a review of the trophic ecology of the barn owl in argentina

J. RaptorRes.34(2):108-119
¸ 2000 The Raptor ResearchFoundation, Inc.
A REVIEW
OF THE TROPHIC
ECOLOGY
BARN OWL IN ARGENTINA
OF THE
M. Is•a•Ei• BEI•I•OC(2
Departamento
de CienciasBiol6gicas,
FCEN-Universidad
de BuenosAires, Ciudad Universitalia,Pab. 2,
BuenosAires1428, Argentina
ABSTRACT.--Ireviewed the literature on the trophic ecology of the Barn Owl (Tyt0 alba) in 40 areasof
Argentina representingsevenvegetationtypesin the species'range. Mammals (rodents,marsupialsand
bats) representedan averageof 90.2 -+ 13.6% (_+SD) of the total prey found in pellets, and most
mammalianpreywere sigmodontinerodents(77.0 -+ 19.6%). Birdswere the mostcommonsecondary
prey, and insects,reptiles and amphibianswere negligible in the diet. The number of mammalian genera
found in pellets (dietary richness)was similar among vegetation typesbut higher in subtropicalthan
in temperate regions, showing a negative correlation with latitude. Mean weight of dominant prey
speciesrangedfrom 12.6-326.0g. Averagefood niche breadth (FNB) wassimilarin subtropical(4.07)
and temperate (4.03) regions. Standardized FNB was 0.33 -+ 0.16 and it was similar among vegetation
typesbut lowerin subtropicalthan in temperateregions.The diet of Barn Owlsmayreflectsomehuman
alterations
to the habitat.
KEYWO}•DS: Barn Owl;Tyro alba; diet;,trophicecologs;
Argentina.
Un resumen de la ecologiatrofica de Tyroalbaen Argentina
RESUMEN.--Serevis6 la literatura acerca de la ecologia tr6fica de Tyroalba (lechuza del campanario)
proveniente de 40 localidadesde Argentina, que representan siete tipos de vegetaci6n en el •trea de
distribuci6n de la especie.Los mamiferos (roedotes,marsupiales,murcirlagos) representaronun promedio de 90.2 -+ 13.6% (_+DE) del total de presaspresentesen regurgitadosy la mayoria de estos
mamiferoseran roedoressigmodontinos(77.0 _+ 19.6%). Las avesfueron la presasecundariam•ts
comfin y los insectos,reptiles y antibiostuvieron una representaci6ninsignificanteen la dieta. E1 nilmero de grinerosde mamlferos representadosen los regurgitados(riqueza) fue similarentre los distintos tipos de vegetaci6n, pero mayor en regiones subtropicalesqueen templadas,presentandouna
correlaci6n negativacon la latitud. E1peso medio de la especiepresa dominante en la dieta vari6 entre
12.6-326.0 g. La amplitud de nicho tr6fico (ANT) promedio rue similar entre regionessubtropicales
(4.07) y templadas(4.03). La ANT estandarizada
totalfue 0.33 _+0.16.La ANT estandarizada
fue similar
entre los tipos de vegetaci6n,pero menor en regionessubtropicalesque templadas.La dieta de Tyto
alba puede reflejar algunasalteracionesantr6picasdel h•tbitat.
[Traducci6n del autor]
Food is a major dimension of ecologicalniches
(Schoener1974). Competitionfor food resources,
resulting in specializationand food partitioning,
has been suggestedas a primary mechanismallow-
predator models in studiesof community ecology
(e.g., Jaksi• 1985, Marti et al. 1993), and the Barn
Owl (Tyro alba) has been frequently included in
major trophic studiesof raptor assemblages
(Marti
ing coexistence of speciesin assemblagesof am-
et al. 1993 and references therein).
phibians(Toft 1985), reptiles(Pianka1973),birds
(Lack 1946) and mammals (Dickman 1988). De-
The Barn Owl is a common and widely distributed nocturnal predator that feeds primarily on
scriptionsof what animals eat are essentialto identify energy paths through trophic websand to understand how species divide food resources.
Furthermore, knowledge of trophic ecologyis fundamental to understand feeding strategies(selective and opportunistic behaviors) and niche dynamics. Raptors have frequently been used as
small mammals. In Argentina, it is one of the most
common and best-studied raptors. It occurs
throughout the country and is particularly common in agrosystems,grasslandsand open areas.
Studies of the breeding biology have been conducted in temperate grasslandsin the eastcentral
part of the country (Fraga 1984, Nores and Gu-
108
JUNE2000
B•
OWL DIET IN ARGENTINA
ti6rrez 1986, Bellocq and Kravetz 1993). Two additional studieshave focusedon feeding strategies
of breeding and nonbreedingBarn Owls,and prey
selectionon rodent species(Bellocq and Kravetz
1994, Bellocq 1998).
Food habits of the Barn
Owl have been studied
in numerous areas throughout its geographic
range (Clark et al. 1978). Over 40 studieshavedescribedthe food habitsof Barn Owlsin Argentina
and several
additional
studies
have been
conduct-
ed in Chile (e.g.,Jaksi• and Yafiez1979, Iriarte et
al. 1990). Here, I presenta synthesisof the trophic
ecologyof the Barn Owl in Argentina based on a
compilationand analysisof publishedinformation
in 40 areasof the country.
MATERIALS
AND METHODS
109
cus, Schizachyrium
consanguineum,
Setaria mendocina).De-
forestationand ranching has partially modified the natural vegetation.
Warm semidesertsinclude the driest landsof Argentina where annual precipitation ranges from 80-200 mm
Typically, they are dominated by thorn-scrub communities with shrubssuch as Larrea divaricata,L. cuneifolia,L
nitida, Montteaaphyllaand Bougainvillae
spinosa.
Bromehads are also common. The main human activity of the
region is ranching. In cold semideserts,mean annual
temperaturerangesfrom 5.0-13.4øCand annual precipitation ranges from 155-500 mm. It is a steppe where
common speciessuch as Malinum spinosum,
Brachyclados
caespitosus,
Junellia tridensand Nassauviaglomerulosaare
adapted to a dry and windy climate. Further details on
thesevegetationtypescan be found in Ragonese(1968)
and Cabrera (1971).
Data Analysis.Pellet analysiswasused by all authors to
study diets of Barn Owls. The objectives of the studies,
however, were varied (Table 1). While some studies fo-
cusedon feeding habits of the owls, others emphasized
ied in over 40 areasrepresentingsevenmajor vegetation small mammal distributions and descriptions of new
types: subtropical rainforest, humid forest, the delta of small mammal races (e.g., Contreras and Rossi 1981). I
the Paran/t River, grasslands,savannas,and warm and used the number of mammalian genera found in pellets
to providean estimateof dietaryrichnessfor eachlocality
cold semideserts(Table 1, Fig. 1).
Subtropicalrainforestsare characterizedby warm and and I estimatedthe frequencyand recorded the average
weight (taken from Redford and Eisenberg,1992) of the
humid weather, with mean annual temperatures of 2021øCand annual precipitationrangingfrom 1500-2000 dominant prey species.
For each localityand prey type, data were summarized
mm. The vegetationhasseveralstrataand a high richness
as
the percent frequency of the total number of prey
of speciesand life forms (e.g., epiphytes).The delta of
the Paran• River has elementsof the subtropicalrainfo- found in pellets. Because most publications provided
rest. However, I considered it as a separate region be- more detailson mammalianprey than on anyother prey,
I was also able to obtain frequencyestimatesof rodent
cause it has distinct environmental
characteristics
and numerous endemisms. The vegetation physiognomyis families in the diets. Although there were relatively redominated by pastures,willows and marshesin the lower cent changesin small mammal taxonomy,I maintained
zones,and by open shrublandsand humid forests(where the old taxonomyasit appearedin literature. As a result,
I refer to Cricetidae (insteadof Sigmodontinae) and Munative speciesare common) in the upper zones.
Subtropicalhumid forestswere originallyxerophyllous ridae (instead of Murinae). Marsupials,bats, birds, repand dominated by Schinopsis
balansaeand Aspidosperma tiles, amphibiansand arthropodswere considereddifferquebracho
in the upper canopy.There are typicallyone or ent categories. Finally, I estimated the average
two subcanopytree stratafollowed by shrubs (e.g., Pro- percentagefrequency for each prey category by vegetasopisruscifolia,Acaciapraecox,Castelacoccinea)
and herbs tion type.
Food-niche breadth (FNB) was estimated for each lo(e.g. Bromeliaserra,Leptochloa
virgata, Gomphrena
pulchella). The landscapehas been widelymodified by logging cality following Marti's (1988) criteria. Thus, mammals
(rodents,marsupialsand bats) were categorizedby genus
and by ranching.
Temperate grasslandshave a moderate climate where whereasbirds, reptiles, amphibians and insects(includannual precipitationvariesfrom 600-1100 min. The most ing other invertebrates)were catagorizedby class.Levins'
common native grassesare Stipa,Piptochaetium,
Aristida, index was used to estimate food-niche breadth as FNB =
Melica,Briza,Bromus,
Eragrostis
and Poa.These highlypro- 1/Ep?, where p, is the proportionof item i in the diet
ductive grasslandshave gradually been converted to ag- Hurlbert's standardization was employed to allow meanriculture over the last two centuries. Currently, the nat- ingful comparisons,and it was calculated asFNBs = (Bob•
ural plant community is composedof both native and -- Bmin)/(Bmax - Bmin), where Bobs = FNB (as calculated
introducedspecies(e.g., Lolium,Briza,and Bromus).Pri- above), Brain = 1 (minimum possibleniche breadth) and
mary usesof the land are for cereal crops and livestock Bm•x = N (maximum possible niche breadth or total
number of prey categories).
breeding.
ANOVA or Kruskal-Wallis (H, when the data set d•d
Savannashave a moderate to dry climate with xerophyllousvegetation,where Prosopis
caldeniais the most not reach homogeneityof varianceseven after transforcommon tree species.Other tree speciesinclude P. flex- mation) tests were used to test for differences in FNBs
uosa, Geoffroea
decorticans,
Jordina rhombifolia,Schinusfas- and in the number of mammalian genera found in diet
ciculatusand Ximeniaamericana.Savannasare open wood- among vegetationtypes.Correlation and regressionanallands with some shrubs (e.g., Condalia microphylla, yseswere used to testfor associations
and explain spatial
Atamisquea
emarginata,
Ephed,zz
triandra,Maytenus
spinosus) variationsof food-niche parameters.All means are preand diverse grasses(e.g., Trichloriscrinita,Elionurusmuti- sented +__1SD.
Habitat Descriptions. The diet of Barn Owls was stud-
110
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B.•
OWLDIET IN ARGENTINA
111
112
BELLOCQ
VOL. 34, NO. 2
usually abundant in the diet. Mammals represented an averageof 90.2 + 13.6% (range = 36.3100%) of the total prey found in pelletsand most
mammalian prey were cricetine (now sigmodontihe) rodents (77.0 +_19.6%, range = 27.5-100%).
Barn Owls took a wide variety of mammalian species depending on the distribution and, presumably, the local availabilityof prey. The differential
predation on rodent speciesin agrosystems
reported by Bellocq and Kravetz (1994) may be attributed to habitat
17-18
13 14-15
24 .16
'•
19
27-28
26
21-23
32-34.
selection.
There
is evidence
that Barn
Owls feed selectivelyon large rodents when prey
abundanceis high during the breeding season,but
not consistentlyduring the nonbreeding season
when rodent abundanceis low (Bellocq 1998).
Small marsupialswere taken by Barn Owlswhen
available.They represented23.8%of the totalprey
in Bonpland but, in Diego Gaynor,they were not
found in pellets or in smallmammal trapping conducted during severalyearsusing severaldifferent
trapping methods.Marmosapusillawasconsistently
found in pellets from the Prosopis
savanna.Marsupials such as Marmosaagilisare abundant in the
humid forest and Massoia (1983) suggestedthat
they may be a preferred prey of Barn Owls in the
delta of the Paranti River. Although marsupials
such as Lutreolina
crassicaudata
occurred
in almost
all study areas, they were only occasionallyfound
in pellets,presumablybecauseof its large sizeand
aggressivebehavior. Other marsupialsrepresented
in diets included Thylamys
agilis,T. elegans
and Monodelphishenseli.
Birdswere the most common secondaryprey but
the percent of birds found in pelletsvaried greatly
among localities. The averagepercent of birds in
the diet was7.2 - 12.8% (range = 0-63.7%). The
following orders were identified in decreasingorFigure 1. Geographicallocation of siteswhere the diet
of Barn Owls was studied in Argentina. Numbers correspond to locationsgiven in Table 1.
RESULTS
AND DISCUSSION
Overall Diet. The Barn Owl is a specialiston
small mammals (Dean 1975, Morton and Martin
1979,Jaksi•et al. 1982,Lenton 1984,Marti 1988).
Mammals were also the most common prey found
in pelletsin all areasof Argentinawith the exception of San Miguel and Castelarwhere passerine
birds were more abundant (Table 2). In Castelar,
C6rdoba, San Miguel and Ensenadita (urban or
suburban areas), birds, Muridae and bats were un-
der: Passerifirmes, Columbiformes, Tinamiformes,
Charadriiformes, Anseriformes and Gruiformes.
Bats were occasionallyfound and represented
<56.6% of the diet. Batsmostcommonlytaken included speciesof Lasiurus,Molossus,
Eumops,Myot•s
and
Sturnira.
Reptiles such as snakesand iguanaswere only
found in pellets from Diego Gaynor,Laguna Bianca, Punta Este and Luan Cura Hu6. Amphibians
were found in pellets from S.M. Tucumtn (Leptodactyluschaquensis),
Campo Ram6n (Batrachia),
Desaguadero(Batrachia), San Miguel (Leptodactylidae, Bufonidae,Ceratophrynidae),Alta Italia,
Villa Regina (Batrachia), Trevelin (Pleurodema
bufonina) and Junin de los Andes.
Ju•E 2000
B,a•,• OWL DIET IN ARGENTINA
113
Arthropods were likely taken opportunistically
and their role in fulfilling energetic requirements
appeared to be negligible in Argentina. Insects
were reported in the diet in five localities.In Lobos, 15% of the pellets collectedyear round contained remains of insectsand the percent of pellets
showing insects was higher in spring-summer
(50.3%) than in fall-winter (1.8%) (Faverin 1989).
g) and Scapteromys
tumidus(146 g) which were the
primary prey found in two subtropical localities
(Desaguaderoand the Ibicuy Islands).
ComparisonamongRegions.The mean percent
frequencyof the dominant prey speciesin the diet
was similar between subtropical (39.5 _+15.0) and
temperate (34.7 -+ 10.2) localities(F = 0.941, P >
0.05). Although the primary prey speciesvaried
within most
Although33.5% of the food itemsfound in pellets among localities,there wasconsistency
in Diego Gaynor were arthropods,their contribu- regions (Table 2).
tion in terms of biomass(<1%) was negligible in
The number of mammalian genera found in
comparisonto that of vertebrates(Bellocq 1990). diet was similar among vegetation types (H =
Massoia and Vetrano (1988a) identified spiders 1.110, P > 0.05), but higher in subtropical(13.6 +and insects (Carabidae, Scarabaeidae, Coccinelli- 4.3) than in temperate (8.8 _+ 2.5) regions (F-dae, Rutelidae, Gryllidae, Acridiidae, Mantidae, 19.008, P < 0.001) (Table 3). In contrast, more
Mantispidae, Cicadidae, Blattidae) in pellets from classesand rodent families were represented in diVilla Regina which represented9.5% of the total em in temperate (3-4 classesand 4-6 rodent families) than in subtropical (2-3 classesand 3 rodent
prey.
Food-niche Parameters. Dietary richness was families) regions.This may be explainedby the dinegativelycorrelated with latitude. The number of versityof primary prey in the wild. Speciesdiversity
mammalian genera found in pelletsvaried from 4- of Cricetine rodents is higher in subtropicalthan
20 (Table 2) and was correlated with latitude (r =
in temperate regions (Redford and Eisenberg
-0.456). I excluded from the analysisone outlier 1992), and they representedover 60% of the diet
correspondingto Junin de los Andes. The coeffi- of Barn Owls in natural regions of Argentina (Tacient of determination
showed that latitude
exble 3). Barn Owls seem to prey on a wide variety
plained 18% of the variation in the number of of Cricetine specieswhen availablebefore explormammaliangenera found in pellets (F = 7.862, P ing other prey taxa.
< 0.01). Given that speciesrichnessis highly and
FNB wassimilar in subtropical(4.07 _ 1.22) and
negativelycorrelated with latitude (Rohde 1992), temperate (4.03 _+1.51) regionsof Argentina (F=
the spatialvariation in dietary richnessmay be par- 0.005, P > 0.05). Marti et al. (1993) obtained simtially explained by differential availabilityof prey at ilar values for temperate and tropical regions of
the regional scale. This expected general pattern South America (Table 4). The averageFNBs was
identified in southern regions of the Neotropics 0.33 (Table3). It washigherthananyvalueestiwas not found among diets of raptor assemblages mated in the southern portion of the Barn Owl's
in northern latitudes (Marti et al. 1993).
range in the Neotropics (Table 4) but similar to
I also expected to find an associationbetween the average value obtained from Marti's (1988)
FNB and latitude, where declining FNB should be compilation of nine areas most of them from
associatedwith increasinglatitude. However, there southern latitudes (FNBs = 0.27, F = 0.27, P >
was no significant correlation between FNB and 0.6). My FNBs value was also similar to the mean
latitude. Average FNB was 4.05 _+ 1.40 ranging value estimated from 19 areas around the world
from
1.37 in Alta
Italia
to 7.49 in Saladillo
and
(FNBs = 0.26, F -- 0.96, P > 0.3) (Marti 1988).
FNBs was 0.33 - 0.16 ranging from 0.05 in Alta
In Argentina, FNBs was similar among natural
Italia to 0.69 in the Toay and Loventu• Districts regions (H = 3.552) (Table 3). However, the av(Table 2). The dominant prey in the diet repre- erage FNBswaslower in subtropical(0.25 - 0.04)
sented>30% of the total prey in 70% of the lo- than in temperate regions (0.35 _ 0.19, F = 4.423,
calities and >40% in 47% of the localities.
P < 0.05). Marti et al. (1993) estimated a higher
Mean weightof the dominantpreyspeciesvaried diet diversityin tropical than in temperate regions
from 12.6-326.0 g (Table 2). The overall geometric of South America.
Management Implications. Diets of Barn Owls
mean of prey weight of Barn Owls in temperate
Neotropical regions was estimated to be 45.1 g may reflect opportunistic feeding becauseof the
(Marti et al. 1993). It was unusual for Barn Owls effects of human impactson their habitat. For into take prey as heavyas Holochilusbrasilensis
(326 stance, the diet of Barn Owls changed after dam
114
B•I•I•OC:(2
VoI•. 34, No. 2
JUNE2000
BARON
OWLDIETIN ARGENTINA
115
116
BELLOCQ
VOL. 34, No. 2
+1 +1 +1 +1 +1 • • +1 +1 +1 +1 • +1 •
+1 +1
+1 +1 +1 •
+1 +1
•
•
+1 +1 +1
+1 +1 •
+1
•
JUNE2000
BARNOWLDIETIN ARGENTINA
Table 4. Food-niche breadth (FNB, based on proportion of species) and standardizedfood-niche breadth
(FNBs) of the Barn Owl in regions of Europe, North
Ainerica and South America. Source: Marti et al. (1993).
FNB
FNBs
Mediterranean
3.84
0.20
West
4.43
0.06
East
2.23
0.03
West
7.88
0.03
Midcentral
9.61
0.29
Eastcentral
1.96
--
Southeastern
3.04
--
North
South
I thank
0.18
0.38
Insular
3.20
0.19
on the Colorado
for comments
on earlier
versions
of
Aires.
--AND
CITED
F.O. KRAVETZ. 1990. Practical
and theoretical
implicationsof perch use for avian predators on rodent populationsß
Ecosur16:61-67.
--
4.28
4.61
Marti
BELLOCQ,M.I. 1990. Composicitn y variacitn temporal
de la dieta de Tyroalbaen ecosistemasagrariospampeanos,Argentina. Vida Silvestre
Neotropical
2:32-35
ß1998. Prey selectionby breeding and non-breeding Barn Owls in Argentina. Auk 115:224-229.
Ainerica
Temperate
Tropical
C.D.
this manuscript. Comments by F.M..Jaksi• and two anonymousreviewersalso helped to improve it. This work was
supported by the Gonsejo Nacional de Investigaciones
Gientfcas y Ttcnicas of Argentina and the University of
LITERATURE
Ainerica
construction
ACKNOWLEDGMENTS
Buenos
Europe
117
AND --'.
1993.
Productividad
de la lechuza
del
campanario (Tyt0 alba) en nidos artificiales en agros-
istemaspampeanos.Hornero13:277-282.
--AND
--'.
1994. Feeding strategyand predation
of the Barn owl (Tyt0 alba) and the Burrowing Owl
(Speotyto
cunicularia)on rodent species,sex, and size,
in agrosystems
of central Argentina. EcologiaAustral4'
River in Casa de Pie-
29-34.
A.L. 1971. Fitogeogratiade la Reptblica Argendra. Montalvo et al. (1985) reported diet compo- CABRERA,
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CLARK,
J.R., D.G. SMITHANDL.H. KELSO.1978. Working
and November 1983, prior to and after dam conbibliography of owls of the worldßNatl. Wildl. Fed
struction (alsoin Noriega et al. 1993). Prior to dam
Sci. Tech., Washington, DC U.S.A.
construction, the diet consistedof 97.3% mammals CONTRERAS,
J.R. ANDM.I. ROSSL1981. Una nueva subesand 2.7% birds.After dam construction,it changed
pecie del rattn colilargo para la provincia de Mento 60.2% mammalsand 39.8% birds. This change
doza: Oligoryzomys
fiavescensoccidentalis(Mammaha,
in diet may have been due to a change in the relative abundance of vertebrate populations or a
higher vulnerabilityof birds due to increasingedge
Rodentia, Cricetidae). l•istoria Natural (Argentina) 1
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Barn Owls are predatorsof small mammal spede Tyroalba (Aves:Tytonidae) en localidadescosteras
cies considered harmful for agriculture and agrode la provincia del Chubut (Repfiblica Argentina).
forestry. Calomys
lauchaand Holochilusbrasilensis
are
Neotropica
35:43-49.
known to damage crops and young tree planta- --,
G.I. MONTALVOANDE.R. JUSTO.1983. Mamfferos
tions, respectively.Furthermore, C. laucha,Oligoryintegrantes de la dieta de Tyro alba (Aves: Strigifbrzomys
fiavescens
and Ahodonazaraecan carry serious
mes, Tytonidae) en la provincia de La Pampa, Argenhuman diseasessuch as hantaviruspulmonary syntina. Iaristoria
Natural (Argentina) 3:187-188.
effect.
drome (Levis et al. 1995) and hemorrhagic fever
(Kravetz et al. 1986). Increasing availability of
roosting sitesfor raptorial birds in cropfieldsresulted
in
decreased
short-term
abundance
of
C.
laucha (Bellocq and Kravetz 1990). Similar results
were found after habitat enhancementfor predators in pine plantations (Mufioz and Murfia 1990)
and nest boxes have been
a useful
tool to increase
productivityof Barn Owls in agrosystems
of Argentina (Bellocq and Kravetz1993).
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, L.J.M., N.G. BASSO,
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Received1 May 1999; accepted30 December 1999