Diet of the Barn Owl (Tyto alba tuidara) in northwestern Argentine Patagonia

5 Pages·2000·1.8 MB·English

by  M S Pillado

.
J Raptor Res, 34(4);334-338
© 2000 The Raptor Research Foundation, Inc.
Diet of the Barn Owl Tyto alba tuidara) in Northwestern Argentine Patagonia
(
Maria Pillado and Ana Trejo
S.
Centro Regional Universitario Bariloche, Unidad Postal Universidad, 8400 San Carlos de Bariloche, Argentina
Key Words: Bam Owl; Tyto alba; diet, Patagonia', Argen- where n^ was the number ofindividuals ofthe ith species
tina. and Wi was the mean weight. We also determined the
mean length of rodents consumed after Jaksic et al.
(1977): MLR = S fiXi/m, where p was the frequency of
Given its wide distribution and sedentary habits, the the i species in the diet, x^ was mean body length, and m
diet ofthe Barn Owl (Tyto alba) has been studied in more the total number of identified rodents. Mean weight of
detail and more extensively than that of any other bird mammals and mean body length of rodents were taken
of prey (Everett et al. 1992). Rodents and other small from the literature (Redford and Eisenberg 1992, Pear-
mammals are the main prey in the diet of Barn Owl in son 1995).
all of its range along with variable proportions of birds, Food-niche breadth (FNB) was estimated using Levins’
reptiles, amphibians, fish, and arthropods (Taylor 1994). (1968) index: FNB — 1/(S p^), where was the propor-
A
The Barn Owl {T. alba tuidara) is widespread in conti- tion of prey taxon i in the diet. standardized niche
breadth value (FNB^J was then calculated, which ranged
nental Argentina and occasionally on islands (Canevari from 0-1: FNBgt = (FNB — l)/(n — 1), where « was the
et al. 1991). Food habits ofthe Barn Owl have been thor- total number of prey categories (Colwell and Futuyma
oughly studied in agrosystems in Argentina (Bellocq 1971). Evenness was calculated by the Shannon-Wie-
(J')
1990, Bellocq and Kravetz 1994), but little is known about ner function as follows: = H'/H'max, where H' was
J'
its diet in southern Argentina. In Patagonia, most studies the Shannon-Wiener function and H'max was the maxi-
have focused on the arid eastern steppes (De Santis and mum value of H'; that is, the logarithm of the number
Pagnoni 1989, De Santis et al. 1993, 1996, Garcia Espon- of species in the sample (Krebs 1989).
da et al. 1998). Our aim was to provide information on
Results and Discussion
the diet of the Barn Owl in a somewhat different area
with more mesic vegetation features and a small mammal A total of 425 prey items was identified from 229 pel-
fauna mixing typical steppe species with others more lets. The mean number of prey/pellet was 1.9 ± 0.9
characteristic of humid forests nearby (Monjeau 1989). (±SD, range = 1-4) and the mean number of rodents/
pellet was 1.8 ± 0.9 (range = 1-4). Barn Owls preyed
StudyArea and Methods
mainly on rodents (95.1%). Hares and insects made up
The study site was located in the Reserve Area of Na- 0.5% and 4.4% of prey, respectively. The two European
huel Huapi National Park, in northwestern Argentine Pa- hares {Lepus europaeus) found in the diet were newborns.
m
tagonia (7r07'25"W, 40°47T4"S) at 700 elevation
above sea level. The area is an ecotone between the arid InsBeyctpsewrecreentalflrienqutehnecyf,amtihley mScoasrtabcaoenisduaeme(dTasbilgem1o)don-
Patagonian steppe to the east and the southern beech
(Nothofagus spp.) forests to the west. The site was domi- tine rodent species were Abrothrix longipilis, Loxodontomys
nated bybunchgrasses {Stipa speciosa) and cushion bushes micropus, and Oligoryzomys longcaudatus. In terms of bio-
(Mulinum spinosurn) with scattered trees (Austrocedrus chi- mass, Loxodontomys micropus was the most important prey
lensis, Maytenus boaria, and Populus nigra). At times, wil- in the diet, followed by Abrothrix longipilis‘And Oligoryzomys
lows (Salixfragilis) formed small gallery forests. longicaudatus (Fig. 1).
Owl roosts were located by observing areas of white- The Barn Owl feeds almost exclusively on small mam-
wash or recording places where pellets were found. Pel-
mals throughout its range, although the proportions of
lets were collected every two weeks fromJune 1993-May
other prey may vary slightly (Taylor 1994). Barn Owls in
1994 at two known roost sites. Pellets were grouped into
calendar seasons, oven-dried in 70°C for 72 hr, and pro- our study preyed almost exclusively on rodents with ju-
cessed following standard methods (Marti 1987). Most venile hares and insects rarely appearing in the diet,
prey were identified to species. Mammalian prey were mostly in spring. We did not find birds, reptiles, nor am-
identified and quantified on the basis of skulls and den- phibians to be important prey as was the case in La Pam-
taries using reference collections and keys (Pearson pa, Argentina (Noriega et al. 1993).
1995). Insects were quantified by counting head capsules Based on the literature, the most important prey of
and mandibles.
Barn Owls in Argentine Patagonia are Eligmodontia mor-
Biomass of each rodent species in the total biomass of
gani and Reithrodon auritus (De Santis and Pagnoni 1989,
the diet was calculated by multiplying mean body mass
ofindividuals by the number ofindividuals in pellets and De Santis et al. 1993, Tiranti 1996, Travaini et al. 1997).
expressed as a percentage of total rodent biomass con- In our study area, neither species represented >4% and
sumed. We calculated the geometric mean of weight of 8% of total prey items, respectively. This was not surpris-
prey (Marti 1987): GMW = antilog (S rajog nj. ing because the habitat characteristics of our study area
334
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December 2000 Short Communications 335
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336 Short Communications VoL. 34, No. 4
Ct A1 Ax Em Ech It Lm 01 Px Ra
Species
Frequency Biomass
(%) (%)
Figure 1. Frequency and biomass of rodent prey species in the diet of the Barn Owl. Biomass is expressed as the
M—
percentage of biomass of each species calculated on total rodent biomass. Ct Ctenomys haigi, Abrothrix longipilis.
Ax A. xanthorhinus, Em Eligmodontia morgani, Ech Euneomys chinchilloides. It Irenomys tarsalis, Lm Loxodontomys
micwpus, 01 Oligoryzomys longicaudatus, Px Phyllotis xanthopyga, Ra Reithrodon auritus.
were not optimal for these rodents, which prefer the variable proportions (Donazar et al. 1997, Trejo and Gri-
more xeric and open habitats of the Patagonian steppe gera 1998). Evenjuvenile hares may not be very suitable
(Pearson 1995). prey for Barn Owls since they are much smaller than
The most common species in the diet, both in fre- Great Horned Owls (Everett et al. 1992). According to
quency and biomass, Abrothrix longipilis, Oligoryzomys lon- Jaksic (1986), this is a common situation in southern
gicaudatus, and Loxodontomys micwpus are good climbers South America where some predators hunt mainly the
and prefer brushy places, although the latter is also more abundant native rodents, often ignoring abundant
found in shallow wet grasslands (Pearson 1983). Taking introduced lagomorphs.Jaksic (1986) attributed this fact
this into account, we inferred that the most frequently- to an “escape in size.’’ Maximum weight ofjuvenile hares
used habitats in the Barn Owls’ hunting range were those is about 300 g (Bonino and Montenegro 1997), which
with good vegetation cover and ample water. puts them beyond the size of prey more frequently con-
Hares were only occasionally eaten by Barn Owls de- sumed by Barn Owls. Rabbits, although smaller than
spite their relative abundance (approximately 4-18 hares, were probably not in our study area.
hares/ha; Novaro et al. 1992), their crepuscular or noc- Mean weights and sizes ofrodents were approximately
turnal habits, and their open nests (Bonino and Monte- the same during the four seasons, suggesting that in our
negro 1997), all traits which might make them vulnerable study area the Barn Owl preyed more upon medium-
to an aerial nocturnal predator like the Barn Owl. There sized {A. longipilis, L. micropus, and O. longcaudatus) than
is only one citation for the Argentine Patagonia record- on the smaller-sized {A. xanthorhinus and E. morgani) ro-
ing predation by Barn Owls on rabbits Oryctolagus cunic- dents in the area. Mean weight of prey of the Barn Owl
(
ulus, 0.1% of total prey, Travaini et al. 1997). In central in Chilean Patagonia is smaller (29.9 g), due to a greater
Chile, the proportion ofrabbits in the diet of Barn Owls consumption of smaller species (Iriarte et al. 1990).
IS also very low (0.03% of total prey, Herrera andJaksic Food-niche breadth is intermediate, as has been shown
1980). In Chilean Patagonia, Iriarte et al. (1990) did not in Chilean Patagonia (Iriarte et al. 1990). This indicated
record predation on hares by Barn Owls although they that the Barn Owls in our study behaved essentially as
were eaten by Great Horned Owls {Bubo virginianus) in specialized rodent predators. Diets of sympatric Great
December 2000 Short Communications 337
Horned Owls have been studied in two sites in north- Everett, M., I. Prestt and R. Wagstaefe. 1992. Barn and
western Patagonia (Don^ar et al. 1997, Trejo and Gri- Bay Owls Tyto, Pholidus. Pages 36-50 in J.A. Burton
gera 1998) and, in both cases, a lower food niche breadth [Ed.], Owls of the world. Eurobook, Italy.
(0.20) was found to be due to lower species evenness in Garcia Esponda, C.M., L.J.M. De Santis, J.I. Noriega,
the diet. G.O. Pagnoni, Moreira, and N.M. Bertellotti
G.J.
— 1998. The diet of Tyto alba (Strigiformes: Tytonidae)
Resumen. En el presente trabajo se estudio la dieta de in the lower Chubut valley (Argentina) Neotropica44-
.
Tyto alba tuidara en el noroeste de la Patagonia argentina. 57-63.
Los roedores sigmodontinos fueron el componente prin- Herrera, C.M. and F.M.Jaksic. 1980. Feeding ecology of
cipal de la dieta, tanto en numero como en biomasa. Las the Barn Owl in central Chile and southern Spain. A
liebres y los insectos fueron poco consumidos. Las espe- comparative study. Auk 97:760-767.
cies de roedores mas consumidas fueron Abrothrix longi-
pilis, Loxodontomys micropusy Oligoryzomys longicaudatus. De Iriarte, J.A., W.L. Franklin, and W.E. Johnson. 1990.
Diets of sympatric raptors in southern Chile./. Raptor
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el area de estudio se desarrollo preferentemente en am- Jaksic, F.M. 1986. Predation upon small mammals in
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