CENSUSING OWLS IN LARGE AREAS: A COMPARISON BETWJ5EN METHODS Iiiigo ZUBEROGOITIA*~& Luisa Fernanda CAMPOS*
SUMMARY.-Censusingowls in large areas; a comparisor~benveen t,nefl~ods.The aim of this study is to obtain an efficient technique for censusing owls in large areas. Up to now. most census work on this group have been cm-ed out by means of enquiries combined with nest sire or tenitory searches. This study added the playback method and tested the efficiency of the three methods. We canied out m intensive census of nocturnal raptors in Biscay (Northern Spain) during four years (1992-1996). During this time we used three different methods: 1- searching for suitable owl nest sites, 2- en- quiries to rival people and colleagues. 3- playback methods. We found that the playback method was most ef- ficient for censusing Tawny Owls (Srr-iraluco). Little Owls (Arhene rtocfua)and Barn Owls (Tyro alba). The response of Tawny and Little Owls to playbacks did not vary throughout the year and was not influenced by weather conditions. However. responses by Barn Owls varied significantly throughout the year, March. April, May and July being the best months for censusing this species with playback methods. Moreover, the territorial behaviour of this species was influenced by weather conditions. The two other methods allowed us to obtain some data that complemented the results obtained by play- back methods, and were necessary to find some secretive and scarce owls like Long-Eared Owls and Eagle Owls. On the basis of these results, the used playback methods as the main technique for censusing owls and for monitoring their populations is strongly advised. However, it must be complemented with the two other methods to obtain complete resul~sFor all owl species. Key words: Censusing methods, distribution. owls, playback methods, Strigiformes.
bum.-Censo de rapoces nocrumas en grandes dreas: una comporacidn entre nrlodos. El objetivo de esle mbajo es el de obtener una tknica eficaz pmcensar rapaces nocturnas en grandes zonas. Hasta aho- ra la mayoria de 10s censos habian sido realizados mediante entrevistas combinados con la bBsqueda de nidos y temtorios. Nosotros aiiadimos el mttodo del reclamo y valoramos la cficacia de 10s tres rnktodos. Hemos realizado un censo intensivo de las rapaces noclurnas en Bizkaia (Norte de Esp~a)a lo largo de cua- tro aiios (1992-1996). Duranle este tiempo utilizamos tres mCtodos de censo: l- bdsqueda directa de nidos. 2- cuestionarios a las gentes de 10s pueblos y a ornit6logos. 3- reclarnos. Cornprobarnos que el reckamo es el me- todo m& eficaz para censar Chbos Comunes (Strix aluco). Mochuelos Comunes (Arhene nocruu) y Lechu- zas Cornunes (Tyto alba). La respuesta a1 reclamo de 10s Chabos y Mochuelos Comunes no vari6 a 10 largo del do, y no estuvo influenciada por las condiciones meteorol6gicas. Sin embargo, la respuesta de las Le- chuzas Conlunes si vari6 significativamente a 10 largo del 60, siendo rnano. abril, may0 y julio 10s meses en 10s que se dio una mejor respuesta territorial de la especie. Ademas, el comportarniento territorial de las Le- chutas Comunes escuvo influenciado por las condiciones atmosf6ricas. Los ob'os dos mAodos nos pennilieron cornpletar 10s resultados obtenidos con 10s reclarnos, y fueron ne- cesarios para localizar algunas de las especies mds escasas y dificiles de derectar como el BlEho Chico y el BJho Real. Con base en estos resultados, aconsejamos utiliw el reclamo como la principal tbcnica para censar ra- paces noclurnas y efectuar seguimientos de sus poblaciones. Sin embargo, para obtener resultados cornpletos de todas las especies. el reclamo debe ser apoyado con 10s otms dos metodos. Palabrar clave: Disuibuci6n. rnktdos dc censo, rapaces nocturnas. reclarnos, Strigiformes.
INTRODUCT~ON 19915)~The ecology and elbology of different spe- cies is being increasingly studied (see Galbraith Owls are one of the most difficult groups of et al., 1992); however, little is known about their birds to study, since most of them are secretive population and conservation status on a large- and difficult to observe (Proudfoot and Beasom, scale basis (see Hagemeijer & Blair, 1997).
* bib. Zoologia. Dpto. hlogia y DinPmica Celular Animal. Facultad dc Ciencias. Universidad del Pais Vasco. Ap. 644. E-48080 Bilbao. l E-mail: [email protected]. There are many countxies where the status of owl species is not yet known. This situation is clearly revealed in some books such as the The study area covered the whole territory of Atlas of Spanish breeding birds (Purroy, 1997) Biscay (2,384 km2), a province in the Western or tbe Atlas of European breeding buds (Hage- Basque Country (Northen Spain). Its territory is meijer & Blair, 1997). hilly md densely populated with extensive ur- One of the main reasons for the paucity of ban and industrialised areas. A description of information about these birds is the census its landscape and climate can be found in hidi methodology. Up to now, the greater part of (1987). census studies have been carried out by means The study was cartied out between Decem- of enquiries and a combination of nest site or ber 1992 and December 1996. We searched for temrory searching (see Pumy, 1997). The en- Tawny Owl Szrix aluco, Barn Owl Tyto alba, quiries method depends on the people being Little Owl Athene noctua, Scops Owl Otus interviewed, mainly on their experience or on scops, Long-eared Owl Asio ohs, Eagle Owl the time that they spnd in the field. Hence, it Bubo bubo, and Short-eared Owl Asio Jam- is very problematic to compare the results ob- meus) temtories continuously, two days per tained in different sites with this method. The week, every month, using three diferent met- second method is infiuenced by the difficulties hods: for detecting each species in the different ha- 1. Nesting sites search. We searched for bitats, as well as by each species' breeding owls in 325 churches, 834 houses and barns, 16 behaviour and the time available for the global derelicted buildings, 2 castles, 3 towers, all the census. Nevertheless, these methods can of- country quarries, cemeteries, schools, caves, fer good results whenever the protacted pe- walls, bridges, cliffs, tree holes, old stick nests riods of searching necessary to carry them out and 150 nest boxes that we set up in 1995. Fo- can be performed, but the cost-benefit rela- llowing Taylor (1992), these potential nest sites tionship for these methods increases notably were examined each year to determine the (see Exo, 1992; Percival, 1992; Bruijn, 1994; number available to owls of all species. the Saurola, 1995). number used for breeding, and other breeding There is a third method based on playback data. techniques. The efficience of this method= 2. Enquiries to rural people and colleagues. logy has been proven with some species Wre We used this method in all the villages, towns the Tawny Owl Srrix aluco (Hirons, 1976; and isolated houses of Biscay. We assesed the Hardy, 1977; Galeotti, 1990; Redpath, 1994), validity of each enquiry (n =713) by visiting the Tengmalm's Owl Aegolius funereus the different places in which the presence of (Holrngren, 19791, the Screech Owl an owl was indicated or suspected. This method Otus ask (Carpenter, 1987; Smith et al., was mainly used for Barn Owls, Little Owls 1987), the Spotted Owl Srrix occidentalis and Scops Owls. We did not use this method (RuLkevich & Gutitrrez, 1996). the Fermgi- for Tawny Owls because they were very com- nous Pygmy-Owl Glaucidiurn brasilianurn mon and their nest sites were difficult to find. cactonun (Proudfoot & Beasom, 1996) and Nevertheless, we noted all the information for the Long-Eared Owl Asio ohrs (Viada, 1994). comparison with the other two methods. Addi- The experience of these authors has proved tionally, we also used the enquiries for sear- that this is a good method for censusing secre ching the wintering species, Long-Eared Owl tive nocturnal birds and its cost-effectiveoess and Short-Eared Owl. seems to be the better than for the other two 3. Playback methods to provoke the tem- methods. tonal vocalization (see Sara 91 Zanca, 1989; In this paper we compare the results obtai- Galeotri, 1990; Redpath, 1994). We used this ned during four years of intensive censuses in method in 2056 different points for all species. Biscay with these three methods. Our main aim Each tape had male, female and owlet voice was to derive from these results a technique recorded in a continuous form. for censusing owls in large areas which combi- Scops Owls were only censused between nes the most efficient methods for each owl April and September, because they do not species. spend the winter in the study are.(Zuberogoitia CENSUSINO OWLS IN LAROE AREAS: A COMPARISON BETWEEN MmiODS 49
&Torres, 1997). On the other hand, the Short- Eared Owl is a wintering species in Biscay (Zuberogoitia & Torres, 1997), but we censu- A total of 1704 temtories of Tawny Owl, sed it throughout the whole year because it bre- 407 temtories of Bam Owl. 272 temtones of eds in neighbouring provincies (see Ombia& Little Owl. 26 temtories of Scops Owl, 6 tem- Fajardo, 1997). Moreover, we used the broad- tories of Long-Eared Owl and 3 temtories of casting methods in the forests of the highest Eagle Owl were found in the study area (Table mountains for seeking Tengmalm's Owl (Ae- 1). goliusfunereus). The success of each method was different Taped vocalizations were broadcasted ac- depending on each species (Table 1). AIl cording to the size of the owl, starting from the Tawny Owls were located using the playback smallest to the biggest. Thus we tried to avoid of temtorial cails. Some of them were located inducing predatory andlor competitive beha- by enquiries and searching for nest sites, alt- viours between species (see Mikkola, 1983). hough we used the playback method to con- Playbacks of each species vocalizations were firm their effectiveness in these tenitories. broadcasted for 5 minutes and. later, a reaction With regard to Bam Owls, 25.5% of temtones was waited for in silence for about 10 minutes. were found by enquiries, 41.3% by searching Although Redpath (1994) suggested 30 min., for nest sites and 33.2% by playback methods we tested this method during the year before (Table 1). this study was started and found, that it was pos- Al1 Little Owls were located first by play- sible to detect most of the temtories within 10 back records; the efficiency was low due to the min of the broadcast (own data unpublished). patchy distribution of Little Owls (see Zubero- The census activity began at dusk. We pla- goitia & Torres, 1997). but in areas where the yed the tape at intervals, in different sites that species exists, the efficiency was even higher were close to each other, so that the cry of owls than 100% (two or more pairs answered to the demarking territones could be heard. These playbacks in many sites). points were separated 200 m in flat fields for Scops Owls, Long-Eared Owls, Short-Emd censusing Little Owls and Scops Owls, and 500 Owls and Eagle Owls were intensively sought m for censusing the test of the species in flat in the study area, the efficiency of each method lands, and between 500 and 1OOO m for censu- being lower than 5%. sing in mountains. We usually played the tape The overall success of the playback method in the highest sites of mountains and hills to did not vary throughout the year for Tawny beiter hear the response of owls. The distance Owls and Liitle Owls, dthough we observed between two points was established according variation for Barn Owls (x2,,=65.03, to the size of the temtory of each species and pc
Several authors proved that this method was weather conditions, but we obtained very good a good technique for locating Tawny Owls. results during the breediug season, from March using it for specific studies (Galeoiti & Pavan, to July. In fact, we found significant differen- 1993; Galeotti, 1994; Redpath, 1994). Also, ces between these breeding months and the other authors have used the same method for rest of the year. The efftciency shown in the re- censusing other owl species (Hirons, 1976; sub, 32.5 %, is the yearly average for al1 the Hardy, 1977; Holmgren, 1979; Carpenter, four years of out study; however, if we consi- 1987; Smith etal.. 1987; Hardy, 1992; Proud- der only the breeding season. the efficiency foot & Beason, 1996; Rinkevich & Guti€rrez, will be higher. Also, we proved that the Bam 1996). However, the territorial behaviour of Owl tesponses depended on the weather con- Barn Owls and Littie Owls seems to be less ditions, so that the overall efficiency of the well documented. Sará & Zanca (1989) argued method can be even better if properly conduc- that playback is a bad technique for Bam Owl ted. census. Our results agree with this claim when in the same way, Sará & Zanca (1989) ar- the species was censused in winter or in bad gued that the best censusing months for the Lit- CENSUSMG OWLS M LARGE AREAS. A COMPARISON BETWEEN MFTHODS
Bam Owl responses to playback calls by month. [Respuesta de las lechrms al reclamo segrín los 1neses.1
No. calls 28 46 71 52 69 68 65 47 40 53 41 30 [N," reclamo.r] % responses 21.43 36.96 45.07 51.92 46.38 19.12 49.32 19.15 17.50 32.07 9.76 6.67 1% de respuesras1
Bam Owl responses to playback calls aceording to pmvalent weather conditions. Other: Fmst, fog. mist, snow- a-.. f.11a-.. [Respuesras de las lechuzas al reclamo según las condiciones mereoro1dgica.r. Otras: helada, niebla. bruma, nevada.] Without clouds Cloudy Rainy Cold Windy Other [Despejado] [Nublado] [Uuvioso] [Frió] [Venrosol [Orrosl No. calls 336 131 142 147 38 26 [N" reclamosl % responses 34.52 37.40 26.05 17.69 13.16 19.23 1% de respuestas] tle Owl are Febmary, March, June and July. scarce in the province (Zuberogoitia &Torres. However, as we proved, Little Owls answered 1997). hence the sampled effon used for them well throughout the year and we did not find was relatively large thus affecting the cost-ef- significant variation depending on the environ- fectiveness of the method. ment conditions. The low efficiency of the playback method The other two methods, enquiries and nest in the Scops Owl case seemed to be due to the site searching, are the most used census techni- few pairs that bred in the study area. as Scops ques for large areas (see ¡.e. Percival, 1992; Owls responded to the playback very well most Bmijn. 1994; Purroy, 1997). These methods, of the times we played it near their territories. however, do not allow us to locate a represen- although their voices were more difficult to tative proponion of owl temtones and their ef- hez than those of the rest of the species. On the ficiency is lower than that of the playback met- other hand, Long-Eared Owls dtd not respond hod. Besides, the sampling effon required is ar al1 to playback calls. Sometimes, when we relatively large. Tawny Owl, Long-Eared Owl found one bird, we played the tape to test its and Scops Owl nests are very difficult to find behaviour, but we never obtained any answer. because they breed in forested areas (see Mik- In fact, Sará &Zanca (1989) and Taylor (1992) kola, 1983; Saurola, 1995; Zubemgoitia & To- argued that the playback technique does not rres, 1997). Hence. these methods are not very seem useful for censusing Long-eared Owls. cost-efficient, although they were useful for the However, Viada (1994) had succesful results Barn Owl that breeds in humanised areas and with this method in Mallorca. In the same way, helped us to find some of the breeding temto- wintenng Short-Eared Owls did not answer to nes of Scops Owls, Long-Eared Owls and Ea- the territorial calls. Since we could not census gle Owls and the wintering presence of Long- Shon-Eared Owls during the breeding season, Eared Owls and Short-Eared Owls. It should the efficiency of this method is yet to be te~ted be noted, however, that the last two species are with this owl species. The scarcity of Eagle Owls caused the low ceedings. pp. 277-281. USDA Forest Service Ge- efficiency of the methods. Hence we could not neral Technical Report RM 142. test the differences between the three of them. Exo, K. M. 1992. Population ecology of Little Owls Penteriani & Pincheda (1991) and J.A. Martl- Athene noctua in Central Europe: A review. In. C.A. Galbraith, 1.R Taylor & S. Percival (U,): nez (per. corn.) considered that the playback The ecology and conservation of European owls. method was not an efficient method for censu- pp 64-75. Peterborough, Joint Nature Conserva- sing this species. tion Commitee. (UK Nature Conservation, No 5). GALBRAITH.C. A., TAYLOR,I. R. & PERCIVAL,S. (Eds.).1992. The ecology and conservation of Europeon owls. Peterborough. Joint Nature Con- servation Commitee. (UK Nature Conservation. We conclude that the playback method is the most efficient for censusing large areas the GALEMTI,P. 1990. Territorial behaviour and habitat selection in an urban population of the Tawny In lhis 'lows the fin- O,I (Sl* L,).Bo[l, Zoo],,57; 596, ding of more territories than the two other met- 1994. Patterns of territory size and defence le- hods during [he same period, although there vel in rural and urban Tawny Owl (Slrit uluco) are problematic species like Long-Eared Owls ppulations. JOU~~of ~oo~ogy (bndon), 234: and Eagle Owls. In any case, the efficiency of 641-658. this method was hig& when canied out in -& PAVAN.G. 1993. Differential responses of re- good weather conditions. mtorial Tawny Owls (Srrix aluco) to the hooting We strongly advise the use of the playback of neighbourds and strangers. Ibir, 135: 300-304. method as the main technique for censusing HAGEMEIIER,W. J. M.& BLAIR,M. J. (Eds.). 1997. The EBCC Atlas of European Breeding Birds. owls and for monitoring their populations. Ho- Their Dhribution and Abundance. T & AD Poy- wever, it must be complemented with heother ser. London. two methods to obtain complete results for all HARDY, A. 1977. Hunting ranges and feeding eco- species. logy ojowls infromlands. PhD. Thesis, Aberdeen University. ACKNOWLEDGEMENTS.-Wethank lose Antonio HARDY,A. R. 1992. Habitat use by farmland Tawny Gonzslez-Oreja, Jose Antonio Martinet, Ricard Gu- Owls Sirir aluco. In, C. A. Galbraith. I. R Taylor ti6rrez and Mario Diaz for their valuable comments. & S. Percival (Eds.): The ecology and conserva- We also wish to thank everybody that took pm in tion of European owls. pp 55-63. Peterborough, the field work, especially Julen Zuberogoitia. Gorka Joint Nature Conservation Cornmitee. (UK Na- Ocio, Tamara Crespo, Gorka Duralde. Lander As- ture Conservation, No 5). torkia, Udimga Abrisketa, Fernando Garitagoitia, I-~oNs,G, J. M. 1976. 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