Winter diet of urban roosting Long-eared Owls Asio otus in northern Italy: the importance of the Brown Rat Rattus norvegicus

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Winter diet of urban roosting Long-eared Owls Asio
otus in northern Italy: the importance of the Brown Rat
Rattus norvegicus
A. Pirovano , D. Rubolini , S. Brambilla & N. Ferrari
Published online: 29 Mar 2010.
To cite this article: A. Pirovano , D. Rubolini , S. Brambilla & N. Ferrari (2000) Winter diet of urban roosting Long-eared
Owls Asio otus in northern Italy: the importance of the Brown Rat Rattus norvegicus , Bird Study, 47:2, 242-244, DOI:
10.1080/00063650009461181
To link to this article: http://dx.doi.org/10.1080/00063650009461181
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©2000 British Trust for Ornithology
The diet of Long-eared Owls Asio otus has
been extensively reviewed1–3 and is generally
well studied in northern Europe,4,5 Britain6,7
and southern Europe.3,8–10 Some indications
of a winter dietary adaptation to man-made
environments have been reported (e.g. a
dominance of the House Mouse Mus domesti-
cus11 or an increase of birds in the diet4). Here
we analyse the diet of urban roosting Long-
eared Owls, to assess whether such adaptation
to an urban environment occurs.
In the city of Milan, northern Italy (45°28′N
9°12′E), a large roost of Long-eared Owls (up to
76 birds) has occurred every winter (October–
April), since at least 1988 (A. Pirovano, unpubl.
data). The study site is located along a public
footpath and in private gardens, with two
sub-roosts 150 m apart. Data from both sites
were pooled for most analyses. The owls hunt
in the suburbs of the city and the adjoining
farmland.12
Pellets were collected between October 1996
and April 1997, pooled by month for analyses
and examined following standard techniques.13
Although the pellets could belong to a non-
independent sample, we have minimized bias
by using an average value for every month.14
Estimates of biomass were derived from the lit-
erature3,15 and from specimens collected in the
study area. As in other studies, birds were
considered as a single category3,5,9,10 and they
were assigned an average mass of 20 g each.3
The weight of predated Brown Rats Rattus
norvegicus, the only rat species in our study
area,16 was estimated by measuring mandible
length and using the regression equation given
by Di Palma & Massa.15 In biomass calcula-
tions, rats were assigned the average monthly
weight.
A total of 2760 prey items was identified in
2054 pellets. Mammals accounted for 91.0% of
the diet in number; the remaining 9.0% was
birds (see Appendix). Diet composition varied
significantly between months (χ2= 395, df = 24,
P< 0.001, computed on numbers of the five
main prey categories, Fig. 1a). Brown Rat is
very important in the diet of Long-eared Owl in
this locality, as is clear from consumed biomass
(overall 65.2%, range 54.4–76.5% per month,
Fig. 1b). Rats are often represented in the diet of
the Long-eared Owl (80% of 18 studies), but
few studies show such a large presence both
in terms of number (20.5%) and biomass
(%N: median = 0.7, range 0.1–4.7%, n= 14
studies; %B: median = 4.8, range 0.5–17.5%, n=
9 studies).
Weight (mean ±sd) of rats eaten was 140.0 ±
30.2 g (range 89.8–224.5 g, n= 260), suggesting
mainly young or subadults in a non-repro-
ductive state.17 Rats predated in autumn and
spring months were lighter than those taken in
winter months (quadratic regression of individ-
ual rat weights on month, F257 = 37.33, P< 0.001,
r2= 0.23) and monthly proportion (%N) of rats
in the diet was negatively correlated with
monthly mean rat weight (data from both
sub-roosts, rs= –0.73, n= 14, P= 0.003). This is
most probably explained by a decrease in avail-
ability of young rats in mid-winter months,18
together with a selection of smaller individuals,
that may be easier to capture compared with
large and aggressive ones.
Bird Study (2000) 47, 242–244
Winter diet of urban roosting Long-eared Owls
Asio otus in northern Italy: the importance of the
Brown Rat Rattus norvegicus
ANDREA PIROVANO1, DIEGO RUBOLINI2*, SILVIA BRAMBILLA3and NICOLA FERRARI3
1Centro Studi Faunistica dei Vertebrati, c/o Civico Museo di Storia Naturale, c.so Venezia 55, 20121 Milano,
Italy, 2Stazione Ornitologica La Passata, Miragolo S. Marco di Zogno (BG), Italy and 3via Marcora 11,
20100 Milano, Italy
SHORT REPORT
*Correspondence author: via Vespri Siciliani 5, 20146
Milano, Italy.
Email: rubolini@mail.inet.it
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©2000 British Trust for Ornithology, Bird Study, 47, 242–244
Winter diet of urban Long-eared Owls 243
The presence of the Wood Mouse Apodemus
sylvaticus in the diet is negatively correlated
with that of the Brown Rat (monthly %N: rs=
–0.75, n= 7, P= 0.052; monthly %B: rs= –0.96,
n= 7, P< 0.001). Despite being numerically
most abundant (37.8%), the Wood Mouse
seems to be an alternative prey to the Brown
Rat, given the dominance of rats by biomass.
The low value of the prey/pellet ratio and
the high value of the average meal (see
Appendix) compared with the literature (mean
±sd, prey/pellet = 2.1 ±0.3, n= 7 studies;
average meal = 45.9 ±6.9 g, n= 8 studies) may
be an index of the energetic advantage of eating
rats: because they are heavier than other prey,
owls need to hunt less often and can obtain a
larger amount of food per hunting trip. This
may explain the choice of an urban winter roost
site.
In conclusion, we confirm the trophic
plasticity of Long-eared Owls in their Italian
wintering range3–9 and highlight their ability to
adapt to an urban environment.
ACKNOWLEDGEMENTS
We thank H. Hauffe, G. Bogliani, P. Galeotti, Dr
M. Marquiss and Dr P. Walsh for useful com-
ments on the manuscript. We also thank B.
Chiarenzi, S. De Michelis, S. Di Martino, L.
Fornasari, T. Londei, F. Noetzli and A. Zilio for
collaboration. The Editor kindly improved the
final version of the manuscript.
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0Oct
20
40
60
80
100 b
Biomass (%)
Nov Dec Jan Feb Mar Apr
0Oct
20
40
60
80
100
Birds
a
Frequency (%)
Other
Wood Mouse Savi’s Pine Vole
Brown Rat
Nov Dec Jan Feb Mar Apr
Figure 1. Monthly diet composition, October 1996 to
April 1997: (a) number as a percentage of the diet for the
five main prey categories; (b) biomass as a percentage
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species with an overall frequency less than 5% in
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244 A. Pirovano et al.
©2000 British Trust for Ornithology, Bird Study, 47, 242–244
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(MS received 12 December 1998; revised MS accepted 5 August 1999)
APPENDIX
Monthly diet composition, October 1996 to April 1997, number of prey items (n), percentage of prey in number (%N) and
biomas (%B) according to prey species.
Oct Nov Dec Jan Feb Mar Apr Total
Prey species %N%B%N%B%N%B%N%B%N%B%N%B%N%Bn%N%B
Rodentia
Apodemus sylvaticus 10.4 4.6 27.2 9.9 23.1 10.0 47.9 27.5 43.2 22.4 45.1 21.3 44.6 23.0 1043 37.8 18.0
Micromys minutus 0.5 0.1 2.2 0.2 3.1 0.3 9.0 1.3 6.7 0.9 3.0 0.4 1.3 0.2 132 4.8 0.6
Mus domesticus 2.7 0.7 1.9 0.4 2.4 0.6 1.9 0.6 2.4 0.7 1.6 0.4 0.6 0.2 56 2.0 0.6
Rattus norvegicus 29.0 71.4 35.0 76.5 23.1 69.3 12.0 54.4 16.1 61.2 22.2 65.6 19.8 60.8 567 20.5 65.1
Muridae spp. 0.9 0.4 1.3 0.5 0.7 0.3 1.3 0.8 2.0 1.1 1.4 0.7 3.8 2.0 43 1.6 0.8
Pitymys savii 9.1 3.7 17.5 5.8 31.0 12.2 21.6 11.3 23.4 11.0 18.6 8.0 22.9 10.7 587 21.2 9.2
Microtus arvalis ––0.6 0.2 1.4 0.6 0.6 0.4 0.7 0.4 0.2 0.1 – – 16 0.6 0.3
Arvicola terrestris 2.3 4.1 1.6 2.3 0.7 1.2 0.4 1.0 0 0 0.2 0.4 – – 15 0.5 1.1
Muscardinus
avellanarius –––––00.2 0.2 – – – – – 0 1 0.0 0.0
Insectivora
Crocidura leucodon 0.9 0.1 –––––––––––– 2 0.10.0
Crocidura suaveolens ––––––0.2 0.0 – – – – – – 1 0.0 0.0
Crocidura sp. 1.4 0.2 – – – – – – 0.8 0.1 0.5 0.1 0.6 0.1 13 0.5 0.1
Sorex araneus 0.5 0.1 – – – – – – 0.1 0.0 – – – 0 2 0.1 0.0
Chiroptera
Pipistrellus kuhlii 10.9 1.5 0.3 0.0 1.7 0.2 ––––0.20.0–– 311.10.2
Chiroptera spp. 0.5 0.1 – – – – – – 0.1 0.0 – – – – 2 0.1 0.0
Aves 31.2 13.1 12.5 4.3 12.9 5.3 4.8 2.6 4.5 2.2 7.0 3.1 6.4 3.1 249 9.0 4.0
Pellet number 133 296 251 465 471 320 118 2054
Prey number 221 320 294 476 851 441 157 2760
Prey/pellet 1.66 1.08 1.17 1.02 1.81 1.38 1.33 1.34
Average meala79.19 63.19 57.31 37.75 73.87 61.94 54.74 59.52
Diet breadthb4.67 4.09 4.51 3.31 3.63 3.40 3.37 4.14
aAverage meal is defined as: (mean prey weight) ×(prey/pellet). bDiet breadth according to Levins’ index, NB = 1/Σpi2, where piis the proportion of the
prey.
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