Biology of Owls

The World Working Group on Birds of Prey and Owls

Biology of Owls


Andris Avotins (Oral)

Nest Site Choice by Tawny Owls Strix aluco in Latvia

Natural (as opposed to artificial) nest site choices by Tawny Owls were investigated in Latvia, Eastern Europe. Most nest descriptions were made since 1990, but some date back as far as the 1960 s. 239 breeding attempts were registered in 192 different sites. Half of them were found in sample areas of owl census in Madona district (90 sq km or 0.14% of the whole of Latvia), where most Tawny Owl nest sites were controlled since 1990 (high intensity of nocturnal observations and daylight controls, including trapping of breeding females).

91% of nests were found in various types of tree holes including stumps, 7% in various buildings, but 2% - in old nests of raptors (Buteo buteo and Accipiter gentilis). Breeding on the ground or in corvid nests is not found. Twig nests of raptors and some unsuitable sites in buildings were used only in a scarcity of tree holes, although some abandoned houses have been used for a long time.

12% of nests were found in decayed tree stumps (aspen mainly). This is an important nest site type in forests. 6% nests found in cavities between two old treetrunks (mainly lime trees).

Up to 70% of nests are found in trees of exotic origin, planted near farms, villages, in parks, cemeteries, or preserved for generations as very old free-standing trees on farmland. Also in forests, some nests are found in old trees left from houses deserted in the last century.

Teici Nature Reserve, Aiviekstes 3, Laudona, Madona distr., Latvia, LV-4862

Email:  Fax: + 371-4807200

Vincent Bretagnolle1, Hardouin Loïc1a, Christian Bavoux2 (Oral)

Guy Burneleau3 and Thomas de Cornulier1

Abundance and Distriburion of Little Owl Athene noctua in western France: methodological aspects for a survey on a large spatial scale.

Our aim is to show that it is feasible, realistic, and important to conduct large-scale sampling of Little Owl abundance and distribution. Using data that have been collected yearly since 1999 in two study areas (total area = 511 sq km) in western France, we compare the results of different sampling strategies, all based on playback methodology, with various grid sizes. We also investigate the effect of some factors on the responses of Little Owls in order to increase the efficiency of the playback. We found that using different calls (e.g. male hoot, chewing or alarm calls) has significant effects on response probabilities as well as seasonal effects. We also compared responses of Little Owls to playback at different distances and population density, and again found statistical differences. We finally show some results of spatial distribution analyses, which reveal that owl distribution is clustered, though depending on the spatial scale used.

1. Centre d’Etudes Biologiques de Chizé CNRS UPR 1934, 79360 Villiers-en-Bois, France.

2. Le Marais aux Oiseaux, Les Grissotières, 17550 Dolus-d'Oléron, France.

3. 2, rue du port de Chiffeu, Mauzac, 17320 Saint-Just-Luzac, France.

1a. Email:

María Del Mar Delgado1, Carlotta Maggio2, Ana asanta Alves3 , (Oral)

Carmelo Escot Munoz3 and Vincenzo Penteriani1

Preliminary data on an Eagle Owl Bubo bubo population in S-W Spain

We present the preliminary results of a study on an Eagle Owl population in Andalusia (south-western Spain). The main aims of our work are to analyse: (1) relationships between Eagle Owl density and rabbit/red grouse availability; (2) breeders' performance as a consequence of individual and territory quality; (3) characteristics of vocal display and individual recognition by call; (4) vocal behaviour, call structure and individual quality; (5) sex determination and young morphological development; (6) movements of breeders within their territories and mating system; (7) features of young dispersal; (8) characteristics of floater settlement areas; (9) process of floater recruitment within the breeding population; (10) sex- ratio.

1. Estación Biológica de Doñana, CSIC, Department of Applied Biology, Avda. María Luisa s/n, Pabellón del Perú, 41013 Seville, Spain. Tel: + 34 954 232340  Fax: + 34 9544621125  Email:

2. Centre for Wildlife Assessment & Conservation, School of Animal and Microbial Sciences, University of Reading, Whiteknights, PO Box 228, Reading RG6 , U.K.

3. EMASESA, Estacion de Ecologia Acuatica "Principe Alberto 1 de Monaco", c/Leonardo da Vinci, s/n, Isla de la Cartuja, 41092 Seville, Spain.

Wade L. Eakle (Oral)

Ecology and Conservation of the Palau Scops Owl: A Literature Review

The Republic of Palau is an archipelago of several hundred volcanic and limestone islands and coral atolls in the western Caroline Islands, located 760 km southeast of the Philippines and 1300 km southwest of Guam in the northwest Pacific Ocean. The endemic Palau Scops Owl (Pyrroglaux podarginus) is thought to occur on all major islands from Babeldaob to Angaur, as well as smaller, high limestone islands in coastal mangrove swamps, interior rainforests, wooded savannas and near villages in lowlands, feeding on insects, other arthropods and earthworms. Nesting in hollow trees and tree holes in dense vegetation, sometimes near the ground, from February-March, the owl has been described from abundant by some authors to highly endangered by others. The only known surveys for the owl were completed in 1945, but recent reports of owls commonly heard in the Rock Islands and in Koror suggest it may be widespread. However, with construction of the 85-km Palau Compact Road on the island of Babeldaob, in accordance with a Compact of Free Association with the United States, sensitive habitats including mangroves and rainforest have been impacted by erosion, dredging and soil disposal. The new road and planned developments on the island may compromise future conservation efforts for the owl and other wildlife species.

US Army Corps of Engineers, South Pacific Division, 333 Market St., San Francisco, CA 94105, USA.

Tel: Private + 1-707-579-9218   Office + 1-415-977-8039/8047.


Wade L. Eakle (Oral)

Assessing the Impacts of Clean Water Act Permits on the Cactus Ferruginous Pygmy-Owl in Arizona

The endangered Cactus Ferruginous Pygmy-Owl ( Glaucidium brasilianum cactorum) ranges from western Mexico (states of Colima and Michoacan) north to southern Arizona, USA, where it is found in Sonoran Desertscrub and Semidesert Grassland communities. Preferred habitats include ephemeral drainages with Saguaro (Carnegiea gigantea) or other columnar cacti, large trees, and a well-developed shrub layer. In response to a lawsuit filed by Defenders of Wildlife in 1997, and subsequent Arizona District Court order issued in 1999, the Corps of Engineers completed in 2002 a GIS-based cumulative impact assessment of Clean Water Act (CWA) permits issued in Pygmy-Owl habitat in southern Arizona from 1994-2000. Aerial photography, permit databases, procedural safeguards, historic and recent Pygmy-owl sightings, and local planning data were used to evaluate causal effects of impacts authorized by CWA permits on Pygmy-Owls and designated critical habitat. The assessment revealed that most impacts to Cactus Ferruginous Pygmy-Owls and their habitat are facilitated by factors outside the Corps of Engineers' CWA regulatory control; permit mitigation requirements effectively minimize habitat impacts; and the indirect effects of low-density residential development may not adversely impact Pygmy-Owls and their habitat in all cases.

US Army Corps of Engineers, South Pacific Division, 333 Market St., San Francisco, CA 94105, USA.

Tel: Private + 1-707-579-9218  Office + 1-415-977-8030  Fax: + 1-415-977-8039/8047


Loic Hardouin and Vincent Bretagnolle (Oral)

Acoustic Recapture as a tool for Conservation in the Little Owl Athene noctua

Vocal individuality is a potential tool for the monitoring of bird species that are otherwise difficult to observe or catch. In this study, we assessed the potential of this technique for the monitoring of Little owls Athene noctua , a common but threatened owl in France and Europe (SPEC 3 category). Our study is based on the analysis of two calls, the male hoot and the chewing call, recorded in western France in 2002 and 2003. We show that using quadratic discriminant analysis, vocal individuality is revealed on male hooting. The cross-validation method (Leave-one-out), which provides an estimate of the robustness of classification, indicated that successful classification was 96%. Variability in male hooting is also studied during the breeding season and between years. The shortcomings and the advantages of using vocal individuality for monitoring purpose in this species are discussed with regard to call variability.

Centre d’Etudes Biologiques de Chizé CNRS UPR 1934, 79360, Villiers en bois, France.


Girish A. Jathar1, Farah Ishtiaq2 and Asad R. Rahmani1 (Poster)

Breeding Success in the Forest Owlet Heteroglaux blewitti: Preliminary Results

Studies were carried out on breeding success of the Forest Owlet ( Heteroglaux blewitti) in 1998-99 and 2001-2002 in Toranmal Reserve Forest of Shahada, in Maharashtra state, India. One pair attempted nesting in 1998 and succeeded in rearing a young. Four pairs attempted nesting in 2001-2002 of which only one succeeded in rearing one individual. The most frequently used nest trees were, Boswellia serrata and Hemidacton exulsum. Measurements of the four nesting trees were taken, the mean nest tree height was 13.37±1.43m, mean nest height was 6.58±0.94 m, while mean girth at nest was 116.25±16.48cm. The mean depth of nest cavity and the entrance size were 50±19.10 cm and 14.25x12.25 cm respectively. Mean clutch size was 2.4±0.48 (n=5). Failure in breeding was observed due to cannibalism, ovicide, infanticide, predation and egg removal from the nest, and burning of nest tree by tribesmen. Encroachments, illicit woodcutting, grazing and superstitions among local tribes are affecting the scarce population of the Forest Owlet. Conservation measures are discussed.

1. Bombay Nat. Hist. Soc., Hornbill House, Dr. Salim Ali chowk, Shaheed Bhagat Sing Rd., Mumbai 400023, India


2.  Smithsonian Institution, Washington DC 20560, USA

David H. Johnson (Oral)

Owls of the World – Inventory Methods, Taxonomy, and Vocalizations

An international team is beginning a multi-year worldwide project to resolve foundational aspects of taxonomy and conservation on the extant owl taxa. The project is focused on six tasks: 1) Develop inventory techniques for locating owls and their nests; 2) Analyze the molecular systematics and phylogeny of owls using mtDNA; 3) Acquire high-quality recordings of owl vocalizations, to assist in species identification; 4) Conduct analysis of the morphological aspects on new owl taxa found; 5) Refine the geospatial distributions and habitat orientations of the owl species, create updated GIS maps, and develop a museum voucher specimen database; and 6) Distribute project information (ca. 250 species accounts) via internet pages. Each Species Account page will include: a) scientific and common name(s), b) pictures of the owl species, c) sound files of their vocalizations, d) a distribution map, e) basic life history and habitat information, f) IUCN status, g) recommended inventory methods, and h) citations and links to more information. Substantial coordination with ornithological societies, universities, raptor organizations, local researchers, and biologists is expected and desired. Products will be developed in English, with summaries in other languages. This paper presents an update on the status of this project.

David H. Johnson, 2344 Summit Lake Shore Rd. NW, Olympia, Washington 98502, USA.

Tel: + 1-360-902-2603  Fax: + 1-360-902-2158  Email:

Jose Carlos Motta-Junior and Adriana de Arruda Bueno (Oral)

Trophic Ecology of the Burrowing Owl Athene cunicularia in Southeast Brazil

The Burrowing Owl (Athene cunicularia) is widely distributed in open areas from southern Canada to southern Chile and Argentina. Despite its being a relatively well-known species, virtually no study was published on its feeding ecology in Brazil. The aim of this research was a comprehensive analysis of the diet of this raptor in six Sao Paulo State localities, Southeast Brazil. The collection and analysis of 938 pellets and 51 pellet debris yielded 10,540 prey individuals distributed in approximately 103 species/morphospecies. Invertebrates were numerically the main prey (66.6-96.8%), mostly represented by termites, orthopterans, beetles and spiders. On the other hand, by estimated biomass consumption vertebrates (mainly small rodents such as Calomys tener) formed the bulk of the diet, yielding 46.3-91.5% of total biomass consumed in each locality. In general, rodents and termites were more consumed in the dry season (April to September), whereas beetles were more preyed on in the rainy season (October to March). The greater predation on these prey may be understood by their higher temporal abundance in the environment. Therefore, this result showed opportunism by the owls. The food niche breadth (standardized Levins measure) was higher in more disturbed localities.

Universidade de São Paulo, Departamento de Ecologia, 055508-900 São Paulo, SP, Brazil.


Jose Carlos Motta-Junior, Cleber Jose Rodrigues Alho (Oral)

and Sonia Cristina da Silva Belentani

Food Habits of the Striped Owl Asio clamator in Southeast Brazil

The Striped Owl (Asio clamator) is a poorly known species ranging from southern Mexico to northern Argentina. Few quantitative data are reported on its food habits, particularly in Brazil. The study goal was a quantitative analysis of the diet of this raptor, including estimated biomass consumption. Field work was conducted in two localities in southeast Brazil, where savannah grassland was the main vegetation cover. Samples of 87 pellets plus 6 pellet debris and 44 pellets plus 9 debris were collected respectively in the Reserve of Sao Carlos University and in Itirapina Ecological Station, roughly 30 Km apart. Pellet analysis for both localities yielded 434 prey individuals, including 47 species/morphospecies. Vertebrates, mostly rodents and birds, were both by number and by biomass the bulk of the diet (82.6-83.5% and 99.4-99.6%, respectively). Invertebrates were only represented by insects. Unlike other owls of similar body mass (335-546 g), the Striped Owl seems to prey regularly on relatively larger prey (>100 g), such as sub-adults and juveniles of Spiny Rats (Clyomys bishopi), Cavies (Cavia aperea) and Opossums (Didelphis albiventris). This can be partially explained by the larger and possibly stronger claws of this owl.

Universidade de Sao Paulo, Departamento de Ecologia, 055508-900 Sao Paulo, SP, Brazil.


O. Schwerdtfeger (Oral)

Reproductive Effort of Tengmalm´s Owl Aegolius funereus under periodical and long-term Changes of Living Conditions

In the course of ecological studies on a population of Tengmalm´s Owl in the Harz mountains (Northern Germany) the breeding biology, population dynamics and feeding ecology have been investigated for 24 years.

In the study area of 200 sqkm nearly all broods (650) have been recorded and nearly all owls have been marked.

The yearly average values of all breeding parameters correlate significantly with the 2- and 3-year-variation of the small mammals' frequency monitored from stored prey. During the first 12 years the population cycles of the field-voles was decisive; in the following years those of the Apodemus species. In the course of time the share of late broods was growingly linked to an increasing fluctuation of adult owls.

Despite these changes the mean reproduction-rate remained constant in the long term of investigation. In spite of the cyclically varying breeding-data the average number of eggs resulting in no fledglings was constant each year. The number of eggs exceeding the loss-figure of 2.4 led regularly to a breeding success. Of course, the investment of eggs takes place on the basis of prey supply.

Quellenweg 4, D-37520 Osterode am Harz, Germany. Email:

Tamás Sike and Attila D. Sandor (Poster)

The Distribution of the Barn Owl (Tyto alba) in Romania taking into account the literature and observational data

There is no comprehensive publication in Romania which gives an overview concerning the Barn Owl's (Tyto alba) countrywide distribution. At the European level, in handbooks and reports, one can find distribution maps but these are only estimated areas. Also these maps are disimilar; there are many differences among them. In order to get a real overview concerning the distribution of the Barn Owl in Romania we collected all the published data which so far exist in Romania, and completed these with our own surveys. Because of the big time differences we have prepared three distribution maps. Analysing the results we can draw the conclusion that except for the highland zone the species can be found everywhere in Romania; the limiting factors can be food and proper breeding sites.

Tamás Sike, Ro-3900 Satu Mare, Averescu 3 C/48, Romania


A. K. Singh (Poster)

A Case of infectious Bursitis in the Oriental Scops Owl

Owl populations have been on a continuous decline worldwide and the specific reasons are still unknown to the research community. This report indicates the prevalence of infectious-bursal disease in the Oriental Scops Owl, recently reported in the Satpuda forests of Betul, in Madhya Pradesh, India. Villagers took one adult female Oriental Scops Owl to the Veterinary Hospital for post-mortem, as it was found dead in the backyard of a farmer's house. The gross pathological findings showed several pinpoint haemorrhages at the junction of the proventricularis, swollen, enlarged and hemorrhagic bursa, and hemorrhagic lesions in the thigh muscles. The macroscopic findings were similar to those of infectious bursitis, which was further confirmed by serological examination. Infectious bursitis has been common in poultry, and is a cause of concern since almost a decade to a very large section of birds ranging from domestic to commercial origin (especially in India). It is more so because domestic birds in rural areas are rarely vaccinated, making them a potential risk factor for birds of prey. The mode of infection could possibly involve birds (especially poultry - alive or dead), which might have acted as a source of infection to the owl.

Dr. A. K. Singh. Government Veterinary Hospital, Khedi, Betul, M.P. India

Tel: Private: + 91-07141-231907  Office: + 91-07141-262291


Paolo Taranto (Poster)

Status of Knowledge of the Barn Owl and Little Owl in Italy: a bibliometric review

This study was undertaken in order to determine the extent of knowledge of the two most common Italian owls, the Barn Owl (Tyto alba) and the Little Owl (Athene noctua). The bibliometric method was used for performing a statistical analysis of all the Italian publications regarding these two species from 1900 to 1999. The Barn Owl has the major number of studies among all the Italian owls. The data analysis shows a disequilibrium on the available information in the case of the Barn Owl, because of the high number of specific publications on the feeding argument (63.4%), in relation to the other arguments (i.e. biology with only 2 papers, 0.9 % of the total). Instead, for the Little Owl the data are more equilibrated. For both species the results show that only partial and inadequate information is available on phenology, biological aspects and systematics that are of basic importance for a correct planning of conservation programmes.

Via Calvart 10, 40129 Bologna, Italy




© 2003 CTM