J RaptorRes.35 (3) :240-246
¸ 2001 The Raptor ResearchFoundation, Inc.
SEASONAL
THE
BARN
AND
GEOGRAPHIC
DIFFERENCES
OWL IN AN AGRO-ECOSYSTEM
IN
MICHELA
IN THE
DIET
OF
NORTHERN
ITALY
BOS• AND FRANCA GUIDALI
UniversitadegliStudidi Milano, Dipartimentodi Biologia,Sezione
Ecologia,via Celoria,26-20133 Milano, Italy
ABSTRACT.--We
studiedthe dietary niche breadth of the Barn Owl ( Tytoalba)in the Po plain of northern
Italy. A total of 1266 pelletswascollectedduring 2 yr in an agriculturalarea locatedbetweenMantovaand
Brescia.Bonesof 4455 prey itemswere identifiedwith smallmammals,in particularrodentsand Insectivora, the mostimportantdietarycomponentand birds (Passeriformes)
comprisingmostof the remainder
of the diet. Arthropodswere occasionally
eatenand amphibiansand batswere rare in the diet. Twothmilies
of rodents(Microtidaeand Muridae) were the mostimportantdietarycomponents.Membersof the order
Insectivorawere complementaryprey, showingan increasein winter when volesdeclined.In summer,a
significantincreaseof birdswasseen.Dietary compositionand niche overlapwere testedon dietsof two
pairsof owls.One pair ate mostlyvolesand the other pair ate birdsmore t?equently.
Nevertheless,
niche
overlapwashigh becauseof the similaritybetweenthe nest sitesused by the two pairs.
KEYWord)s:Barn Owl;Tyto alba; diet;,seasonal
variation;nichebreadth.
Dit•rencias estacionalesy geogrfificasen la dieta de Tytoalbaen un agroecosistemas
del norte de Italia.
R•SOmEN.--Estudiamos
la amplitud del nicho alimenticiode Tytoalbaen el plano del R/o Po en el norte
de Italia. Un total de 1266 egagropilasfueron recolectadasdurante 2 aftosen el firea agricolalocalizada
entre Mantovay Brescia.Los huesosde 4455 presasfueron identificadoscomo pequefiosmamiferosen
particular los roedorese insectostheron los componentesmas importantes es la dieta. Las aves(Passeritbrmes)constituyeronel restode esta.Losartropodosfueron ocasionalmente
ingeridosy losantibios
y murci61agosrara ve. Dos familias de roedores (Microtidae y Muridae) fueron los componentesmas
importantes.Los miembrosdel orden Insectivoraconstituyeronuna presa complementariamostrando
un incremento
en el invierno
cuando
los roedores
declinaron.
En el verano
hubo
un incremento
de
aves.La composicitnde la dieta y el traslapedel nicho fueron probadosen la dietasde dosparejasde
lechuzas,una igirio masque todo ratones, mientras que la otra comi6 avescon mas frecuencia.Sin
embargo el traslapedel nicho fue mayor debido a la similaridadde los sitiosdel nido utilizadospor
estasdos parejas.
[Traducci0n de Ctsar M•trquez]
The Barn Owl (Tyto alba) is a widely-distributed
owl with euryphagousfeeding habits (Lovari et al.
1976, Contoli 1981, Galeotti 1992). Because95% of
the variabilityin its diet is due to environmentalfactors (Spitz 1981), analysisof its diet can provideinformation on the availabilityof small mammal prey
speciesin a particulararea, even when few pellets
are available (Contoli 1981). Its specializationon
mammal speciesdecreaseswhen the abundance of
its main prey decreases(Herrera 1974) and it increasesthe diversityof its diet to new prey items of
lower energy content. Consequently,changesin its
&et can be used to reflect real changesin the small
mammal fauna available to the owl (Marti 1986).
Moreover, analysisof Barn Owl diets can be used
as a tool in the researchfor the management and
protection of important habitats. Indeed, small
240
mammals may represent a biological indicator of
biocoenosisin terrestrial ecosystems
becausethey
occupyvarious trophic levels,and then their presence or absence gives information on degree of
biotopealteration (Contoli 1975).
Many studies have examined the diet of Barn
Owls in Italy (Gontoli 1980, 1981, Gontoli et al.
1983, Torre 1983, 1987, Pandolfi and Santolini
1987,1988,Boldreghiniet al. 1988,BiginiandTurini 1992, 1995), but few data are available on the
diet of the speciesin the Po plain in northern Italy
(Groppali 1987, Vicini and Malaguzzi1988). The
aim of our studywas to provide new information
on the niche breadth of the Barn Owl (Tyto alba)
and to contribute to the knowledge of the presence and the distribution of its prey speciesin an
area in the Po plain.
SEPTEMBER 2001
BARN OWL DIET IN NORTHERN ITALY
STUDY AREA AND METHODS
The studywas conducted in a 3500 ha area located between Mantova and Brescia.Approximately 90-95% of the area was intensivelycultivated
with few patchesof the original mixed woodland
habitat (Querco-Carpinetum)remaining (Sestini
1963, Ingegnoli 1993, Pignatti 1994). Agricultural
fields consistedof cereals,corn, sunflower (40%),
vegetablesand fodder (35%), and wheat (5%).
The area also included the Chiese River and many
canals, as well as woody areas, grasses,permanent
pastures,buildings,and abandonedfields.
The area was subdivided in three zones, 3-5 km
apart, representing three different vegetation
types.Severalbuildingssuchasfarmsor complexes
of houseswere alsopresent.It contained two Barn
Owl nest sites and we collected pellets at six collection sites (two silos and four hay lofts) in the
two nest areas. Zone 1 (2 km2) was characterized
by cultivatedfields (corn, sunflower,wheat), waste
herbaceous fields (Convolvulusarvensis,Calystegia
sepium,Malva spp.), Platanusacerifoliaand Sambucusebuluson the edges of the ditches, and small
woodlots of Robinia pseudoacacia.
Wetlands were
abundant along the Chiese River, many ditches,
241
ways possible because diagnostic bones were lost
and skullswere fragmented. Biomasswas calculated basedon averageweightsof prey obtained from
the literature (Toschi and Lanza 1959, Toschi
1965, 1986, Brichetti 1976). For Rattus,we used the
averagemassgiven by Di Palma and Massa(1981).
We estimatedthe percent frequencyand percent
biomassfor each prey speciesbased on the total
number of specimensfound and the biomassvalue
of each specimen.To determine seasonalvariation
in the diet, we calculatedthe frequencyof the families and classesof prey in each seasonand tested
the differences with a chi-squaretest using the average frequency valuesbetween the two years. Finally, we tested the correlation (Spearman correlation coefficient) between the seasonal trends of
prey frequencies.
To determine niche breadth, we first tested the
relative importance of the prey categoriesand the
diversityof the diet using a Kruuk and Parish'sdiagram (1981). It is basedon the comparisonof the
percent frequency on the X-axis and percent biomass on the Y-axis. The
relation
(X'Y/100)
equaled the bulk of each prey categoryin the diet
and all points with equal X*Y values were connected by a set of isopleths. For both frequency
mole drains, and an artificial lake. This zone conand biomass,the averagevaluesbetween the 2 yr
tained one nest site and two roosts.Zone 2 (1 km9)
were used.To have another measureof the trophic
was surrounded by sunflowerfields, woodlandsof diversity, we also calculated the Shannon index
Robinia pseudoacacia,
Populus alba and Crataegus (H') (Margalef 1968) and the Levins' normalized
monogyna
and included two roosts300-400 m away (B) index (Levins 1968).
from the river. Zone 3 (1 km 9) had more homoWhen comparing diets, we considered only the
geneousvegetationwith corn fields and meadows samplescollectedat the two sitesthat were located
and it included one nest site. Zones 1 and 3, within
the municipalitiesof Remedello and Acquafredda,
differed in their vegetation. Zone I had grassland
and pastures and the habitat was more varied.
Zone 3 consistedalmost completelyof corn fields
4200 m apart in two areaswith different vegetation.
A total of 460 pellets was collected at Site I (N =
1756) and 449 were collected at Site 2 (N = 1579).
Niche overlap between the two nest siteswas tested
using a Percentagesimilarityindex (Pso,Schoener
(ISTAT 1990).
1970) basedon the equations:
We visitedthe sitesevery2 wk, at the middle and
H' = -• i pi In Pi B = (l/R) • pi9
at the end of every month, from January 1993December 1994. A total of 1266 pellets was collected. Pelletswere analyzedusing the dry method
(Contoli 1980) and bones of prey specieswere where Pi = ni/Nand R = dietary categories.
identified by using dichotomouskeysfor mammals
(Chaline et al. 1974, Pucek 1981, Amori et al. 1984, RESULTS AND DISCUSSION
1993) and birds (Moreno 1985, 1986, 1987). Prey
We identified a total of 4455 prey items with a
were enumerated
based on the minimum
number
total biomassof 91 021.02 g (Table 1). Mammals
of individuals (Southern 1954). Pelvis fragments (Rodentia, Insectivora,and Chiroptera) were the
were also used to estimatethe quantity of prey, be- most important prey speciesin the diet (F = 90.46,
cause skulls were missing in some cases.A com- B = 79 550.8 g, %B = 87.40). The two principal
sylvaticus)
plete determination of avian remains was not al- prey species( Sorexaraneusand Apodemus
= - o.(2
242
Bos• AND GUIDAL!
VOL. 35, NO. 3
Table 1. Number of specimens(N) and fi7equency(F) of prey in Barn Owl diets by seasonin the Po plain, Italy
(pooled valuesfor 1993 and 1994).
DEC.-FEB.
PREY
N
MAR.-MAY
F (%)
N
F (%)
JUNE-AuG.
N
F (%)
SEPT.-Nov.
N
F (%)
Mammals
0
0.00
1
0.20
2
0.08
2
0.27
Soricinae
Sorex araneus
Talpa •ropaea
125
118
14.99
14.15
78
69
15.73
13.91
212'
175'
8.92*
7.36*
106
91
14.17
12.17
Crocidurinae
249**
29.86**
128
25.81
484
20.36
148
19.79
Crocidura leucodon
Croddura suaveolens
M•crotidae
Microtus arvalis
Microtus savii
Muridae
84
161'*
180'
119'
46
196
10.07
19.30'*
21.58'
14.27'
5.52
23.50
41
78
134
102
25
113
8.27
15.73
27.02
20.56
5.04
22.78
226
245
702
483
156
631
9.51
10.31
29.53
20.32
6.56
26.55
69
77
207
138
54
196
9.22
10.29
27.67
18.45
7.22
26.20
125
15.78
Apodemus
sylvaticus
14.99
76
15.32
456
19.18
118
9
1.08
6
1.21
30
1.26
13
1.74
33
14
3.96
1.68
20
3
4.03
0.60
81
15
3.41
0.63
33
13
4.41
1.74
64
61
7.67
7.31
50
23
10.05
4.64
235
252**
9.88
10.60'*
74
58
9.89
7.75
3.74
Mus domesticus
Micromysminutus
Rattussp.
Other
Btrds
mammals
Passer domesticus
31
3.72
13
2.62
114**
4.80**
28
Passer montanus
8
0.96
1
0.20
25**
1.05'*
11
1.47
Other
2
0.24
1
0.20
17
2
0.26
20
2.40
8
1.61
96**
0
0
0.00
0.00
1
1
0.20
0.20
0
21
birds
Unidentified
birds
Rana sp.
Arthropoda
71
4.04**
0.00
89
17
0
8
2.27
0.00
1.07
* Frequencies lower than expected.
** Frequencies higher than expected.
were not habitat specialists.However, some prey
species such as Crocidurasuaveolens,
Crociduraleucodon,Microraysminutusand Microtus arvalis were
associatedwith open and cultivatedhabitatswhile
others including Neomysfodiens,Neomysanomalus
cated that the studyarea had not been affectedby
chemical fertilizers (Contoli 1981). Every species
that occurred in the study area occurred in the
diet indicating that Barn Owls had large hunting
territories and had both crepuscularor nocturnal
and Arvicola terrestris and Mus musculus and Rattus
foraging habits.
spp. were associatedwith canals and ditches and
Birds represented a minor component in the
human habitations,respectively.Burrowingand ar- diet (F = 8.84, B = 11 396.22 g, %B = 12.52). The
and
boreal speciessuch as Talpa europaea
and Muscar- principal prey specieswere Passerdomesticus
dznu• avellanariuswere taken rarely, while fbrest Passermontanuswhich were very common in the
specialistssuch as Clethrionomys
glareolusand Apo- studyarea. Arthropods (ClassesChilopoda and Indemusflavicolliswere not found in the diet. Sur- secta) and amphibians were only occasionally
prisingly,wc had a record of a Suncusetruscus,
a found.
specieswhich is seldomreported in Lombardy (OtBarn Owls specialize in eating small mam•nals
Iolini and Aceto 1996).
throughout their range and the variety of small
Our sampleincluded speciesrelated not only to prey taken appearsto vary with availabilityand envariousecosystems
but also to varioustrophic levels vironmental and geographicalfeatures.In North
with rodents being first-order consumers and America, Clark and Bunck (1991) found an inverse
shrews,moles, and rats being higher-levelconsum- correlation between shrewabundanceand precipers. Moreover, we observed a high diversity and itation. In wetter habitats, shrews were more comabundance
of insectivorous
mammals
which
indi-
mon in the diet while, in drier areas, rodents were
SEPTEMBER 2001
BARN OWL DIET IN NORTHERN ITALY
243
more common. Unlike Herrera (1974) who found
a low diversity of mammalian prey in the diet of
Barn
Owls in the southern
found
an inverse
correlation
Mediterranean,
between
latitude
we
and
diet diversity.In other studiesin Africa (Heim de
Balsac 1965, Goodman 1986) and South America
(Jaksic and Y•fiez 1979), rodents were the principal component of the diet of the Barn Owl.
Apparently, the presenceof shrewsin the diet is
variable and depends on environmental and geographical features.Birds, amphibians,arthropods,
fish, and bats have been reported as prey in some
areas when preferred small mammalian prey species declined. Very narrow Barn Owl diets have
been reported by Lenton (1984) for Malaysiaand
by Morton and Martin (1979) for Australia.In both
cases, the Barn Owls colonized
0
10
20
3o
40
50
60
70
80
90
100
Frequency
of preycategories
Figure 1. Kruuk and Parish'sdiagram of the estimated
bulk of prey categoriesin the diet of Barn Owls in the
Po plain, Italy. Isoplethsconnect points of equal relative
bulk
in the Barn
Owl diet.
areas where the
small mammal fauna consistedof few speciessuch
as Rattusspp. and Mus musculus.
Mammals were the most important component
of the diet in all seasons,
alwaysexceeding80%. In
spring, they reached their greatest frequency.
There were significantseasonaldifferencesof prey
frequencies (X2•5= 113.79, P < 0.01). In winter,
Crocidurinae, mainly Crocidura suaveolens,occurred significantly more often and Microtidae,
mainly Microtusarvalis,occurred lessthan expected. In summer, Soricinae decreasedin frequency
and birds, mostlyPasserspp., increasedsignificantly. Other mammals found in the diet included Neomysfodiens,Neomysanomalus,Suncusetruscus,
Microtus multiplex, Arvicola terrestris, Muscardinus
avellanariusand Pipistrellus
pipistrellus.
Other birds
Our analysisof niche breadth confirmed the euryphagoushabitsand the wide trophic niche of the
Barn Owl. The Kruuk and Parish diagram (Fig. 1)
revealed that Microtidae and Muridae were principal prey (hyperbola of 10%), while all the other
prey categorieshad only a marginal role (hyperbola of 1%) and none of them representedmore
than 50% of the diet. Both the indexes
of niche
breadth had high values (H' = 1.648, B = 0.58).
The Shannonindex washigher than previouslyreported by other authors in other regions (e.g., 1.13
[Lovari et al. 1976]; 1.37 [Amori and Pasqualucci
1987]; 1.39 [Petretti 1977]). Our index was was
very similarto indexesof 1.67 reported by Groppali (1987) and 1.63 reported by Vicini and Malafound in <3% of the sampleincludedHirundorus- guzzi (1988) in the Po plain and to the average
tica, Riparia riparia, Melanocorypha
calandra,Alauda value of 1.7 calculatedfor Italy by Contoli (1988).
arvensis,Sturnusvulgaris,Motacillaflava, Hippolais
polyglotta,Cardueliscarduelis
and Carduelischloris.
The predation on small mammals that we observedmayhavebeen a reflectionthe reproductive
cyclesof the prey species(Torre 1983, Boldreghini
et al. 1988, Bigini and Turini 1992, 1995). We recorded the highest predation rates when juvenile
dispersal occurred (February-March for Insectivora, summer for Microtidae,
and autumn for Mu-
ridae). It is possiblethat the increase in summer
vegetation may have also contributed to the decline
in small-sized
insectivores
which
became
dif-
ficult to find. Our analysisconfirmed the inverse
trends for frequencyof Crocidurinae and Muridae
(r = -0.84, P < 0.001). As for birds, our data con-
These resultsconfirmed the presenceof a diversified small mammal population with varioustrophic
levels represented which could reflect a complex
trophic relationshipand an apparentlymature and
stableecosystem(Margaleff 1975).
At site 1, Microtidae, in particular Microtusarvalis, were twice as abundant as in Site 2 and Mu-
ridae and birds were more frequent in the diet
than expectedin Site 2 (Table 2). These differences could have been due to the differencesin vegetation
between
the two sites. Site 1 was located
in
an area with variouskinds of agriculturewith fields
and woodlands on the edges of canals. The area
surrounding Site 2 had lesshabitat diversitywhich
may explain the absenceof Microtidae in the diet
firmed that their capture fluctuated in accordance and the occurrence of Muridae in the diet. At Site
2, the decrease of Microtidae forced Barn Owls to
to the availabilityof mammals.
244
Bos•. AND GUIDALI
VOL. 35, NO. 3
Table 2. Number of specimens(N) and frequency (F) of prey in the diets of Barn Owls at two nestssites(pooled
values for 1993 and 1994).
SITE 1
SITE 2
N
Mammals
N
F
1678'
95.56%*
1310'
82.96%*
581
33.09 %
523
33.12 %
I
0.06%
3
Insectivora
Talpidae
Soricinae
Crocidurinae
Rodentia
F
0.19%
192
376
1096
10.93%
21.41%
62.41%
170
336
786
10.77%
21.28%
49.78%
Microtidae
638*
36.33%*
303*
19.19%*
Microtus arvalis
Muridae
529*
413'
30.13%*
23.52%*
142'
449*
8.99%*
28.44%*
321*
20.33%*
283*
16.12%*
Gliridae
Apodemus
sylvaticus
4
0.23%
1
Chiroptera
0
0.00%
I
0.06%
65*
27*
3.70%*
1.54% *
260*
125 *
16.47%*
7.92 % *
8*
0.46%*
25*
1.58%*
2
28*
0.12%
1.59%*
15
95*
0.94%
6.02%*
B•rds
Passerdomesticus
Passer montanus
Other birds
Unidentified
birds
0.06%
* Frequenciessignificantlydifferent between the _twosites.
increasetheir predation on the other availablespecies,in particular birds (X24-- 233.4, P < 0.01).
The increasein predation on complementaryprey
categorieswasconfirmed by both indexesof niche
breadth, that were higher in Site 2 (H' = 1.686, B
= 0.623) than in Site 1 (H' = 1.51, B = 0.29).
The Percentagesimilarityindex (Pso= 0.73) indicated a high niche overlap in the diets of Barn
Owl at the two sites.This finding wasexpectedbecause the two sites had similar climatic, environ-
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of birds and Microtidae
in the diet at both
sites was
a reflection of the adaptabilityof Barn Owls to differing prey availabilities.
to thank
D. Fontaneto
and
S. Scali for
com-
ments and suggestions
on improvingthe manuscriptand
G Barbieri and S. Bandera for infbrmation concerning
the study area.
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Received30 September1999; accepted15 April 2001