109 Senckenbergiana biologica | 86 | 1 | 109­–125 | 4 figs., 2 tabs. | Frankfurt am Main, 15. vI. 2006 Population numbers and status of land birds of the Juan Fernández Archipelago, Chile (Aves: Falconiformes, Columbiformes, Strigiformes, Caprimulgiformes, Passeriformes) INGO HAHN, UWE RÖMER & ROBERTO P. SCHLATTER Abstract Results of a recent land bird population survey of the Juan Fernandez Archipelago are pre- sented. To additionally gain indications for possible tendencies, two censuses with a seven year interval were carried out (19­9­4/9­5 and 2001/02). During the 111 line transect counts 249­6 bird records were made, covering the eleven resident species and the three islands (Robinson Crusoe, Alejandro Selkirk, Santa Clara). On base of this total bird numbers were calculated, including ad- ditional data of distribution and ecology, for each island, habitat type, and species. For the entire archipelago (9­380 ha) in 2001/02 a total of 15 175 land bird specimens is calculated (1.62 ind./ha). Non-endemics were more abundant than endemics (63:37%). Robinson Crusoe shows the highest overall land bird density (2.56 ind./ha), however, mainly basing on non-endemics (70%). Contra- rily Alejandro Selkirk hosts much lower individual numbers (0.65/ha), but endemics take a higher proportion on the overall land bird community (65%). Three of the endemic taxa possess very low total population sizes: Falco sparverius fernandensis, Sephanoides fernandensis, and especially Aphrastura masafuerae with only about 140 individuals. Although differences between the two study periods generally may be related to natural fluctuations, the populations of the mentioned taxa probably have suffered strong decline since human impact began in 1574. Alteration of habi- tats, introduction of predators, and immigration of competitive birds seem to be the main threats for these endemics. Basing on theseeschweizerbartxxx sng-results, the archipelago represents not only the most important endemic bird area of Chile and in the south-east Pacific Ocean, but also the most threatened one including several endangered endemics. Conservation management should include monitoring of the bird populations, basic research of their ecology (especially reproduction), and removal of introduced (plant and animal) species. Eradication campaigns should start with goats on Alejandro Selkirk, rabbits on Santa Clara, and cats in Robinson Crusoe’s settlement. K e y w o r d s : Alejandro Selkirk, biosphere reserve, island birds, bird census, bird habitats, endangered species, national park, nature conservation, Robinson Crusoe, transect method. Números poblacionales y estatus de las aves terrestres del Archipiélago de Juan Fernández, Chile (Aves: Falconiformes, Columbiformes, Strigiformes, Caprimulgiformes, Passeriformes) R e s u m e n : Se dan a presentar resultados de un estudio poblacional de las aves terrestres del Archipiélago de Juan Fernández. Para adicionalmente ganar indicaciones de tendencias, dos censos han sido realizado con un intervalo de siete anos (19­9­4/9­5 y 2001/2002). 249­6 contactos de aves han sido registrado en las 111 transectos lineares, cubriendo las once especies residentes y las tres islas (Robinsón Crusoe, Alejandro Selkirk, Santa Clara). Con esa base números totales de aves han sido calculado, incluyendo datos adicionales de distribución e ecología, para cada isla, tipo de hábitat, e especie. Para el archipiélago entero (9­380 ha) en 2001/02 un total de 15 175 especimenes de aves Dr. Ingo HAHN, Departamento de Ecología, Pontificia Univ. Católica de Chile, Casilla 114-D, Santiago, Chile. — Current address: Institute of Landscape Ecology, Robert-Koch-Strasse 26, D-48149­ Münster, Germany; [email protected] Dr. Uwe RÖMER, Faculty of Biology, Universität Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany; [email protected] Prof. Dr. Roberto P. SCHLATTER, Instituto de Zoología “Ernst F. Kilian”, Facultad de Ciencias, Universidad Austral de Chile, Ca- silla 567, Valdivia, Chile; [email protected] © E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller), 2006, ISSN 0037-2102 110 terrestres han sido calculado (1,62 ind./ha). Non-endémicos estaban mas abundante que endémicos (63:37%). Robinsón Crusoe tiene la densidad total mas alta de aves terrestres (2,56 ind./ha), pero mayoramente por base de non-endémicos (70%). En contraposición Alejandro Selkirk hospedas números de individuos mucho mas bajos (0,65/ha), pero endémicos toman una porción mas grande de la comunidad total de aves terrestres (65%). Tres de los taxa endémicos tienen poblaciones totales muy bajas: Falco sparverius fernandensis, Sephanoides fernandensis, y especialmente Aphrastura masafuerae con solo 140 individuos. Aunque las diferencias de los dos periodos de investigación posiblemente están en relación con fluctuaciones naturales, las poblaciones de las tres formas mencionadas probablemente han sido sufriendo disminuciones fuertes desde el inicio del impacto humano en 1574. Alteración de hábitat, introducción de predadores, e invasión de aves competitivos probablemente representan los grandes peligros para esos endémicos. Basando en esos resultados, el Archipiélago de Juan Fernández no solamente representa el área mas impor- tante para aves endémicos en Chile y en el sureste del Océano Pacifico, pero también el área mas en peligro incluyendo varias especies endémicas. Manejo de conservación debe incluir monitorio de las poblaciones des aves, estudios básicos de su ecología (especialmente reproducción), y la erradicación de especies introducidos (de plantas e animales). Programas de erradicación deben enseñar con cabras en Alejandro Selkirk, conejos en Santa Clara, y gatos en la población humana de Robinsón Crusoe. Introduction Although islands cover only about 2% of the terres- special conservation threats. Indeed, all endemic land bird trial surface, they host an important part of the global taxa of the archipelago are principally threatened (GLADE biodiversity: for instance 14% of all mammal species 1993): the superficies is very limited, massive habitat al- (ALCOVER et al. 19­9­8) and more than 30% of several bird teration has been detected, and preliminary observations families (CLEMENTS 2000). Islands became known as indicate low abundances. For single species a high extinc- speciation centres and, in geographical isolation, large tion risk already has been expected (BROOKE 1987, BOurNE numbers of endemic species evolved (JOHNSON eschweizerbartxxx& sng- STAT- et al. 19­9­2, HAHN & RÖMER 1996, ROY et al. 19­9­9­). TERSFIELD 19­9­0). Birds are especially well represented on However, still little is known about the populations islands, as they are favoured to reach them through their of the islands’ avifauna. One species, the Juan Fernan- ability to fly. dez Firecrown (Sephanoides fernandensis (KING 1830), However, naturally most islands are of small size, of- Caprimulgiformes, Trochilidae), already has been stud- fering only very limited potential for distribution, and do ied and counted (although quite some time ago). Most not provide many retreat areas for their bird species. But land bird species only have been estimated roughly with- it was theoretically speculated that island birds generally out method, others never been in the focus of quantita- possess higher population densities than their mainland tive registration at all. But the basis for any successful pendants, mostly because species richness was detected conservation management is the knowledge of reliable to be lower (MACARThur & WILSON 19­67, MACARThur population numbers. Thus, census covering as many et al. 19­72). Practically it turned out that human impact individuals as possible of the total island population has highly severe consequences for island ecosystems: becomes necessary. For comparing and detecting spe- many of the autochthonous island populations of previ- cies of special conservation concern, generally all land ously abundant bird species strongly declined and now bird species need to be included into census programs. are threatened (COLLAR et al. 19­9­4, STATTErsFIELD & CAP- This would also lead to results of basic scientific interest PER 2000) or have already become extinct through this which, in turn, might be of significance for practical con- influence (DIAMOND 19­82, KING 19­80, 19­85). About two servation action. For example it is not known, to which thirds of all recently threatened bird species live on is- extent autochthonous and allochthonous habitat types lands and amongst 217 extinct bird taxa worldwide, 200 are inhabited by endemic birds on the one hand, and by had been restricted to islands (IMBODEN 19­85). non-endemics on the other hand. Differences in regard In Chile about 50% of the endemic avifauna is re- to entire islands, island regions, or habitat types may be stricted to a number of small islands (especially Juan detected. On the Juan Fernandez Islands this might bring Fernandez), which cover less than 1% of the county’s insight in differentiation of the two main islands, Rob- surface (ARAYA et al. 19­9­5, compare also STATTErsFIELD et inson Crusoe and Alejandro Selkirk, which show about al. 19­9­8). Thus, these islands are of overwhelming impor- the same superficies and geographical latitude, but are tance for the country’s endemic avifauna, but might face separated by a sea distance of ca. 170 km. 111 Through comparative repetition after several years, alpine vegetation is found. The islands belong to the few especially if including identical investigators,