those of
Morris (1979), Hamilton & Neil (1981) and Santhanakrishnan(1987), who also reported the higher preference of medium weight class of
almost all rodent prey species. The
possible reasons for the selection of medium weight class could be a function
of easy handling and swallowing the prey and efficiency in carrying the prey to
its nests as reported by Colvin (1984). Since the energy gain of the Barn Owl depends upon the energy spent, the
process of capturing larger prey may not compensate the energy gain (Colvin
1984) and thus the owls may have avoided spending higher energy in capturing
the large prey. According to Krebs
(1978) the large prey items are not always optimal for a predator, because they
require more handling time than smaller items. Similarly, the least preference shown to small weight class could also
be due to the lower energy gain relative to the energy spent on hunting the
smaller prey, as reported by Taylor (1994). The intermediate class prey may give optimal reward, as they provide
better balance of capture/handling time for energy gained (Davies 1977). Hence, the preference of barn owl to medium
weight class individuals in all the prey species might be based on
profitability (Ille 1991), which in turn depends on
hunting, handling and subduing of the prey (Colvin & McLean 1986) resulting
in a strategy of compromise between all the above factors. Further in the present study, Barn Owls
mostly swallowed whole prey when the prey size was small. In the medium sized prey, owls fed mostly the
head and the body portion, while in the large sized prey, the owls consumed
only head portion. The reason for the
selective feeding on various parts in different weight classes of the prey
could be the function of required quantity of food and the gut capacity along
with nutrient quantity available in different parts of the prey. For example when the prey size is larger than
the gut capacity, owls select parts that contain higher quantity of energy, as
owls preferred the head in larger size- class prey, which had higher protein
than other parts of prey as shown by the study. Therefore, the selective partial feeding in larger and medium sized prey
could be attributed to higher profitability, as speculated by Ille (1991). So it
can be reasonably concluded that the barn owl’s preference to medium size prey
and selective feeding on certain body parts in the case of large prey, could be
due to their relatively higher energy profitability.
References
Barnes, H. & J. Blackstock(1973). Estimation of lipids in marine animals tissues: Detailed investigation of the sulphophosphovanillin method for total lipids. Journal
of Experimental Marine Biology and Ecology 12: 103–118.
Bellocq, M.I. & F.O. Kravetz(1994). Feeding strategy and predation of the barn
owl (Tyto alba)
and the burrowing owl (Speotyto cunicularis) on rodent species, sex and size in
agro-system of Central Argentina. EcologiaAustralia 4(1): 29–34.
Colvin,
B.A. (1984). Barn owl foraging behaviour and secondary
poisoning hazard from rodenticide use on farms. PhD Thesis. Bowling Green State
University, Bowling Green Ohio, 326pp.
Colvin, B.A., P.L. Hegdal &
W.B. Jackson (1984). A
comprehensive approach to research and management of common Barn owl
populations, pp 270–282. In: McComb, W. (ed.). Proceeding of Workshop on
management of non-game species and ecological communities. 1984 June 11-12; University of Kentucky, Lexington, 564pp.
Colvin, B.A. & E.B. MacLean (1986). Food habits and prey specificity of the common barn owl
in Ohio. Ohio Journal of Science 86 (1): 76–80.
Davies, N.B. (1977).
Prey selection and the search strategy of the spotted flycatcher (Muscicapa striata):
A field study on optimal foraging. Animal Behaviour25: 1016–1033.
Errington, P.L. (1967). Of Predation and Life. Iowa
State University Press. Ames, IA, USA., 277pp.
Fast, S.J. & H.W. Ambrose (1976). Prey preference,
hunting and habitat selection in the barn owl. The American
Midland Naturalist 96: 503–507.
Griffith, D. (1980). Foraging costs and relative prey size. The American
Naturalist 116: 743–752.
Hamilton, K.L. & R.L. Neil (1981). Food habits and bioenergetics of a
pair of barn owls and owlets. The American Midland Naturalist106: 1–9.
Ille, R.
(1991). Preference of prey size and profitability in
barn owls in central Chile and southern Spain: A comparative study. Behaviour 116: 180–189.
Jacob, J. (1974).Quantitative measurements of food selection: a modification of the forage ratio
and Ivlev’s electivity index. Oecologica14: 413–417.
Krebs, J.R. (1978).Optimal foraging: decision rules for predators, pp 2–63. In Krebs, J.R. &
N.B. Davies (eds.) Behavioural Ecology: an
evolutionary approach. Blackwell Publication, Oxford 420pp.
Leonardi, G. & G.L. Dell’Arte(2006). Food habitats
of barn owl (Tyto alba)
in a steppe area of Tunisia. Journal of Arid Environments 65:
677–681.
Lowry, O.H., N.J. Rosebrough,
A.L. Farr & R.J. Randall (1951). Protein measurements with the folin-phenol
reagent. Journal of Biological Chemistry 193: 265–275.
Magrini, L.
& K.G. Facure (2008). Barn owl (Tyto alba) predation on small mammals and its role in the
control of hantavirus natural reservoirs in a periurbanarea in southern Brazil. Brazil Journal of Biology 68(4): 733–740.
Mikkola, H.
(1976). Owls killing and
killed by other owls and raptors in Europe. British Birds 69: 144–154.
Morris, P. (1979).Rats in the diet of Barn owl (Tyto alba). Journal
of the Zoological Society of London 189: 540–545.
Neelanarayanan, P. (2007a). Technique for estimation of barn owl (Tytoalba stertens Hartert,
1929) prey biomass with special reference to mandible length-body weight ratio
of small mammals. Zoos’ Print Journal 22(1): 2519–2521.
Neelanarayanan, P. (2007b). Diet of barn owl Tyto alba stertens Hartert, 1929 in a portion of Cauvery Delta, Tamil Nadu,
India. Zoos’ Print Journal 22(8): 2777–2781.
Neelanarayanan, P., R. Nagarajan& R. Kanakasabai (1995). The common barn
owl, Tyto alba:
A potential predator of rodent pests. Pestology19(9): 34–37.
Newsome, A. (1990). The control of vertebrate pests by vertebrate predator. Trends in Ecology and Evolution 5(6): 187–191.
Roe, J.R. (1955). The
determination of sugar in blood and spinal fluid with another reagent. Journal of Biological Chemistry 20: 335–343.
Santhanakrishnan, R. (1987). Studies
on population, food habits and nesting of Barn owl, Tyto alba (Scopoli) in a
portion of Cauvery basin. M.Phil Thesis, A.V.C. College, BharathidasanUniversity, Tiruchirapally. (Unpublished).
Schoener, T.W.
(1969). Models of optimal
size for solitary predator. The
American Naturalist 103: 277–313.
Sokal, R.R. & F.J. Rohlf(1981). Biometry. Second Edition, W. H. Freeman and Company, New York. 859pp.
Taylor, I.
(1994). Barn owls:
Predator Prey Relationship and Conservation. Cambridge
University Press, 304pp.
Yom-Tov, Y. & D. Wool
(1997). Do the content of barn owls pellets accurately represent
the proportion of prey species in the field. Condor 99: 972–976.