Predation on reptiles and birds by the common buzzard, Buteo buteo ...

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2086

Predation on reptiles and birds by the common buzzard, Buteo buteo, in relation to changes in its main prey, voles Vidar Selås

Abstract: It has commonly been assumed that predators whose main prey are voles reduce their predation on other prey when voles are abundant. This assumption may, however, not be valid for slow-moving and ground-dwelling prey species with habitat demands similar to those of voles. In southern Norway, the field vole, Microtus agrestis, is an important prey for the common buzzard, Buteo buteo. Two other important groups of prey are birds and reptiles. In a forested study area with cyclic vole populations, prey remains and pellets were collected from buzzard nest sites in 2 peak vole years and 3 low vole years. Birds were most important as prey when vole populations were low. Reptiles, on the other hand, were most common as prey in peak vole years, possibly because the buzzards then concentrated on hunting ground-dwelling prey in habitats with high vole populations. In the study area, this was mainly clearcuts, which were commonly used also by basking reptiles. The vole-hunting adder, Vipera berus, may be especially vulnerable to buzzard predation in peak vole years because it is probably attracted to the vole-rich patches utilized by the buzzard. Résumé : On croit généralement que les prédateurs qui se nourrissent surtout de campagnols chassent moins les autres animaux quand les campagnols sont abondants. Cette supposition peut cependant n’être pas valide dans le cas de proies qui vivent sur le sol ou de proies à déplacements lents dont les exigences écologiques en fait d’habitat sont semblables à celles des campagnols. Dans le sud de la Norvège, le Campagnol des champs, Microtus agrestis, est une proie importante de la Buse variable, Buteo buteo. Les oiseaux et les reptiles sont aussi des groupes de proies importants pour les buses. Dans une région boisée à populations cycliques de campagnols, les restes de proies et les boulettes fécales ont été récoltés aux alentours de nids de buses pendant 2 années à densité élevée de campagnols et 3 années à densité faible. En temps de densité faible, les oiseaux constituent les proies les plus importantes. En revanche, le maximum de reptiles dans le régime est enregistré en temps de densité maximale de campagnols, peut-être parce que, dans ces conditions, les buses concentrent leurs efforts de chasse sur des animaux qui vivent au sol, là où abondent les campagnols. Au site de l’étude, il y a surtout des zones de coupe à blanc utilisées régulièrement par les reptiles qui se chauffent au soleil. Les vipères, Vipera berus, qui chassent les campagnols, sont sans doute particulièrement vulnérables à la prédation exercée par les buses durant les années où la densité est élevée, parce qu’elles sont probablement, elles aussi, attirées par les zones riches en campagnols que fréquentent les buses. [Traduit par la Rédaction]

Introduction

2093 Selås

It has commonly been assumed that in areas with cyclic vole populations, predators whose main prey are voles reduce their predation on other prey when voles are abundant (e.g., Angelstam et al. 1984; Järvinen 1985; Lindström et al. 1987; Marcström et al. 1988). However, this assumption may not be valid for all species preyed upon by such predators. First, a relaxation in predation pressure on a given species as a result of the predators’ functional response to vole numbers may be compensated for by a simultaneous numerical response (immigration and reproduction) by the predators. Birds of prey, especially, are able to increase rapidly in number through immigration as vole numbers increase (Newton 1979). Second, increased vole populations may influence the hunting mode of predators (Andersson 1981). Other small Received November 21, 2000. Accepted September 26, 2001. Published on the NRC Research Press Web site at http://cjz.nrc.ca on December 7, 2001. V. Selås. Department of Biology and Nature Conservation, Agricultural University of Norway, P.O. Box 5014, N-1432 Ås, Norway (e-mail: [email protected]). Can. J. Zool. 79: 2086–2093 (2001)

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ground-dwelling prey species (e.g., shrews and frogs) may thus experience an increased risk of being detected by predators. And third, the response to voles may also involve a change in hunting habitats selected by predators, making prey species with habitat demands similar to those of voles more vulnerable to predation. A predator should select foraging habitats that give the highest energy gain in relation to search and handling time (Stephens and Krebs 1986). Prey species with habitat preferences different from that of voles may thus be less vulnerable to predation in peak vole years. If voles and other prey are not spatially separated, the assumption of relaxed predation on other prey in peak vole years requires either (i) that predators are nonselective hunters, causing a dilution effect on the latter prey (if stable in numbers), or (ii) that the latter prey are not selected because they are less profitable to hunt (low energy gain per attack in relation to handling time per attack; Stephens and Krebs 1986). The first condition may not be true, however; optimal foraging theory predicts that predators will be able to rank prey, such as frogs, reptiles, birds, and mammals, according to profitability. The second condition is probably true for most bird prey, which are, in general, more agile than voles and tend to be difficult to

DOI: 10.1139/cjz-79-11-2086

© 2001 NRC Canada


Selås

catch for predators that mainly hunt ground-dwelling prey (e.g., Newton 1979). There may, however, exist prey species with higher profitability than voles, but which are of minor importance for most predators because they are so rare that the predators cannot spend time searching for them. If predator densities are higher in years with high vole populations, such rare prey species may be preyed upon more often in those years than in years when voles are scarce. In Fennoscandia, the common buzzard, Buteo buteo, is a common raptor species at lower altitudes and in some forested areas with cyclic vole populations (Haftorn 1971). The buzzard is a typical generalist predator, preying on a large number of mammalian, avian, reptilian, and amphibian species (e.g., Andersson and Erlinge 1977; Spidsø and Selås 1988; Mañosa and Cordero 1992; Graham et al. 1995; Jdrzejewska and Jdrzejewski 1998). In Norway, Spidsø and Selås (1988) found that the common buzzard showed a functional response to cyclic field vole, Microtus agrestis, populations. There was also a numerical response, as more buzzard fledglings were produced when voles were numerous, partly because more pairs succeeded in raising young and partly because a higher number of offspring was produced per successful nesting attempt. Spidsø and Selås (1988) found that buzzards brought more birds to their nests in years with low vole populations, and birds were probably the most important prey in such years. Reptiles, on the other hand, seemed to be somewhat more common among prey delivered to nests in peak vole years, but because few buzzard territories were investigated in both a peak and a low vole year, definitive evidence for this pattern is lacking. Few amphibians were detected as prey at nests, possibly because they were not detected by the analyses used. In Finland, Suomus (1952) found that amphibians, like birds, were taken less frequently when vole densities were high. If detected by a buzzard, amphibians and some reptile species, such as snakes, are probably easy to catch and thus of higher profitability to it than are mammals and especially birds. They should therefore not be avoided as prey despite high vole densities, unless they have lower energetic or nutritional value or occur in habitats that are less suitable for vole hunting. The latter may be the case for amphibians, which are most common in wetland habitats. Amphibians are easiest to hunt at their spawning sites in spring (Tubbs 1974). Later in summer they are mostly active at dark, when buzzards are not hunting. Even though some reptiles also hunt primarily at night, they may still be vulnerable to buzzard predation because of their habit of basking during the day. To test the hypothesis that birds and possibly amphibians, but not reptiles, are taken less frequently by buzzards in years when vole populations are dense, I conducted a new study on buzzard prey in the area investigated by Spidsø and Selås (1988). In this second study it was possible to obtain a larger sample because of ongoing investigations on buzzard breeding density. The hypothesis was tested by comparing the total number of prey from different taxa found per buzzard nest in peak and low vole years during both study periods.

Methods The study area is situated in the southeastern part of Aust-Agder

2087 County in southern Norway, within the boreonemoral zone. Most of the area is covered by forests, with scattered lakes and bogs. The forests are mostly of poor and intermediate productivity, managed to provide a mosaic of clearcuts, regrowths, and older stands. More detailed descriptions of the area are given by Spidsø and Selås (1988) and Selås (1997). There are two common vole species in the study area, the field vole and the bank vole, Clethrionomys glareolus. The former is connected with grass-rich habitats, in the study area mainly clearcuts, while the latter is most common in old forest habitats (Hansson 1978). Field voles predominate in the diet of the common buzzard (Spidsø and Selås 1988), probably because this raptor is better adapted for hunting in open landscapes than in closed forests (Tubbs 1974). It may also be important for the diurnal buzzard that field voles are more often active during daytime, and that they are slower and thus probably easier to capture than bank voles (Hansson 1987). The two most important reptile prey species in the study area are the slow-worm, Anguis fragilis, and the adder, Vipera berus (Spidsø and Selås 1988). The small slow-worm feeds mainly on earthworms and slugs, while adult adders feed mainly on voles (Nilson and Andrén 1992a, 1992b). In Norway, slow-worms and adders are more common in clearcuts than in old forest stands because of differences in microclimate (Nilson and Andrén 1992a, 1992b). Adders are partly nocturnal hunters, but pregnant individuals, particularly, bask frequently during summer (Andrén 1985) and are thus likely to prefer open habitats. The grass snake, Natrix natrix, and smooth snake, Coronella austriaca, also occur in the study area, but they are less common than the adder. Of 51 snakes observed by the author and co-workers in the study area from 1993 onwards, 59% were adders, 27% were grass snakes, and 14% were smooth snakes. The smooth snake feeds on other reptiles (Nilson and Andrén 1992b) and may thus prefer the same habitats as the slow-worm and adder. The grass snake feeds mainly on amphibians and is most common in wetland habitats (Nilson and Andrén 1992b). Among the large number of bird species preyed upon by buzzards, thrushes appear to be the most important (Spidsø and Selås 1988). The blackbird, Turdus merula, song thrush, Turdus philomelos, and redwing, Turdus iliacus, are all common and widespread in the study area. They utilize a wide range of forest habitats, but prefer habitats with some cover rather than open areas such as clearcuts (Haftorn 1971). Also the tree pipit, Anthus trivialis, is a common buzzard prey (Spidsø and Selås 1988). This species occurs in both open forests and clearcuts (Haftorn 1971). To sample rodent availability, small mammals were snap-trapped in the study area in autumn (September–October) each year from 1982 to 1989 (Spidsø and Selås 1988) and in an area approximately 10 km northeast of the study area during 1990–1995 (Kålås et al. 1991, 1992, 1994, 1995; Kålås and Framstad 1993; Framstad 1996). In the study area, two permanently established line transects were used, each with 100 traps set out 5 m apart for 2 days. In the other area, 89–100 small quadrats (4 × 4 m), each with 5 traps, were set out at least 10 m apart along eight transects for 3 days. A trapping index was calculated as the number of individuals caught per 100 trap-nights (Fig. 1). During 1982–1994, K.O. Selås (personal communication) noted all vole observations made during fieldwork in spring and summer in the study area. In most years, his observations were in accordance with the snap-trapping indices from autumn. Throughout the 1980s and 1990s, vole populations peaked at 3- to 4-year intervals (Fig. 1). The data on buzzard diet used in the present analyses were collected during two periods, 1985–1987 and 1993–1994. For both study periods, only buzzard-nesting territories that had been investigated in both a peak (1985, 1994) and a low (1986, 1987, 1993) vole year were used. This included three territories from the first study period: one where prey remains and pellets were collected in 1985 and 1986 and two that were investigated in 1985 and 1987. © 2001 NRC Canada

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Can. J. Zool. Vol. 79, 2001

Fig. 1. Fluctuations in vole populations in the study area in Aust-Agder, southern Norway. The snap-trapping index gives the number of voles trapped per 100 trap-nights in autumn, based on 400 trap-nights within the study area each year during 1983–1989 (䊏) and 1335–1500 trap-nights near the study area each year during 1990–1995 (Q). The broken line gives the annual number of voles observed by one person during fieldwork within the study area during 1983–1994. In 1989, vole populations were unusually high in spring and early summer, but crashed before snap-trapping in autumn, which explains the discrepancy between the two indices.

Number of voles

20 15

10 5 0

1983 1985 1987 For the second study period, prey remains and pellets were collected from eight nesting territories in both 1993 and 1994. Two of these territories were investigated in the first study period also. None of the nest sites visited in a single year were closer to each other than 1 km. Prey items were identified from remains and pellets collected in nests, below nests, and at plucking posts close to nests (usually