ACTA ORNITI lO LOGICA Vol. 27 (1993) No. 2
Jacek GOSZCZYŃSKI, P io tr JABŁOŃSKI,. G rz e g o rz LESIŃSKI, Jerzy ROMANOWSKI
Variation in diet of Tawny Owl Strix aluco L. along an urbanization gradient
Goszczyński J . , Jabłoński P. , Lesiński C.. , Rom anow ski J. 1993. Variation in diet of T aw ny O w l Strix aluco L. along an u rbanization gradient. Acta orn. 27:113-123
The diet of T aw n y Owl as a function of urbanization w as stu d ied in Central P oland in 1976-1984. The pellets w ere collected from 24 sites in habitats along an u rbanization grad ien t ran g in g from the large, relatively u n d istu rb ed K am pinos Forest (1441 prey items), through small w o o d lo ts su rro u n d in g W arsaw (686) and into the city of W arsaw (2567). Diet varied d u e to urbanization: the p r o p o r t i o n s of m am m als and am p h ib ian s w ere lowest in the city center (respectively 11.3% and 0%), but the proportion of birds, largely I louse Sp arro w Passer domestiais increased from 2.9% in the forest to 88.7% in the city. N iche breadth and seasonal variation declined with urbanization. Relationships betw een diet composition and habitat stru ctu re of o w l's h u n tin g range in the city, as well as on the seasonal diet variation from owls h u n tin g in forests, let to conclude that the o w ls w ere able to exploit locally a b u n d a n t p rey from a variety of habitats. A lthough the highest densities of Field M ouse Apodeinus agrarius w ere noted in parks and cemeteries, o u r data suggest th at T aw ny O w ls catch this species m ore effectively in small g ardens and secondary gro w th areas. The p re d atio n on Field M ouse by a pair of ow ls b reeding in an urban cem etery w as low. A pproxim ately 3.2% of the p o p ulation w ere rem oved by the owl predation, considerably less than in a m ore rural population.
A gricultural University of W arsaw , D ep artm en t of C am e M énagem ent, Rakowiecka 26/30, 02-528 W arsaw , POLAND.
Institute of Ecology PAS, 05-092 Łomianki, POLAND.
INTRODUCTION
The diet of Taw ny Owl has been the su b ject of n u m ero u s papers (e.g. Uttendörfer 1939, M ärz 1954, Bogucki 1967, Ryszko- wski et al. 1971, Smoenk 1972, W endland 1972, 1984 and others), but data on varia
tion in diet as a function of urbanization are scarce, especially for different habitats com pared d u rin g the same time period (Schnurre 1961, Beven, 1964, W endland 1980). In W arsaw and its surroundings, the diet of T aw ny Owl has not been studied,
b ut there have been n um erous studies on mam m als (Andrzejewski et al. 1978, Babiń- ska-Werka et al. 1979, Goszczyński 1979) and birds (Luniak et al. 1964, Luniak 1981, 1 c>82, Luniak et al. 1986). They showed that the city differs from not urbanized habitats in species composition and in densities of potential prey of T aw ny Owl: there are fewer m am m al species in the city b ut some of them, as well as passerine birds like House Sparrow, can attain high densities.
Thi s al lo ws the s tu d у о f a genera list pred a - tor response to spatial changes in potential prey densities and species composition.
114 J. Goszczyński, P. Jabłoński, G. Łosiński, J. Rom anow ski
According to foraging theory (Krebs ct al.
1983) owls should restrict their hunting activity mainly to patches with locally high prey densities, abandoning other habitat patches in the city. Thus, we should expect narrow ing of the diet spectrum with incre
asing urbanization.
We shall illustrate h o w quantitative differences in theriofauna and avifauna as
sociated with urbanization influence the diet of a polyphagous predator, Tawny Owl. We present data on the diet of owls in locations with different levels of u rbani
zation and in different seasons in and a r o u n d W arsaw , Central Poland. We also analyse the influence of habitat structure
on the diet and evaluate mortality of Field Mouse resulting from T aw ny Owl p r e d a tion at one location.
We thank Jan Finowski and Marek Kel
ler for critical com m ents on the m a n u s c ript. Philip Stouffcr im proved the English in the m anuscript as well. Part of the data were gathered as Master of Science project of the D epartm ent of Ecology, University of Warsaw.
STUDY AREA AND METHODS
We collected pellets at 24 sites from 1976-1984. Each site represented a diffe
rent level of urbanization d e p e n d in g on its
KAMPINOS FOREST
1-24
G
WARSAW
Fig. 1. S tudy area with m ark ed sites of pellet collection. A - Built-up areas w ithin the a d m in istrativ e borders of W arsaw , В - Forests, С - Vistula river, D - Borders of the central zone, E - Borders of the in te rm ed iate zone, F - Borders of the su b u rb a n zone, G - Sites of pellet collection: 1. Kobendza Oak, 2. N art reserve, 3. Z am czysko reserve, 4. Debly reserve, 5. Z aborów Leśny reserve, 6. Kaliszki, 7. Palmiry, 8. Bialy Grad, 9. Młynisko, 10. Grabowy, 11. Dziekanów Leśny, 12. Lipków, 13. Młociny Park, 14. Bielany Park, 15. W aw rzyszew , 16.
Kaskada Park, 17. Bródno Cemetery, 18. O rthodox Cemetery, 19. Pow ązki Cemetery, 20. E vangelical-A ugsburg Cemetery, 21. Skaryszew ski Park, 22. Łazienki Park, 23. Powiśle Park, 24. Saski Park.
Rye. 1. Teren badań z zaznaczonym i miejscami zbioru w ypluw ek.
Variation in T aw n y Owl dit4 115
distance to the city center, from n o n u rb a nized habitats through suburban areas in to c ity of W a r s a w (Fig. l ) . T h e le a st distu rb ed was Kampinos National Park, a large forest area n o rth w e st of W arsaw w here we collected pellets at 12 sites. These were located at the edge of the forest and in its interior. In the area of smaller w ood- lots s u rro u n d in g the citv, w e collected p e l
lets from two forest parks: in Młociny Forest (further from the city) and in Biela
ny Forest (closer to the city).
Within W arsaw we classified our col
lection sites into three progressively more urbanized zones: suburban, intermediate and central. Pellets were collected at 4 sites in the su b u rb a n zone, at 4 sites in the inter
m ediate zone and at 2 sites in the center of the city (Fig. 1).
Prey items w ere identified from skull remains, teeth and other skeletal remains such as the os ilium of am phibians and humerus of Talpa europaen. In some cases, w hen bird skulls were so dem aged that identification was impossible, only the size of prey was evaluated. W e identified 1441 prey items from pellets collected in Kam
pinos Forest, 686 from forests surrounding W arsaw, an d 2567 from Warsaw. Insect remains w ere detected in small num bers and om m itted from analyses. Results are presented as frequencies of occurrence of prey items.
We estimated the predation of Taw ny Owl on Field M ouse based on data on population dynam ics of the m ouse in the O rthodox Cem etery (Wolska street) area.
The owls' food from that cemetery was calculated as biomass percentages. This enabled the evaluation of nu m b er of mice eaten by T aw ny Owl d uring one year and consequently the p re d ato ry impact on the prey population.
The Sim pson index of food niche b re adth was estimated by formula:
where: pi - proportion of each prey item in the diet.
Similarity of the diet composition b e tween seasons was estimated with the Mo- risita index modified by Horn (1966):
n
2 2 > . ѵ / C = ---- !---
i l 11
I
-Vi2+ I .
V /2
1 1
where: х і- percentage of i-th item in the diet in one season, and ѵ і - percentage of i-th item in the second season. The value of the index varies from 0 to 1, where 1 means that diet does not differ in seasons.
To examine the relationship between diet and habitat within an ow l's hunting range, we assum ed arbitrarily that Tawny Owl hunts within a circle with a radius of 5(H) m from the place where pellets were collected. If pellets w ere collected from se
veral nearby points, the central point of this area was chosen as the center of the circle. We estimated the proportion of se
veral habitat types within the circles at 11 sites in Warsaw. We distinguished the fol
lowing habitat types: 1) built-up areas, in cluding: la ) residential areas, and lb ) industrial and commercial areas, 2) open areas, including: 2a) lawns and squares, some containing small woodlots, 2b) small gardens and secondary grow th areas, and 3) parks and cemeteries.
We calculated a linear regression of the frequency of common prev species in the diet on the percentage of di fferent habi tats in the circular h u n tin g range using the stepwise variable selection m ethod. Thus, the final regression formula only included those habitats with regression coefficients that differed significantly from zero.
116 J. Coszczyński, P. Jabłoński, G. Lewiński, J. R om anow ski
RESULTS
General description of the diet
C onsum ption of m am m als and a m p h i
bians decreased and consum ption of birds increased w ith increasing urbanization (Tab. 1 ). In the interior of Kampinos Forest 82% of prey was m am m alian and 3% was
avian, b ut in the center of the city the p a t tern was reversed: 11 % m am m als and 89%
birds. Niche b re adth also declined with urbanization (Tab. 1). As urbanization in creased Taw ny Owl preyed less on Micro- tidnc and Insectivora, and more on Miiririae.
Among the Muridac, there was a strong increase in consum ption of Field Mouse and two typical synanthropic species: Mus
Tabic 1. C h an g es in diet com position of Taw ny Owl in urbanization grad ien t (1-S as in Fig. 2). Frequencies of species in the total n u m b e r of prey items. "+" - the re m a in s found only once (frequency < 0.05).
Tabela 1. Z m ian y składu p o k a rm u puszczyka w gradiencie urbanizacji (1-8 w zrastający stopień urbanizacji - patrz ryc. 2).
Prey type Kampinos Forest W oodlots
su rro u n d in g city The city of Wa r^aw
1 2 3 4 5 6 7 8
Chiroptera 0.1 0.1
Tul pa europaea 0.8 0.4 0.5 0.6 0.6 0.5 1.5
Sorex araneus 17.1 14.4 2.1 1.7 0.6 +
Sorex m im itus 9.0 4.8 1.8
Neomys fodiens 19.5 0.8 0.3
Cleth rionomys glareolus 11.9 25.5 35.4 14.6 8.0 0.9
Pitymys subterraneus 1.1 12.0 6.0 0.9 4.5
Microtus occoiiomus 9.1 9.2 2.4 1.1 0.4
Microtus arvalis 0.5 11.4 6.6 4.6 4.5 7.1 4.0
Microtus iigtestis 2.9 7.0 1.3
Ari'icolti terres tris 1.5 0.4
Microniys m im itus 1.1 1.8 1.7 +
Afiodemus agrarius 0.6 0.4 3.9 15.1 26.8 13.3 7.9 4.9
Apodemus flavicollis 6.5 8.1 3.2 14.0 7.4 0.8
Apodemus syïva tiens 0.3 3.7 8.9 0.9
M tts musculiis 0.2 1.8 2.1 0.6 2.1 4.4 10.2 3.8
Rnttus norvégiens 0.4 0.3 0.3 0.6 1.3 2.5 2.6
Muscardimis avellanarius 0.5 0.4
Sciurits vulgaris + +
Passer domesticus 2.6 8.0 23.2 44.6 51.7 53.1
Passer moil tanus 0.5 2.0 3.9 3.6 3.1 1.3
/lzvs (other species a n d
un d eterm in ed ) 2.9 7.0 9.2 8.0 15.2 21.3 14.0 34.3
Amira 15.0 4.4 16.5 15.4 1.2 0.1
Pisces +
Vertebrata (undeterm ined) 0.3
N u m b e r of prey items ( Vertebrata) 789 271 381 350 336 996 1493 78
The Sim pson index of a niche
breadth 7.89 7.71 5.59 8.48 5.94 3.69 3.24 2.47
Variation in Taw ny Owl diet 117
6 0 -,
4 0 -
2 0 -
4 0 -
20 -
20 ]
40 ”1
20 -
8 0 6 0 4 0 -1 20 20
Insectiv ora
Forest rodents M. av e ll an a r iu s C. giar eo lu s A. f l a vic oll is
Sy na ntropic rodents <M' mus culus, R. no rv eg icus )
--- r ----I--- 1 I--- '
□
A.agrarius
P.s ubterraneus T
P.d om esticus Ш
O th er bird sp e c ie s □
J С Anura
1 2 3 4 5
Increasing urbanisation
Fig. 2. Variation of p re y frequen
cies along an u rbanization g ra
d ien t 1-8: 1 - In terio r of the forest, 2 - Edge of the forest d i
stant from city, 3 - Edge close to city, 4 - S u burban forest park Młociny, 5 - Forest p a rk Bielany, 6 - O utskirts of W arsaw , 7 - In
term ediate zone, 8 - City center Rye. 2. Z m iany frekwencji z d o byczy w pokarm ie puszczyka ze w z r o s t e m s to p n ia u rb a n iz ac ji (1-8).
musculus and Rattus norvégiens. Field M ou
se was particularly frequent in pellets from forests bordering on W arsaw and from su b u rb a n areas. Typical forest species Clet- hrionomys giareolus, Apodemus flavicollis, Muscardinus avellanarius and species with preferences for wet habitats (Soricidae,Mic- rotus oeconomus) were rarely found in pel
l e ts f r o m u r b a n a r e a s . A l m o s t n o insectivores except Talpa europaea and sin
gle Sorexaraneus were caught in urban h a b i t a t s . F o u r s p e c ie s of r o d e n t s w e r e registered in the center of Warsaw, 6 sp e cies in the intermediate zone, 9 in the su b u rb s , 7-9 in the forests b o r d e r i n g on W arsaw and 13 in the Kampinos Forest (Tab. 1).
A m o n g avian prey, H ouse Sparrow was the most frequently caught, and its c o n su m p tio n increased with increasing urbanization (Fig. 2). Other birds (Passer montanus, Parus major, Sturnus vulgaris, Turdus merula, Turdus philomelos, Carditelis chlor is) were less frequently preyed upon (Tab. 1). Columba domestica comprised a
suprisingly small part of the diet. A m p h i
bians w ere a common diet item in forests b u t rarely found in the city (Fig. 2).
Se asona l variation of the diet
We analysed seasonal variation at four l o c a t i o n s u s i n g d a t a from 1976-1978 (Fig. 3). At the Młociny Forest site and on other sites in Kampinos Forest (not shown) am phibian consumption was most variab
le, with a pronounced peak in the spring.
In forests bordering on W arsaw the p ro p o rtion of House Sparrow increased consid
erably in the winter, while in the city the proportion was high and steady all year.
The frequency of insectivores in the diet varied regularly at most sites: it was the lowest in winter and the highest in s u m mer + autu m n . Other seasonal trends dif
fered am ong sites.
For each of the 4 sites in Fig. 3 we cal
culated 3 values of the Morisita index of sim ilarity: s p r in g w a s c o m p a r e d w ith s u m m e r + a u tu m n , s u m m e r + a u tu m n with winter and winter with spring. The
118 J. Goszczyński, P. Jabłoński, С. Łosiński, J. Romanow ski
Fig. 3. Seasonal diet variation at chosen sites. W - Winter, S - Spring, SA - S u m m er and A u tu m n
Rye. 3. Sezonow e zm iany po k a r
m u p u sz c z y k a na w y b ra n y ch stanow iskach. W - zima, S - w iosna, SA - lato i jesień
mean for these three values was the lowest in Młociny Forest (C = 0.71), interm ediate in Bielany Forest (C = 0.90) and the highest within the city (C = 0.97 at Orthodox C e
metery, С = 0.98 at Łazienki Park). Thus, seasonal differences decreased with incre
asing urbanization (Fig. 3).
W e m a d e s im ila r c o m p a r i s o n s for Kampinos Forest, woodlots surrounding Warsaw, and the city using data from the entire study period. Since we had no p re cisely dated pellets from the Kampinos Fo
rest from s u m m e r + a u tu m n , we only com pared the winter diet with the spring one. Seasonal variation was most p ro n o unced in Kampinos Forest (C = 0.78), w h e re it r e s u lte d from g re a t v a ria tio n in proportions of rodents, insectivores, H o u se Sparrow and amphibians. Seasonal va
riation in forests bordering on the city (C = 0.88) was mainly d u e to variation of am phibians and House Sparrow. Seasonal v a r i a t i o n w a s th e lo w e s t in W a r s a w (C = 0.98).
Habitat structure versus diet c o m p o s it io n in Warsaw
The frequency of House Sparrow con
sumed increased with an increasing p r o portion of built-up areas in the hunting range (Fig. 4A). The frequencies of Mus inusculus and liattus norvégiens increased with increasing proportion of industrial and commercial areas (Fig. 4B). This rela
tionship explained 42% of the variance in frequencies of both prey species.
There was a significant relationship b e tween the frequency of Field Mouse in the diet and the percentage of open areas in the owl's territory: y = 0.004 + 0.29x, R2 = 0.67, p < 0.05, df = 10. It was mainly caused by the positive relation of small g ardens and secondary grow th areas on the frequency of captured mice (Fig. 4 0 . This habitat type accounted for 59% of the variance.
Nearly 75% of the variance in frequen
cy of Mierotus nrvalis was explained by the positive influence of open areas (Fig. 4D).
As in the case of Field Mouse, this rela-
O R T H O D O X
C E M E T E R Y LA Z IE N K l"
B IE L A N Y F O R E S T M LO C NY
H IN S E C T IV O R A □ O T H E R B IR D S P E C IE S
Ш R O D E N T IA ШЭ P A S S E R S P. □ A M P H IB IA F R E Q U E N C Y (%)
i n n W S SA W S SA W S SA W S SA
PARK PARK
F O R E S T PARK
Variation in T aw ny Owl diet 119
% P.domestlcus In diet
100
Y- 0 .6 9 •X ♦ 24.66
80 r - 0 . 7 2 eo
40
20
00 10 20 30 40 60 eo 70
% of built-up area In hunting range
% of M.muaculue and R.norveglcua in diet
в
Y • 0.29*X + 6.00
p < 0.06 r-0.66
о 10 20 30 40 60 eo 70
% of industrial and commercial areas in hunting range
% of M.arvalis in diet Y - О.Зб^Х - 1.71 О < 0.001 r-0.86
o#о 10 20 30 40 60 60 70
% of A.agrarius In diet Y• 0.2Q*X + 6.26 p < 0.01 r-0.77
о 10 20 30 40 60 60 70
% of small gardens and secondary growth areas % of open areas In hunting range In hunting range
Fig. 4. Relationships betw een p ro p o rtio n s of som e prey species in diet and percentage of certain habitats in the h u n tin g range of T aw n y Owl in Warsaw.
Л) Passer domesticus - built-up a r e a s , B) M as musculus and Rnttus iiorvegicus - industrial and commercial areas, C) A ^ d e m u s agrarius - small g a rd e n s and secondary grow th areas, D) Microtus arvalis - open areas
Rye. 4. Relacje m ied zy frekwencją niektórych g a tu n k ó w ofiar a pro cen to w y m udziałem różnych środow isk w obrębie a reałów łowieckich puszczyka.
tionship could mainly be explained by the significant positive relationship with the percentage of small g ard en s and seconda
ry g row th areas: y = 0.02 + 0.09x, R2 = 0.64, p < 0.05, df = 10.
Exploitation of Field M o u s e b y T a w n y O w l
Succesi vc estimations of rodent density (Goszczyński 1979, Babińska-Werka et ni.
1981 ) and owl pellet collections were m ad e at the O rthodox Cemetery. Approximately 450 individuals of Field Mouse w ere born
per 1 ha in one breeding season. After a d ding the adults present before the begin
ning of the season, we estimated a p o p u lation of about 480 in d ./h a . Thus the 12 ha cemetery had a total population of about 5760 individuals. The Orthodox Cemetery bordered on a park, industrial areas, exten
sive grasslands, and the narrow belt of small gardens. Since no Field Mouse was detected in these neighbouring habitats durin g trappings, we assum e that all mice caught by Taw ny Owl came from the ce
metery. The cemetery was permanently in
120 J. Goszczyński, P. Jabłoński, G. Łosiński, J. Romanow ski
habited by one pair of owls, which had two nestlings in 1977. Based on a daily food dem and of 54 g for adults and 46 g for nestling (Ryszkowski et al. 1973), and assu
ming that nestlings and fledglings stay on their natal territory for 6 m onths (Southem 1954), w e estimated an annual biomass of 55.98 kg consumed by one pair and their offspring. Field Mouse constituted 8% of biomass consumed (4478 g). Based on a mean body weight of 24 g for Field Mouse, the owls consumed about 187 individuals, 3.2% of the population.
DISCUSSION
O ur results as well as those from other similar studies (Tab. 2) illustrate that urban Taw ny Owl fed mainly on birds, less fre
quently on m ammals, and very rarely on amphibians. The nu m b er of potential prey species, especially rodents, is lower in u r ban areas (Andrzejewski et al. 1978), but densities of synanthropic prey can be very high - e.g. Passer domcstiais (Tomialojć 1970, Dyer et al. 1977). Thus the food niche of Taw ny Owl is narrow er in urban areas than in other habitats, which was our g e neral expectation based on foraging theory (Krebs et al. 1983).
The diet of Tawny Owl in the center of W arsaw was very similar to that reported from London (Table 2). In both cities H o u se Sparrow m ad e u p about 50% of the owl's diet, although a great proportion of House Sparrow (72.3%) was reported from the city of Poznan (Bogucki 1967). Other bird species also increased in the diet of urban owls in Berlin (Wendland 1980) and London (Beven 1964). Not surprisingly, the m am m al species absent from the diet in W arsaw were those reported to be u n com m on or absent within the city: Sorex mimitus, Microtus agrestis, Muscariiiuus avellanarius (Andrzejewski et al. 1978).
Amphibians were relatively common prey items outside the city, especially in Kampinos Forest, where they dom inated some samples, but their frequencies diffe
red greatly am ong sites, probably d e p e n d i n g o n t h e a v a i l a b i l i t y of m c s i c conditions. High frequencies of a m p h i bians were also recorded in Białowieża Fo
rest - 37.9% (Ruprecht & Szwagrzak 1987) and at one location in G erm any - 36.0%
(Schaefer 1975).
The distinct increase in dietary d iversi
ty on the borders of W arsaw (Tab. 1) can be interpreted as an ecotone effect. In this zone there m ust be a relatively high n u m ber of prey species, both those typical for developed areas and those typical for wild habitats. T aw ny O w l has been charac
terized as a highly polyphagous species based on its substancial seasonal variation in diet (Southern 1954, Goszczyński 1981).
Our results illustrate that it is able to explo
it a variety of developed areas because of its dietary plasticity. We found the greatest seasonal differences in Kampinos Forest, the least disturbed site. But presumably because of decreased seasonal diversity in prey as urbanization increased, interseaso- nal diet differences decreased (Fig. 3).
11 appears that owls were able to exploit locally ab u n d a n t prey (Goszczyński 1981).
For example, Microtus arvalis (Fig. 4D) and Talpa eutvpaea were caught mostly bv owls foraging in open areas. The extent of urban developm ent correlated with the frequen
cy of House Sparrow in the owls diet (Fig.
4A), reflecting an increase in s p a r r o w ab u n d a n c e in built-up areas (Tomialojć 1970, Dyer et al. 1977), which may become especially attractive for hunting by owls because of large n u m b ers of roosting s p a r rows (Górska 1975). O ther birds were ca
ug h t m ainly in areas with greater tree cover, where their densities may be well over 100 p a irs/1 0 ha (Luniak 1981). Mus
Variation in T aw n y Ow l diet 121
Table 2. Frequencies of m am m als, birds a n d am phibian^ in all vertebrates in the diet of T aw n y Owl in W arsaw, its en v iro n s a n d in different places in Europe. References: A and E - Schnurre 1961, 13 - Southern 1934, С - Serafiński 1954, D and 11 - this paper, F - Beven 1964, G - Bogucki 1967
Tabela 2. Porów nanie u d ziału ssaków , ptaków i płazów w pokarm ie puszczyka na terenach miejskich i pozamiejskich.
N o n urb an areas: Berlin A
Oxford 13
W estern Poland С
W arsaw D
M am m als 73.1 94.9 92.5 82.0
Birds 10.0 5.1 5.1 2.9
A m p h ib ian s 14.5 0 2.3 15.0
Berlin London Poznań W arsaw
C enters of towns:
E F G II
M am m als 13.1 9.0 8.4 11.3
Birds 82.0 91.0 90.1 88.7
A m p h ib ian s 4.9 0 1.5 0
musculus and Rattus norvégiens, typical u r ban pests, were caught most frequently in a r e a s w h e r e n o n r e s id e n ti a l b u i l d in g s (Fig. 4B) dominated.
Although the highest densities of Field Mouse wore noted in parks and cemeteries (Babińska-Werka et al. 1979) Goszczyński 1979, o u r data suggest that owls catch this species m ore effectively in small gardens and secondary growth areas (Fig. 4C). The ground there is often covered by low vege
tation, probably increasing the hunting ef
ficiency of owls (Southern & Lowe 1968).
Data from sampling of Field Mouse p o pulation in the Orthodox Cemetery s u g gest that Taw ny Owl predation was very low (3.2% of total biomass in season). In agricultural habitats in western Poland Tawny Owl reduced this species by 33%
(Ryszkowski et al. 1973). Since the density of Field Mouse was much lower in western Poland, these results suggest that owl p re dation is not a numerical response to prey density. In spite of relatively high densities of mice on Orthodox Cemetery the re m a i
ning prey species (mainly House Sparrow) are so n u m e ro u s that they may dom inate the prey base and hence diet of the owls, or
habitat structure at the cemetery makes rodents more difficult to capture. U nfor
tunately, direct verification of the first h y p o t h e s i s is i m p o s i b l e , b u t h i g h e r frequency of Field Mouse in the diet of owls at Bielany and Młociny, where this rodent is less common, provide indirect su p p o rt for the first hypothesis. Babińska- Werka et al. (1981) also su p p o rt the first hypothesis, since they found no differen
ces in abundance of shelters for rodents between sites in forests and in cemetery.
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STRESZCZENIE
[Zm ienno ść skład u pokar mu p u sz czyka w gradiencie urbanizacji]
Badania nad zmiennością składu p o karmu puszczyka p ro w a d z o n o w W a rsza
w ie i jej o k o l i c a c h . M ie jsc a z b i o r u w ypluw ek dobrano tak, aby poszczególne stanowiska re p rez en to w ały wzrastający stopień urbanizacji, od stosunkow o mało zmienionych terenów Puszczy K a m p in o skiej, przez podmiejskie lasy W arszawy, r ó ż n e strefy m iejskie, aż do c e n tr u m (ryc. 1).
Stwierdzono spade k konsumpcji p ła zów i ssaków oraz wzrost konsumpcji p t a ków przez puszczyki w m iarę wzrostu stopnia urbanizacji (tab. 1, ryc. 2). W P u sz
czy Kampinoskiej udział ssaków wynosił 82%, a ptaków ok. 3%, n atom iast w cen
trum miasta te proporcje były odwrotne:
ptaki stanow iły 89% w szystkich zjada
nych ofiar, a ssaki zaledwie 11%. Szero
k o ść n i s z y p o k a r m o w e j p u s z c z y k a zmniejszała sie w kierunku centrum m ia
sta (tab. 1). Wśród ssaków malało spożycie Microtirfac i Insectivora, a z w ię k sz a ł sie
Variation in Taw ny O w l diet 123
udział Miiritlcie: myszy polnej i 2 synantro- pijnych g a tu n k ó w - myszy dom ow ej i szczura w ędrow nego. Wśród zjadanych p ta k ó w gw a łto w n ie wzrastało spożycie wróbla d o m o w eg o (ryc. 2).
Wzrost urbanizacji zmniejszał różnice sezonowe w składzie pokarmu i sprzyjał ujednoliceniu pożyw ienia w skali roku (ryc. 3). Analizy zw iązków między s tru kturą terytoriów zajmowanych przez p u szczyki a składem pokarm u poszczegól
nych par wykazały, że: a) ze wzrostem p o w i e r z c h n i zajętej p r z e z z a b u d o w ę w z r a s t a s p o ż y c ie w r ó b la d o m o w e g o (ryc. 4A), b) udział z a budow y przemysło
wej i u słu g o w ej w obrębie tery to riu m wpływ a na frekwencję myszy domowej i szczura w ę drow nego w pokarmie p u s z
czyka (ryc. 4B), c) istnieje istotny zw iązek między frekwencją myszy polnej i nornika zwyczajnego w pokarmie sowy a u d z i a łem pow ierzchniow ym działek i zieleń
ców ora z tere nów o tw a r ty c h w areale łowieckim puszczyka (ryc. 4C i 4D).
W p r z y p a d k u j e d n e g o s t a n o w i s k a (Cmentarz Prawosławny) oceniono r e d u kcję populacji myszy polnej przez sowy. Z p rz eprow a dzonych wyliczeń wynika, że puszczyki usuw ały zaledwie ok.3% w s z y stkich osobników tego g atunku, co jest presją bardzo małą w p orów naniu z r e d u kcją na innych pozamiejskich terenach.
Wyniki pracy potwierdzają w y k a z y wany przez innych autorów opo rtu n izm p o k arm o w y puszczyka (tab. 2).
