J. RaptorRes. 31(1):16-25 ¸ 1997 The Raptor ResearchFoundation, Inc. NEST-SITE SELECTION BY FOUR SYMPATRIC FOREST RAPTORS IN SOUTHERN NORWAY VmAa SELAS Departmentof Biologyand Nature Conservation,Agricultural University of Norway, P.O.Box 5014, N-1432 Ks, Norway ABsTR•CT.--Differencesbetween 0.1 ha nest-siteplots of Honey Buzzards (Pernisapivorus), European Sparrowhawks(Acdpiter nisus), Northern Goshawks(A. gentilis)and Common Buzzards (Buteobuteo) were comparedto randomlysampled 0.1 ha control plotswithin a 400 km2 area with 80% forestand <2% agriculturalland in southernNorway. At Honey Buzzardnest sites,forests were more productive than in control plots and there wasa higher proportion of spruce,older trees and a higher tree density at Northern Goshawknest sitesthan in control plots. Nests of European Sparrowhawkswere also in siteswith higher tree densitythan expected.Common Buzzardnest siteswere situatedin steeperterrain than control plots and more often had a southern aspect.For sparrowhawks,nesting in forestswith high tree densitymay be an adaptation to avoid goshawksand pine martens (Martesmartes) which are their main nest predators. For the larger species,nest-site selection may be a responseboth to nest predation risk, microclimate,foraging habitat and food supply. KEYWORDS: HoneyBuzzard; European Sparrowhawk; Common Buzzard; Northern Goshawk; Accipiter nisus; Accipiter gentilis;Buteo buteo; Pernis apivorus;forest;, nest-site selection; Norway. Seleccitn del nido de cuatro rapacesde bosquesin que no aparean en el sur de Norway. RESUMEN.---Diferenciasentre 0.1 ha parcela de sitio de nido de Pernisapivorus, Acdpter nisus, A. gentilis y Buteobuteo fueron comparadoscon muestrasalazar 0.1 ha parcelasmanejadas dentro de una area de 400 km2 con 80% bosquey <2% tierra agricolaen el sur de Norway.En nidos de Pernisapivorus, los bosquesfueron mas productivo en las parcelasmanejadas y habia una proporcitn alta de Picea,firboles maduros y densidad alto de firboles en nidos de A. nisustambi•n estaban en sitios con densidad alta de firbolesmas de 1oque esperfibamos.Nidos de B. buteoestaban situados mas en terreno abrupto que en parcelasmanejadas y con frecuencia tenla aspectodel sur. Para A. nisus, nidos en el bosque con densidad alta de firboles puede ser un adaptacitn para evitar A. gentilisy Martes martesque son su principal depredador de nido. Para la especiemas grande, la seleccitn del nido puede ser reaccitn a riesgode depredador al nido, microclima,hfibitat de forraje y suministrode cornida. [Traduccitn de Rafil De La Garza, Jr.] Breeding pairs of raptors use relatively large ar- hand, dense foliage may decreasethe possibilityfor eas, and thus have a good opportunity to select breeding birds to detect and escape from preda- nesting placesthat maximize the probability of suc- tots (G6tmark et al. 1995). Thus, selection of nest cessfulbreeding and lifetime reproduction (New- site may be a trade-off between concealment and ton 1979). Interspecificdifferences in nest-sitese- opportunities to escapeor attack predators, which lection may be due to differences in body size and also depend on flight ability, body size or other flight performance of different species,but it can characteristicsof the species.Selection may alsobe also be due to interspecificdifferences in nest pre- affected by a trade-off between current and future dation risk, climatic conditions during breeding reproduction, since short-lived specieswith large and feeding habits (Newton 1979,Janes 1985), or brood sizes have more to lose when nesting at- to interspecificcompetition for nest sitesand ter- tempts fail than long-lived species with smaller ritories (Newton 1979). brood sizes. For several bird species,dense foliage close to Cover may also be an important factor since it the nest both reduces the rate of detection, and can shield nests from wind or rain and limit ex- impedes the ability of predators to hunt in the vi- cessive nocturnal radiation loss or excessive diur- cinity of the nest (Martin 1993). On the other nal heat-gain from solar radiation (Walsberg1985). 16 M_•CH 1997 NEST-SITE SELECTION BY FOUR RA_PTORS 17 Protection from thermal extremesmay be the most 400 km •and is situated 50-300 m a.s.l. and 10-30 km important factor in nest-siteselection by medium- inland from the coast, in the boreo-nemoral zone (Abra- hamsen et al. 1977). The climate is suboceanic,and snow and large-sizedraptors where nest predation is low usually covers the ground from December-April. The (Newton 1979,Janes 1985). At higher latitudes,the studyarea is hilly and sharplyundulating. It is dominated timing of breeding in these birds should be im- by forests(80%), with scatteredlakes (10%), bogs (5%) portant, since early breeders are faced with more and lessthan 2% agriculturalland. Forestsare character- ized by a fine-grained mosaicof young-, medium- and severeclimatic conditionsthan thosespecies which old-aged coniferous, mixed and deciduous stands,with begin nesting later in spring. Scotspine (Pinussylvestris), Norway spruce (Piceaatnes), If prey are not evenlydispersed throughout the oak (Quercusspp.), aspen (Populustremula) and birch landscape,raptors should selectnest sitesclosest to (Betulaspp.) the dominant tree species. the best hunting areasin order to reduce time and Forestrybased on clear cutting, replanting and thin- ning of the regrowth was introduced to the area in the energy connectedwith foraging. Thus, local varia- 1950s.At the time of my study,approximately 30% of the tion in the availabilityof food may influence the area had been clear-cut,with most regeneration<20 yr. nest-siteselection (Janes1985), and explain inter- The area is dividedinto numeroussmall ownerships with specific differences in nesting habitat. management of forests controlled by each of the land owners.Most of the properties are managed to provide In Fennoscandia,four raptor specieshunt and a mosaicof forest types.Thus, there is a heterogeneous nest in forest-dominatedlandscapes. The Europe- environmenton a small scale,but a homogeneous,frag- an Sparrowhawk (Accipiternisus; mean body mass mented environmenton a large scale. male 150 g, female 260 g) is the main predator on The study area was searchedfor nest sites each year small birds (Sulkava 1964a, Selfis 1993), while the (cf. Forsman and Solonen 1984), and habitat variables were describedat one nest site in each nestingterritory Northern Goshawk (A. gentilis;mean body mass located. If possible,the nest site used in 1988 was select- male 870 g, female 1330 g) primarilyfeeds upon ed. In territories where the 1988 nest was not found, I larger bird speciesand mammals (H6glund 1964, usuallydescribed the nest site usedin 1989. Alternatively, Sulkava 1964b, Widan 1987, Selfis 1989). The Com- the nest site used closest in time to 1988 or 1989 was described.The breeding densityof goshawksincreased mon Buzzard (Buteobuteo; mean body massmale during the time of the study.To get a larger number of 740 g, female 1100 g) is a generalistpredator that nest sitesof this species,I first selectedone nest site from respondsfunctionally to changesin populationsof each of the nesting territories used since 1985. Then, I its vole (Microtusspp.) prey (Suomus1952, Spidso selectedone nest site in each of the 11 new nesting ter- ritories establishedduring 1986-88, even though these and Sel•s 1988), while the Honey Buzzard (Pernis territories substitutedfive of the existingones. Since the apivorus;mean body massmale 750 g, female 910 goshawksin the five old territories had all been illegally g) mainly feeds on the larvae and pupae of social shot by game keepers, I regarded the data to be inde- hymenoptera species(Holstein 1944, Hagen and pendent. Thus, I describeda total of 48 nest site plots of Bakke 1958, It/imies and Mikkola 1972). the sparrowhawk,30 of the goshawk,50 of the Common Buzzard,and 21 of the Honey Buzzard. Severalauthors have describednest sitesused by Control plotswere describedduring 1989. Aerial pho- sparrowhawks (Tinbergen 1946, Holstein 1950, tographsof the studyarea taken in spring 1989 (scale1: Hald-Mortensen1974), goshawks(Holstein 1942, 15000) were covered by a grid with 100 numbered inter- Dietzen 1978, Link 1986), Common Buzzards sectionsof which two were randomly selectedas control plots. Out of 122 selectedpoints, 80 (65.6%) were locat- (Holstein 1956, Kntwer and Loske 1980, Solonen ed in forests>20 yr old and 25 (20.5%) were in forests 1982, Jedrzejewskiet al. 1988, Hubert 1993) and <20 yr old (clear-cutsand regrowth), while 9 (7.4%) Honey Buzzards (Holstein 1944, Arecoif et al. were on lakes,4 (3.3%) on bogs,and 4 (3.3%) on agri- 1994) in Europe. However, no one has compared cultural land or developed areas. Measurements were made only in control plotsin habitatsapparently suitable nest-siteselection of sympatricpopulations of these for raptors (i.e., forests >20 yr old, N = 80). speciesin a continuousforest habitat. My aim was Each of the nest site plots and the control plots cov- to study the importance of different habitat vari- ered 0.1 ha within a circle with a radius of 17.8 m. In ableson nest-siteselection of thesespecies by com- nest site plots, the nest was in the center of the circle. paring habitat variablesfrom plots at nest siteswith The followinghabitat variableswere used: those from plots placed randomly in the study 1) Site type, determined from the plant community area. (Kielland-Lund 1981, 1994). Plots dominated by Bar- bilophozio-Pinetumor Vaccinio-Pinetumwere classifiedas STUDY AREA AND METHODS siteswith