Volume 23 - N. 2
December 1999

Avocetta
Volume 23 - N. 2



  1. Revised distribution and status of diurnal birds of prey in Portugal

    Palma L., Onofre N. & Pombal E.

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    Until 1982, when the first comprehensive reports on the status of the Portuguese birds of prey were published (Palma 1985, Rufino et al. 1985) only some anecdotal and partial accounts from the first half of the century were available (Coverley no date, Tait 1924, Reis Júnior 1934). The present report presents a revision and updating of the information on the distribution and status of diurnal raptors in the country.

  2. Population density and dispersion of the Eagle Owl Bubo bubo in the Province of Trento (central-eastern Alps, Italy)

    Marchesi L., Pedrini P. & Galeotti P.

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    An Eagle owl population was studied from 1994 to 1997 in a 672 km2 study area in the Province of Trento (central-eastern Alps, Italy). The censuses population consisted of 11-12 pairs and a solitary male. 95% of the territories was occupied during each of the years of the study period. Mean density was 1.83 territories/100 km2. Mean nearest neighbour distance was 3.8 km. Territories were regularly dispersed within the study area, as indicated by the high value of the G statistic.

  3. Does bib size act as a status badge in territorial Reed Buntings (Emberiza schoeniclus)?

    Rubolini D., Boto A. & Bogliani G.

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    The hypothesis that bib size acts as a signal of status was tested on territorial Reed Bunting males. In accordance with previous results, we predicted that if males display their status through bib size, a territory invasion by a large badge model would elicit a more aggressive response from the territorial individual. This was tested by presenting two stuffed models with the extremes of badge variation (smallest and largest, as found on 26 museum specimens) to territorial males of unknown badge size and by observing and comparing the responses. A third model, a male in winter plumage, was used as a control. Our hypothesis was not confirmed, but there was a tendency in the predicted direction: territorial males were more aggresive towards the large badge model even if not significantly so. Bib size may then play a minor role in honest signalling in the Reed Bunting, and other cues such as song structure and/or song rate might be more important. It must also be stressed that bib size variability could be involved in individual recognition.

  4. Egg dimensions of the Roller Coracias garrulus in farming areas of the Southwest Iberian Peninsula

    Avilés J.M., Sánchez J.M. and Sánchez A.

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    Egg dimensions of the Roller Coracias garrulus in farming areas of Extremadura were studied in 1989 and 1990. Egg dimensions of Rollers tend to be smaller in the south compared to the north. Differences in egg dimensions were not found in the studied population in relation to years. Climatic conditions during the laying period did not influence egg size. No seasonal decline in egg size was detected and, similarly, we did not detect a clear trade-off between egg size and clutch size.

  5. Distribution and habitat of the Dartford Warbler Sylvia undata in the Eastern Tuscany

    Tellini Florenzano G. & Lapini L.

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    The distribution of the Dartford Warbler in Eastern Tuscany was investigated visiting all types of potentially inhabited shrubland. The species seems to be rather well distributed in the study area, but only on sufficiently extensive (>5 ha) Erica scoparia heathlands. In this habitat it reaches high densities (12.7 singing males/10 ha) in good years, but, after cold spells, the population crashes dramatically. These heathlands are disappearing, so this important part of the species’ Tuscan population is vulnerable.

  6. Estimating fresh mass of small mammals in owl diet from cranial measurements in pellets remains

    Canova L., Yingmei Z. & Fasola M.

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    Owl diet is commonly studied through pellet analysis, and it is usually described as prey frequency or biomass. However, frequency does not indicate the relative trophic importance of prey. Biomass estimates, based on mean mass data from the general literature, do not take into account size, sex and geographical variations. We suggest a regression method for better estimating individual mass from cranial bones and mandibles of small mammals, and we provide regressions for 6 species, frequently preyed upon by owls in Italy. For each prey species, at least one variable explained a significant proportion of sample variation and was significantly related to the mass. The bone length-fresh mass relationship is useful only when well-preserved bones are available. Usually, cranial bones in pellets are severely damaged and the number of feasible measurements is limited, therefore multiple regression methods based on several variables are of limited use.

  7. Vigilance and flocking behaviour of tactilely foraging Dunlins Calidris alpina

    Desholm M., Wegeberg A. M. & Mouritsen K.N.

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    Field observations of the relationship between vigilance, foraging, and flocking behaviour in Dunlins Calidris alpina were carried out under optimal field conditions. The results show that central Dunlins within larger flocks devote less time to vigilant behaviour and more time to feeding (higher probing rate) than more peripheral conspecifics. The most coherent explanation for the difference in vigilance level between peripherally and centrally foraging Dunlins is the individual birds´ perception of a higher predation risk when exposed. It is argued that vigilance for predators in Dunlins constrains foraging activity rather than vice versa. Hence, the adaptive significance of flocking in Dunlins may be an increased feeding activity without jeopardising predator surveillance.

  8. Mean range size of the species, bird richness and ecogeographical factors: data from Italian Peninsula and Islands

    Battisti C. & Contoli L.

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    slands are, generally, inhabited by widespread and generalist bird species. This phenomenon, well known in literature and here extended to a peninsular gradient, is confirmed in Italy for breeding landbirds, and for the sedentary ones, elaborating data of the Atlases. The mean range size values of the species are higher in all the studied islands compared to the mainland. The lesser the island area the higher is the mean range size of the species found. Stenotopic species, rare and localised, are present with lower frequency in small islands, the contrary occurs for widespread species. Along the Italian peninsula a high inverse correlation between mean range of distribution of the species and a number of variables of the latitudinal transects (number of species, highest altitude, area, latitude) has been found. This may be explained, mainly, by the different orophysiographical and environmental availability which affect, at the same time, the bird richness and, in particular, the distribution of the rare and localised species.

  9. Population density and demographic trend of the Scops Owl Otus scops in the Northern Apennine (Oltrepò Pavese, Northern Italy)

    Sacchi R., Perani E. & Galeotti P.

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    From 1992 to 1994 European Scops Owls (Otus scops) were censused in the Oltrepò Pavese, a hilly area (448 Km2 in extent) south of Pavia (Northern Italy), using the playback of tape recorded songs. In the first year 37 territories were found for an average population density of 0.08 terr./Km2 (8 territories over 100 Km2). However, Scops Owls were not regularly distributed throughout the area, but they concentrated in the town of Voghera and in the central part of the hilly area. In 1993 we observed a dramatic decline of the Scops Owl population since 21.4% of territories were not re-occupied. The decline was confirmed in 1994, when 3 other territories were lost for a total decrease of 32.1% relative to the first year of the study. The intensification of agricultural practices (namely vineyards) and the intensive use of pesticides were probably the main factors responsible for this decline.

  10. Autumn migration of raptors on Capri

    Jonzén N. and Pettersson J.

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    In this paper we present the first documentation of raptor migration on Capri, Italy. During the autumns of 1994 and 1995 daily observations were made. The two most common species were HONEY BUZZARD (Pernis apivorus) and MARSH HARRIER (Circus aeruginosus), but the total number was low compared to the Strait of Gibraltar and the Bosphorus. The high numbers from Gibraltar is, however, the result of full-scale counts across the entire width of the Strait. Hence, the numbers are not comparable. A large part of the HONEY BUZZARDS and MARSH HARRIERS migrated in late September – early October, when observations are scarce or lacking at the Strait of Gibraltar and the Bosphorus. We suggest that these late migrating HONEY BUZZARDS and MARSH HARRIERS have a more eastern origin compared to the birds observed earlier in the season.

  11. Predation on geckos (Gekkonidae) by urban Tawny Owls (Strix aluco)

    Manganaro A., Salvati L., Ranazzi L. and Fattorini S.

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    Il regime alimentare dell’allocco (Strix aluco) è stato studiato attraverso la raccolta di borre in 11 territori situati in giardini, parchi e boschi nell’ambiente urbano di Roma. Su 3750 prede, i gechi sono stati catturati regolarmente in 9 territori (82%) e costituiscono una componente importante nella dieta del rapace in due territori (contribuendo rispettivamente per il 17,5% e per il 7,2% in numero). La predazione sui gechi è stata riscontrata tutto l’anno, ma aumenta sensibilmente durante il periodo riproduttivo.

  12. Book reviews

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