Breeding of Tawny Owls Strix aluco in rural and urban habitats in ...

Bird Study (2008) 55, 216–221

Breeding of Tawny Owls Strix aluco in rural and urban habitats in southern Finland TAPIO SOLONEN1* and KIMMO AF URSIN2 1Luontotutkimus

Solonen Oy, Neitsytsaarentie 7b B 147, FIN-00960 Helsinki, Finland and 2Svärdfältintie 2, FIN-04130 Nikkilä, Finland

Capsule The annual average breeding frequency, clutch size, offspring production and chick survival of Tawny Owls did not differ between rural and urban nesting territories. Aims To determine whether the general intensity of human habitation in the territory affects breeding. Methods Clutch size, offspring production, breeding frequency and prey abundance were determined from 210 rural and 60 urban nesting territories monitored between 1994 and 2006. Results Fluctuations in the annual average clutch size did not differ between habitats. Clutch size and offspring production paralleled each other in rural habitats but not in urban ones. Annual average clutch size followed the regional spring abundance of small mammals in rural Tawny Owls but not in urban ones. The breeding frequency was higher after mild winters in rural environments but not in urban ones. Conclusion Over an extended time period, rural and urban habitats were largely of equal quality. In urban environments, however, owls seem to be less affected by the pronounced regional abundance fluctuations of small mammals and weather conditions of the preceding winter that largely govern the breeding of owls elsewhere.

From the point of view of breeding birds, man-made rural and urban environments may differ in many respects other than the direct effects of human activities. Factors such as local food supply, wintering conditions, bird density (competition, prey supply) and predation may considerably affect the occurrence and breeding of birds (Marzluff 2001). Studies at various localities in Europe have shown that some bird species produce fewer eggs and fledglings in urban habitats than they do in rural ones (Perrins 1965, Berressem et al. 1983, Cowie & Hinsley 1987, Schnack 1991, Hõrak 1993, Solonen 2001). However, urban habitats may be superior for other species, such as birds of prey and corvids, because such species are often free from persecution there, and have an abundant year-round food supply (Newton 1986, Vuorisalo et al. 2003, Kelcey & Rheinwald 2005, Rutz et al. 2005, Chace & Walsh 2006). Tawny Owls Strix aluco are common inhabitants of rural and urban habitats in Europe (Cramp 1985). The species commonly occupies habitats in the vicinity of human settlements near the northern limit of its range in Finland, which was colonized only about a hundred *Correspondence author. Email: [email protected]

© 2008 British Trust for Ornithology

years ago (Solonen 2005b). More natural conditions, especially luxuriant mixed and deciduous forests, as well as shores of eutrophic waters, commonly offer all the essential resources for breeding (Cramp 1985, Solonen 1993, 2005b). Tawny Owls avoid large forested areas (Solonen 1993). A suitable nesting hole is an essential prerequisite for breeding, which is otherwise strictly governed by the adequate availability of food, usually small voles (Solonen & Karhunen 2002). In the fluctuating feeding conditions of higher latitudes, breeding of Tawny Owls seems to be more frequent and successful in territories that provide a constant stable food supply (Solonen & Karhunen 2002). We examine whether the general intensity of human habitation in the territory affects the breeding of Tawny Owls in Finland. Our goal was to determine whether the annual average breeding frequency, clutch size, offspring production and chick survival of Tawny Owls differ between rural and urban nesting territories. MATERIAL AND METHODS Study area

The study was conducted between 1994 and 2006 in

Breeding of rural and urban Tawny Owls

Uusimaa, southern Finland (60°N, 25°E). The main study area was situated in the counties of Sipoo and Pornainen, and in the municipalities of Helsinki and Vantaa, but some data from the surrounding municipalities Espoo, Nurmijärvi, Tuusula, Hyvinkää, Askola and Porvoo were also included (Solonen 1993, Solonen & Karhunen 2002). The main study area of more than 500 km2 consists of low-lying rural habitats of mixed fields and forests near the southern coast of Finland, as well as the capital district of about one million inhabitants, the most urban area in the country. The built-up areas were mainly due to relatively recent urban sprawl, quite fragmentary and still largely surrounded by more or less rural habitats. Strictly urban, densely built-up city environments covered relatively restricted areas, including several small and some larger city parks. Monitoring of owls

Territories and nests of Tawny Owls were located mainly in early spring by listening for hooting males and later by checking the known potential nest-sites. Territories were mapped by joining up locations of hooting males into clusters. Tawny Owls occupied the study area relatively evenly (Solonen 1993), largely following the availability of suitable nest-sites. The majority of these consisted of the more than 300 artificial nest-boxes erected throughout the study area. In a few cases, territories were established in other artificial nest-sites or natural cavities. For the present study, the nesting territories (sensu Newton 1979) were classified subjectively into urban (including suburban) and rural ones, on the basis of the dominant habitat types of the surroundings within a radius of about 1 km from the principal nest-site. However, we had no detailed data on the boundaries of territories or hunting areas, or on the real habitat use of the birds studied. The average nearest-neighbour distance of Tawny Owls in the district was about 2 km (Solonen 1993). The rural habitats included agricultural and silvicultural terrains, whereas the urban ones consisted of built-up areas and park-like forests. Urbanized areas were defined as those where land use had closed gaps to less than about 200 m. Of the 270 nesting territories of Tawny Owls located during the study, 210 (77.8%) were categorized as rural. Annual breeding frequency was measured as the proportion of occupied nests to the number of occupied territories recorded. The nests found were usually visited three or four times during the breeding season to

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gather data on clutch size and offspring production. Determination of clutch size was based on at least two visits within two to three weeks if the stained appearance of the eggs or the beginning of hatching did not reveal that incubation had already lasted for at least several days. Offspring production was generally measured as the number of nearly fledged (about three to four weeks old) young in the nest (Solonen 2005b), but on a few occasions a reliable record of the number of fledged young was accepted. Chick survival was indicated by the proportion of the offspring production (brood size) to the clutch size. The data used in this study were derived from 319 rural and 106 urban nesting attempts (Table 1). Predictors of differences in breeding performance

We searched primarily for differences in breeding frequency, clutch size, offspring production and chick survival between Tawny Owls nesting in rural and urban territories. Independent variables that might explain additional variation in the breeding parameters (Solonen 2005b) were the annual fluctuations in food supply measured by the regional abundance of small mammals (T. Solonen & P. Ahola, unpubl. data), and winter weather conditions characterized by the winter index of the North Atlantic Oscillation (NAO) (Jones et al. 1997, Osborn 2004). Positive values of the winter NAO indices (inclusive months December, January, February and March, CRU 2006) indicate milder and wetter winter weather in northern Europe (Hurrell 1995). The annual small-mammal abundance indices (indiTable 1. Annual numbers of territories and nests of rural and urban Tawny Owls. Territories

Nests

Year

Rural

Urban

Rural

Urban

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

45 49 39 50 65 34 30 42 45 56 36 54 67

14 9 9 11 12 9 10 19 17 20 16 31 30

30 29 14 27 28 22 19 21 24 26 21 25 33

8 4 6 6 7 7 7 12 8 7 7 14 16

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viduals/100 trap-nights), including shrews, voles and mice, were based on snap trappings of 384 trap-nights each (T. Solonen & P. Ahola, unpubl. data). Trapping was conducted each spring (May) at 64 standard points of three traps along four catching lines for two nights in the municipalities Lohja and Kirkkonummi (by the organization Kimpari Bird Projects), some tens of kilometres west of the main study area. To enable statistical analysis, small-mammal densities below the detection limit of the trapping method were replaced by a value of 0.001, corresponding to a density of about one-half of the minimum detected. Statistical methods

Because the data compared did not always meet the requirements of equal variance, the differences between groups were tested by non-parametric Mann–Whitney rank sum test. Relationships between variables were examined by Spearman correlation and least-squares linear regression. P-values higher than 0.05 were considered insignificant. Statistical procedures followed Sokal & Rohlf (1981). Calculations were performed by SigmaStat 3.1 and MATLAB 6.5 statistical software.

Figure 1. Relationship of the annual fluctuations of clutch size (●, rs = 0.714, P = 0.005, n = 13) and offspring production (● ●, rs = 0.264, P = 0.372, n = 13) of Tawny Owls between rural and urban habitats in southern Finland.

RESULTS

Annual average breeding frequency, clutch size, offspring production and chick survival of Tawny Owls did not differ between rural and urban nesting territories (Table 2). Annual fluctuations in average clutch size were largely similar between habitats, while those in offspring production were not (Fig. 1). Annual clutch size and offspring production paralleled each other in rural habitats, but not in urban ones (Fig. 2).

Figure 2. Relationship between the annual average clutch size and brood size (offspring production) in rural (●, rs = 0.578, P = 0.037, n = 13, linear regression F1,11 = 11.592, P = 0.006) and urban (● ●, rs = 0.258, P = 0.382, n = 13, linear regression F1,11 = 0.830, P = 0.382) Tawny Owls in southern Finland.

Table 2. Annual average breeding frequency, clutch size, offspring production and chick survival of Tawny Owls in rural and urban habitats in southern Finland in 1994–2006. Variable

Habitat

Mean

sd

Median

Breeding frequency Breeding frequency Clutch size Clutch size Offspring production Offspring production Chick survival Chick survival

Rural Urban Rural Urban Rural Urban Rural Urban

53.1 52.8 3.72 3.64 2.54 2.47 68.3 68.3

9.1 11.9 0.44 0.48 0.36 0.65 7.3 17.8

53.3 53.3 3.79 3.50 2.62 2.67 69.1 75.3

T13,13

P

182.5

0.74

181.5

0.78

172.5

0.90

162.0

0.51

Breeding frequency = number of nests found/number of occupied territories (%); clutch size = number of eggs in a clutch; offspring production = number of nearly full-grown young in nest; chick survival = survival from egg to chick (%). The results of Mann–Whitney rank sum tests (T) between habitats are given.


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The slopes of the regressions, however, did not differ significantly (F1,24 = 0.019, P = 0.891). Annual average clutch size followed the regional spring abundance of small mammals in rural Tawny Owl territories, but not in urban ones (Fig. 3). The slopes of these regressions did not differ significantly from each other (F2,22 = 0.045, P = 0.606). After mild winters the breeding frequency of owls was higher in rural environments but not in urban ones (Fig. 4). There was a significant difference between the slopes of these regressions (F2,22 = 3.581, P = 0.045). DISCUSSION

Our study suggests that in Finland rural and urban habitats are equally productive for breeding Tawny Owls. In urban environments, however, breeding owls seem to be relatively free from the pronounced regional abundance fluctuations of small mammals and the weather conditions of the preceding winter that largely govern the breeding of owls elsewhere. It seems that the degree of urbanization is not an important factor affecting the breeding of Tawny Owls; both rural and urban habitats are occupied and successful if the nest-site and food requirements are fulfilled. Thus, even the extremes of single farmhouses in agricultural environments and large, densely built-up areas may provide comparable breeding conditions for Tawny Owls. However, urban owls, in general, probably gain from the relatively stable food resources (such as rats and Common Pigeons Columba livia) as well as the warmer microclimate of towns compared to rural habitats. Some studies show that Tawny Owls’ diets include a higher percentage of birds in urban and suburban areas (Galeotti et al. 1991, T. Solonen unpubl. data). In line with our results, Ranazzi et al. (2000) found no significant differences in the breeding of rural and urban Tawny Owls in Italy. However, patterns of reproduction of birds in response to urbanization are not consistent, and only a few species have shown no response (Marzluff 2001). The reproductive success of most species breeding in urban habitats, including birds of prey, seems to increase with increasing settlement density (Diermen 1996, Boal & Mannan 1999, Millsap & Bear 2000, Marzluff 2001). Clutch size and offspring production in urban habitats might be higher and less variable than those in rural ones due to the high and stable supply of prey associated with human activity and a consequently weaker dependence on other kinds of prey.

Figure 3. Relationship between the regional spring abundance of small mammals (individuals/100 trap-nights) and clutch size in rural (●, rs = 0.719, P = 0.005, n = 13, linear regression F1,11 = 5.236, P = 0.043) and urban (● ●, rs = 0.270, P = 0.362, n = 13, linear regression F1,11 = 0.402, P = 0.539) Tawny Owls in southern Finland.

Figure 4. Relationship between the preceding winter weather conditions (winter NAO index) and breeding frequency (nests per active territories recorded) in rural (●, rs = 0.648, P = 0.016, n = 13, linear regression F1,11 = 13.788, P = 0.003,) and urban (● ●, rs = –0.236, P = 0.424, n = 13, linear regression F1,11 = 1.069, P = 0.323) Tawny Owls in southern Finland.

Birds of prey are commonly considered as inhabitants of remote districts far away from human settlements. This is assumed to be due to persecution but also to deterioration of habitats due to human activities (Newton 1979). However, various species have spontaneously colonized even densely built-up areas (Kelcey & Rheinwald 2005, Chace & Walsh 2006, Rutz et al. 2006). The quality of feeding and breeding habitats may have deteriorated in the surrounding countryside and by contrast improved in urban areas (Rutz 2006). © 2008 British Trust for Ornithology, Bird Study,

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Independent of habitat quality, fluctuations in food supply and weather conditions frequently affect breeding birds of prey (Newton 1979, Sasvári & Hegyi 2002, Solonen & Karhunen 2002, Solonen 2005a, 2005b). A major determinant affecting breeding seemed to be the weather conditions of the preceding winter (Solonen 2004, Dalbeck & Heg 2006). The present study suggests that urban habitats may act as buffers against fluctuations in both food supply and weather conditions. If available nest-sites do not limit the population size, the density and breeding success of birds are largely determined by food-related factors, including competition and predation (Perrins 1979, Newton 1980, 1998). Breeding success might be affected not only by the amount, but also by the quality of available food (Wendland 1984, Zalewski 1994, Solonen & Karhunen 2002). Local food supply in turn is largely determined by relatively predictable local habitat and annual weather factors. The outcome of habitat selection in birds seems to be a compromise between profits and disadvantages encountered. The primary habitats of most species can be found in rural environments. Increased urbanization has led to local increases in avian and mammalian density and biomass (Millsap & Bear 2000, Ranazzi et al. 2000, Sorace 2002), and high densities of potential prey have attracted various birds of prey to hunt and consequently breed in urban environments (Kelcey & Rheinwald 2005, Chace & Walsh 2006, Rutz et al. 2006). In the case of Tawny Owls (Ranazzi et al. 2000, this study), urban life seemed to be a good but not superior alternative to the rural one. ACKNOWLEDGEMENTS Our fieldwork was helped by many people, not least by our family members. Special thanks are due to Jari Pynnönen for his valuable contribution. We also thank Atte Bonsdorf, Johan Bäckström, Seppo Jäppilä, Jonna Karhunen, Jorma Kohonen, Kai Kuusisto, Andreas Lindén, Risto Pekkarinen, Esa Pienmunne, Rainer Salo, Klaus Silfverberg, Urpo Silvonen, Jorma Turunen, Jarmo Vajesoja and Bengt Ådahl. Rob Bijlsma, David Glue, Hannu Pietiäinen, Peter Sunde and Mark Whittingham read a draft of the manuscript and made useful suggestions. Marcus Walsh kindly checked the English. REFERENCES Berressem, K.G., Berressem, H. & Schmidt, K.-H. 1983. Vergleich der Brutbiologie von Höhlenbrutern in innerstädtischen und


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(MS received 29 August 2007; revised MS accepted 4 December 2007)


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