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Annual variation and breeding success of a threatened insular population of Common Raven Corvus corax (Tenerife, Canary Islands)
Abstract and Figures
During 1996 and 2006 we studied the population size and breeding success of Common Raven in Teno massif, where the majority of Tenerife reproductive pairs reside. Breeding population size remained more or less stable during the study period, varying between four (2001) and nine (2005–2006) pairs. Fledging rate (fledged chicks/successful nests) in 2006 was 2.63 and eight of the nine pairs bred successfully (89% of the nests producing surviving offspring). During 1996–2005, the mean fledging rate of 36 broods randomly selected was 2.72. Implementation of a food supply program to increase the survival rate of non reproductive birds is needed urgently along with other conservation measures.
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... Breeding population size was estimated through intensive territory mapping during 2016 (from 15 March to 8 July), covering the entire breeding season from courtship to fledging (Nogales 1995;Siverio et al. 2007). Based on our previous field experience on Tenerife (i.e. ...
... Based on our previous field experience on Tenerife (i.e. Rodríguez et al. , 2010Rodríguez et al. , 2019Siverio et al. 2007Siverio et al. , 2010, we selected 86 observation sites situated at vantage points distributed across the island (Figure 1) to check known breeding territories, or search for potential nesting sites (suitable cliffs and woodland areas) using binoculars and field scopes. A total of 138.6 hours of direct observation were performed (sessions ranging from 15 to 390 min). ...
... We therefore approached the area to confirm whether it was indeed a nesting site. These were identified according to the following evidence: a) detection of occupied nests, b) observations of adults transporting nest material or food for chicks to a particular site, c) hearing begging calls from non-fledged young, and/or d) observation of recently fledged broods (see Siverio et al. 2007 for a similar procedure). The locations of breeding territories were georeferenced to the nest when known, or the most suitable site judged capable of holding a nest. ...
Until the middle of the 20th century, the Common Raven Corvus corax was an abundant bird in the Canary Islands. However, during recent decades, populations have decreased dramatically in the most densely populated islands, apparently because of changes in land use and other anthropic activities. In the current study, we estimated raven population size and described its breeding distribution on Tenerife, the largest and most populated island. Non-breeding birds were roughly estimated in autumn 2015, then breeding territories were mapped using intensive field research in 2016. The non-breeding population was composed of a minimum of 50 birds, but the real figure could be approximately 65. In total, 28 breeding pairs were located (1.4 pairs 100 km−2), mostly (82.1%) in the Teno massif (28.5 pairs 100 km−2). Non-breeding and breeding populations increased from 2009 to 2016 by 7.6% and 12.8%, respectively. Because no specific conservation actions had been carried out, nor apparent change in threat
regime detected, the reasons for these increasing numbers are unclear. Although casual observations suggested that by 2019 it had continued growing to approximately 37 breeding pairs, quantitative studies on human-related mortality factors are urgently required to guarantee the conservation of this still fragile population.
... Buzzards' data correspond to the 2007 season (Rodríguez et al. 2010b), and for the most abundant species, the common kestrels, nest locations were obtained during 2005-2010 given that the most remote and rugged sectors were surveyed at least 1 year to cover all available habitats for this species. We are aware that raptor density may strongly fluctuate among years due to variability in climate or feeding resources (Newton 1979), but prey availability and climatic stability in the Canary Islands produce no large annual variation in raptor breeding densities (see Siverio 2006;Siverio et al. 2007;Rodríguez et al. 2010b;Siverio et al. 2010a). We may have missed a few nest sites, especially of the abundant kestrels, but we are confident that such omissions represent less than 5% of total nesting sites. ...
... Our results highlight the high conservation value of Teno for birds of prey. It is a very important stronghold for the threatened studied raptors: 1) the unique ospreys breeding pairs of the island, which constitute more than 30% of the Canarian breeding population, are bound to the Teno coastal cliffs (Rodríguez et al. 2013); 2) the first Barbary falcon breeding pairs of Tenerife were discovered on Teno in the early 1990s, since then the insular population has spread through the island reaching more than 35 pairs at present (31% of them breeding in Teno; Siverio et al. 2009Siverio et al. , 2010a; 3) the common raven was formerly distributed through the island, but during the last four decades it has suffered a sharp decline and the bulk of the breeding insular population survived restricted to Teno (Lorenzo 2007;Siverio et al. 2007). In addition, many endemic plants and invertebrates occur there (Reyes-Betancort et al. 2008;Martín 2010), and some vertebrates maintain their more important or unique insular breeding populations there, such as for example the Canarian spotted lizard Gallotia intermedia, the Manx shearwater Puffinus puffinus, or the rock sparrow Petronia petronia (Rodríguez et al. 2014). ...
The specific spatial distribution and habitat association -strongly influenced by environmental factors or competitive interactions- are major issues in ecology and conservation. We located and georeferenced nesting sites of five cliff-nesting raptors (Egyptian vulture Neophron percnopterus, common buzzard Buteo buteo, osprey Pandion haliaetus, common kestrel Falco tinnunculus, Barbary falcon Falco peregrinus pelegrinoides), and common raven Corvus corax on one of the most biodiverse hotspot within the Canary Islands (Teno, Tenerife). We used generalised linear models to evaluate the factors affecting abundance, richness and intra- and interspecific interactions. Raptor abundance increased with slope, shrub-covered area, and habitat diversity, and decreased with altitude, and forested and grassed areas. Richness increased with slope and decreased with altitude. Threatened species (osprey, Barbary falcon and raven) occupied cliffs farther away from houses and roads, and more rugged areas than the non-threatened species. The models suggested that the probability of cliff occupation by buzzards, falcons and ravens depended only on inter-specific interactions. Buzzard occupation increased with the distance to the nearest raven and kestrel nests, whereas falcons and ravens seek proximity to each other. Teno holds between 75-100% of the insular breeding populations of the most endangered species (osprey and raven), indicating the high conservation value of this area. Our study suggests that the preservation of rugged terrains and areas of low human pressure are key factors for raptor conservation and provide basic knowledge on the community structure and habitat associations to develop appropriated management actions for these fragile island populations.
... In general, the density and breeding parameters of many subspecies of Peregrine Falcon have been studied in depth (Mearns and Newton 1988, Mendelsohn 1988, Ratcliffe 1993, Zuberogoitia et al. 2002, Rizzolli et al. 2005, Verdejo and López-López 2008). With the exception of the Canaries, where their biology and ecology have received some attention (Delgado et al. 1999, Rodríguez and Siverio 2006, Rodríguez et al. 2007), the acquired knowledge of the Barbary Falcon throughout its distribution range is still scant (Ferguson-Lees and Christie 2001). In this regard, many of the data available regarding Morocco, its area of occupation closest to the Canaries, are confusing since they could also pertain to other sympatric races of Peregrine Falcon (Thévenot et al. 2003). ...
... In this study, the importance of Teno is already noteworthy (given the high availability of suitable cliffs and other factors) as being one of the areas of the island with a greater density of Barbary Falcon. Thus, the availability of these cliffs favours the abundance of falcons and other rupicolous species, both here (Rodríguez et al. 2007, Siverio et al. 2007, Rodríguez et al. 2010, present study) and in continental environments (Newton 1988, Gainzarain et al. 2000, Jenkins and van Zyl 2005, Rizzolli et al. 2005). On the other hand, annual differences in distances between neighbouring nests detected in Teno are because they were greater when there were fewer pairs at the beginning of the study. ...
Territory spacing and breeding rates of an insular population (north-western Tenerife, Canary Islands) of Barbary Falcon Falco peregrinus pelegrinoides was studied from 1993 to 2008. The population increased constantly since the outset, from two pairs in 1993 to 12 in 2008. Mean density was 5.48 pairs per 100 km and mean nearest neighbour distance was 3 119 m. The regularity of the spatial distribution pattern of the nests, observed in most years, may be maintained in the future despite the expectation that new pairs may occupy still-vacant territories. Considering the 79 breeding attempts analysed, the mean number of fledged young per territorial pair was 1.92, per laying pair was 2.0 (n = 76), and per successful pair was 2.24 (n = 68). No significant variations were observed between the annual mean number of fledged young per laying pair, nor between the number of fledged young of pairs according to density in a 5 km radius. All fledglings (brood size one to four) left the nest in the month of May. In order to avoid affecting breeding success, sporting activities practised in the breeding areas must be correctly managed by the appropriate authorities.
Este libro pretende contribuir a rellenar el vacío existente en la literatura científico-divulgativa acerca de los bosques termófilos de Canarias, sin duda el ecosistema del archipiélago peor conservado y menos conocido para el gran público. Magníficamente ilustrada, la obra comienza con una introducción a la distribución, riqueza, origen, historia y estado actual de los bosques termófilos, para ir profundizando posteriormente en las diferentes formaciones (sabinares, acebuchales, palmerales, etc.) que lo integran. Finalmente, se ofrece un capítulo sobre la avifauna de estas formaciones y se aborda la restauración ecológica llevada a cabo en el macizo de Teno (Tenerife), objetivo del proyecto LIFE Hábitat, que ha posibilitado este libro.
The Teno range of mountains, located in the northwest Tenerife, Canary Islands, is still a refuge for the flora and fauna of the island. Until now, at least four native species of reptiles, 232 of birds (both breeding and migrant), and six mammals have been recorded for that site. For some of these animals, such as Canarian Spotted Lizard (Gallotia intermedia), Manx Shearwater (Puffinus puffinus), Osprey (Pandion haliaetus), Rock Sparrow (Petronia petronia) and Northern Raven (Corvus corax), these cliffs, gorges, tablelands or forests hold a good part or all of their insular populations. This illustrated and annotated checklist synthesizes and updates the available information on terrestrial vertebrates for the entire northwestern area of Tenerife. A current and comprehensive list of extant species supports effective management strategies and conservation projects as well as the efficient dissemination of information regarding this Natural Heritage. More info at http://www.gohnic.org/noticias/nuevas-publicaciones-gohnic/
A popular article on an important site for local biodiversity, not protected by law, situated in the south of Tenerife Island (Canaries).
During the 2009 breeding season (February-June) we studied the population size of Common Raven on four islands of the Canarian archipelago. A total of 44 pairs were censused: ten in Gran Canaria, 12 in
Tenerife, 17 in La Palma and five in La Gomera. Most of these pairs have their territories in ravines, gullies and sea cliffs, coinciding with or close to areas where livestock is still being raised (herds of goats and sheep) or presence of feral goats. The size of the floating population is unlikely to exceed that of the breeding birds. We also discuss real and potential threats.
1) et Patrick BERGIER (2) (1) 353 chemin des Mendrous – 34170 Castelnau-le-Lez (France) michelthevenot@wanadoo.fr (2) Go-South -4 Avenue Folco de Baroncelli – 13210 Saint Rémy de Provence (France) www.go-south.org pbergier@yahoo.fr Disponible en ligne (Available online) : 13 septembre 2008 Cette nouvelle livraison de nos 'Eléments de bibliographie ornithologique marocaine' regroupe une sélection d'articles traitant de l'avifaune du Maroc. Parmi ceux-ci, nous avons distingué, comme dans la livraison précédente, ceux traitant spécifiquement de ce pays de ceux de portée plus générale mais concernant aussi le Maroc. Un bref résumé informatif suit certains d'entre eux dont le titre n'exprime pas de façon évidente le lien avec le Maroc. Un troisième paragraphe présente une sélection de travaux récents relatifs à d'autres pays proches (Espagne et Iles Canaries, Portugal, Algérie, Tunisie et Mauritanie en particulier), en lien direct avec l'avifaune marocaine. Rappelons que la majeure partie de la bibliographie ornithologique marocaine disponible fin 2001 a été référencée dans 'The Birds of Morocco' (Thévenot, Vernon & Bergier 2003. British Ornithologist Union Checklist Series 20). Depuis, la majorité des nouveaux titres apparus jusqu'à fin 2005 ont été listés dans nos 'Eléments de bibliographie marocaine' – 1 (Bergier & Thévenot 2004 – Go-South Bull. 1 : 7-12), 2 (Thévenot & Bergier 2005 – Go-South Bull. 2 : 44-51) et 3 (Thévenot & Bergier 2007 – Go-South Bull. 4 : 32-41). Une 'Bibliographie ornithologique marocaine' est maintenue à jour à la rubrique 'Moroccan Bibliography' du site www.go-south.org. Nous serions reconnaissant à toute personne ayant connaissance de publications récentes non signalées dans nos 'Éléments de bibliographie ornithologique marocaine' de bien vouloir nous en faire part ; de même nous vous remercions d'avance de bien vouloir nous signaler toute erreur ou imprécision qui existerait dans les références présentées.
Several field studies on common ravens Corvus corax were carried out between
1985 and 2004 in Białowieża Primeval Forest (E Poland), one of the best preserved lowland
temperate forests in Europe. The surveys focussed on spatial aspects (nest site selection,
territory size, territory use), reproduction (breeding success) and foraging patterns (food
habits, carrion use). Nests (n = 87) were regularly distributed over the entire forest and located
in coniferous forest. More than 90% of the nests were built in pines Pinus sylvestris. On
average, 61.5% (SD ±15.4) of all breeding pairs were successful, rearing 2.8 (SD ± 0.18)
fledglings per year. The occupied raven territories covered 14.1 km². Inside the territory
boundaries, more than 76% of the exposed baits (n = 57) were used by ravens, even under
dense forest canopy. Adult ravens were highly efficient in locating carrion; 79.2% of the exposed
baits were detected and used by ravens within the first 24 hours after exposition. Ravens
scavenged carcasses (n = 214), mainly of ungulates, intensively during the coldest
months, abruptly decreasing the use of carcasses when spring arrived and other food resources
became available. Diet analysis showed that carrion and plant material were the
dominant food items of ravens in Białowieża Forest. Up to 82% of the analysed pellets contained items obtained from scavenging. Adult ravens had a more diversified diet than
immatures, which consumed more items related to scavenging than to predation. The proportion
of squirrels in the diet of adult ravens was remarkably high. Ravens in Białowieża
Forest showed efficient strategies to exploit the resources available in the highly productive
temperate forests, where they did not depend on artificial food subsidies.
As the Brittany population of Raven has been declining since the nineteen eighties, this species has been studied in order to identify its current population size and the causes of its recent decline. The post-1970 population increase, following conservation measures, was short-lived; as today only 30 pairs breed with ony approximately twenty being successful in any one year. Declining food sources and disturbance caused by badly designed coastal paths seem to be the main cause of this decline.
We examined the dispersal and gregariousness of 10 ravens from one feeding flock through daily and nocturnal monitoring for 70 days and nights over an area of approximately 5,000 km2. One of the birds took a mate and established residency within this area. Another passed through the area on infrequent intervals having an apparently much larger range. Two stayed only a day or less. All but one bird wandered widely, taking up residency lasting for one to several weeks in ranges from 190-3, 100 km2. The 10 birds from the feeding flock did not stay together, although up to two occasionally overlapped at nocturnal roosts. The birds roosted and dispersed independently of one another with no indication of any "flock" cohesiveness. There was a great variation of response from "vagrants" to "residents", with one grading into the other through temporary residences and periods of wandering.
Common ravens (Corvus corax) are human-subsidized scavengers and predators in the Mojave Desert. They have increased dramatically in number and have been implicated as contributors to the decline in desert tortoise (Gopherus agassizii) populations. Known patterns of increased fledging success near human developments suggested that food was the most likely resource subsidy received by ravens. Because ravens are opportunistic foragers with a generalist diet, we predicted that the types of resource subsidy provided by different kinds of human developments should be reflected in measures of diet composition of breeding ravens. We estimated diet composition from contents of raven pellets collected at nests and related diet composition to distance of the nests from roads and point sources of resource subsidies, such as towns or landfills. Ravens that nested close to point subsidies far from major roads had the greatest incidence of trash in their diets. Ravens that nested close to roads but far from point subsidies had a low incidence of trash and a higher incidence of presumably road-killed mammals and reptiles. Ravens far from both roads and point subsidies had more plant material and arthropods, and ravens close to both roads and point subsidies had more birds and amphibians. Diet diversity was not related to distance from roads or developments. Fledging success was correlated with diet composition, such that birds with diets consistent with trash or road-kill subsidies fledged the greatest number of chicks. Our results suggest that ravens forage opportunistically on foods available near their nests, and different kinds of human developments contribute different foods. Improved management of landfills and highway fencing to reduce road-kills may help slow the growth of raven populations in the Mojave.
Nature is the international weekly journal of science: a magazine style journal that publishes full-length research papers in all disciplines of science, as well as News and Views, reviews, news, features, commentaries, web focuses and more, covering all branches of science and how science impacts upon all aspects of society and life.
Data are provided on the density and distribution of an oceanic island population of Ravens (El Hierro, Canary Islands). The density of both the breeding population (ca. 35 pairs/100 km2) and that censused from a vehicle (35 birds/100 km) is the highest recorded in island environments and the second highest in the species' entire distribution area. The high density is probably due to an abundant food supply and numerous suitable nesting sites. Relief and altitude influenced the distribution of breeding territories which showed a high level of constancy from one year to the next.
