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SAB 019 1999 P250-257 the Decline of the Pampas Meadowlark

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SAB 019 1999 P250-257 the Decline of the Pampas Meadowlark Studies in Avian Biology No. 19:2.50-257, 1999. THE DECLINE OF THE PAMPAS MEADOWLARK: DIFFICULTIES OF APPLYING THE IUCN CRITERIA TO NEOTROPICAL GRASSLAND BIRDS PABLO LUIS TUBARO AND FABIAN MARCELO GABELLI Abstract. The current status of the Pampas Meadowlark (StumeZZudejilippii) was analyzed in terms of the new criteria adopted by the World Conservation Union. Estimated values of the species ’ Extent of Occurrence and Area of Occupancy were 8,000 square kilometers and 150 square kilometers, respectively. The total population size of the species was estimated to be approximately 7,500 adults. Considering its Extent of Occurrence and population size, this species qualified as Vulnerable by World Conservation Union criteria, but because of its small Area of Occupancy and possible fluctu- ations in its preferred habitat-natural grasslands-the species should be placed in the Endangered category. LA DECLINACI6N DE LA LOICA PAMPEANA: DIFICULTADES PARA LA APLICACI6N DE LOS CRITERIOS DE LA UICN A LAS AVES NEOTROPICALES DE PASTIZAL Sinopsis. Se analizo el estatus actual de la Loica Pampeana (Sturnelh defilippii) en funcion de 10s nuevos criterios adoptados por la UICN. Los valores estimados para la Extencion de Ocurrencia y el Area de Ocupacion son 8.000 km* y 150 km2, respectivamente. El tamaiio total de la poblacidn fue estimado en 7.500 adultos. Por su Extencidn de Ocurrencia y tamafio poblacional la especie califica coma Vulnerable, pero considerando su pequefia Area de Ocupacion y posibles fluctuaciones en el habitat preferido (pastizales naturales) la especie debe ser ubicada en la categorfa En Peligro. Key Words: Argentina; grasslands; IUCN; pampas; Pampas Meadowlark; Sturnelh dejilippii; World Conservation Union. Neotropical grasslands in South America are Formerly distributed from southern Brazil and widespread and include a variety of types. For Uruguay to central Argentina, the Pampas example, high-altitude grasslands can be found Meadowlark has declined since the beginning of in the Andes, Sierras Pampeanas, and Puna pla- the twentieth century. In this paper we describe teau. Low-altitude grasslands occur in the Ve- the species ’ present status. We also discuss the nezuelan llanos, pampas of Argentina, and cam- applicability of the new criteria of the World pos of Uruguay and southern Brazil (Cabrera Conservation Union (IUCN) for assigning threat and Willink 1980). Lowland grasslands such as status (Mace and Stuart 1994) to neotropical the Pampas have experienced extensive pressure grassland species in general and to the Pampas from agriculture and cattle-farming (Soriano et Meadowlark in particular. al. 1991), and concern about the conservation of Argentine grassland birds is growing (Bucher METHODS and Nores 1988). This study is based on field work done in Argentina The Pampas Meadowlark (Stumella dejilippii) in November 1992 and 1993 and in January, May, and is typical of many bird species living in lowland December 1996 in the southwest of the province of grasslands. This is a highly dimorphic species, Buenos Aires and adjacent areas of the province of La especially during the austral spring and summer Pampa. when mature males exhibit a red throat and Field work took place on an 11,800-km route on breast contrasting with a dark body. Females are primary and secondary roads. We looked for Pampas much duller, with a reduced, paler red breast Meadowlarks from a vehicle moving at low speed (< area. Pampas Meadowlarks are gregarious, even 50 kmihr) and with windows down. Pampas Mead- during the breeding season, which typically ex- owlarks were easily detected because males have a tends from October to December. Males ’ terri- conspicuous flight display and flight song. We made numerous stops along the route, as many as one every tories are small and tightly packed in areas of 0.5-1.5 km, in areas where previous information or suitable habitat, forming what we call reproduc- research suggested the species might be present, as tive groups. The nest consists of an open cup well as at any site judged to be appropriate habitat. built on the ground and concealed under grasses. The minimum geographical area (hereafter referred In January, at the end of the breeding season, to as Extent of Occurrence, or EO; Mace and Stuart individuals congregate in large flocks, hereafter 1994) covered by the Pampas Meadowlark was esti- referred to as nonreproductive groups, until the mated using a grid of 100 km2 superimposed on a map following spring. (Mercator projection) with the location of reproductive 250 PAMPAS MEADOWLARK DECLINE--T&mm and Gab& 251 groups found during field work. We drew a minimum them S. ambigua, S. trichotoma, S. neesiana, S. cla- convex polygon, which included the outermost points razi, P. montevidense, and P. stipoides. We also found of distribution of reproductive groups, and counted the Bromus unioloides and Hordeum parodii. The struc- number of squares included inside the polygon. The ture of natural grasses had a typical “tussock” physi- number of squares multiplied by 100 yielded the EO ognomy. Planted pastures and croplands typically in- in square kilometers. The Area of Occupancy, or A0 cluded Avena barbata, A. sariva, Bromus mollis, Lo- (the fraction of the EO effectively occupied by the lium multiflorum, alfalfa (Medicago sativa), Tvifolium species; Mace and Stuart 1994), was calculated as the repens, Vicia angustifolia, V. sativu, and wheat (Trit- sum of the areas of the sites where the species was icum aestivum) as dominant species. The main differ- found or estimated to be present. ence between planted pastures and croplands is that We used additional sources of information to deter- planted pastures are not cropped at the end of the an- mine the species’ original distribution and the timing nual growing cycle but remain in place for several of its range contraction. These included ornithological years. collections at the Museo Argentino de Ciencias Natu- We used a graduated ruler to assess the height of rales “Bernardino Rivadavia” in Buenos Aires and at the vegetation at 25 sample points inside each habitat the National Museum of Natural History, Smithsonian type. These samples were located along a transect Institution, Washington, D.C; the database of the grass- crossing the area and were separated by 10 m. At each land birds of eastern Argentina, Asociaci6n OmitoK- sample point we measured the height of the vegetation gica de1 Plata (R. Fraga, pers. comm.); and a biblio- touching the ruler to the nearest centimeter. graphic survey, including the complete collection of We compared vegetation height between habitat the journal EZ Hornero from 1918 to 1995. The Inter- types using the Student t-test. Paired t-tests on vege- national Council for Bird Preservation (ICBP)/IUCN tation height were performed between preferred sites book Threatened Birds of the Americas (Collar et al. that were occupied by Pampas Meadowlarks and ad- 1992) also provided useful information. jacent unoccupied sites, or when we compared the In areas where we detected Pampas Meadowlarks same sites at different times. All statistical tests were during field work, we conducted a point count using a two-tailed. visual scanning method (Lehner 1979, Bibby et al. RESULTS 1992) and estimated the area used by the reproductive group. The frequency of vocal and flight displays in- RANGE CONTRACTION creased with the size of the reproductive group (Goch- The former range of the Pampas Meadowlark feld 1978), so small groups were less detectable, or included most of the Pampas grasslands from appeared smaller in size than they were. the southern part of Brazil (Rio Grande do Sul, To assess the relationship between the visual scan- Santa Catarina, and Paran states), Uruguay (de- ning count and the real size of a reproductive group, partments of Canelones, Paysand& Cerro Largo, at four sites we made exhaustive searches coupled with song recordings and territory mapping. These searches Flores, San Jo&, Tacuarembb, and Montevideo), revealed that our scanning method underestimated the and eastern and central Argentina (including the size of reproductive groups by about 50%. We there- provinces of Entre Rios, Buenos Aires, Santa Fe, fore applied a correction factor, which consisted of Cbrdoba, San Luis, and La Pampa; Fig. 1). doubling the number of counted birds, to estimate the Since 1900 the distribution of this species has number of individuals per site. decreased by 90%. It is now virtually extinct in The procedure for estimating the total Pampas Brazil, Uruguay, and most of the northern and Meadowlark population size was as follows. First we central Argentine pampas (Fig. 1). Pampas divided the EO into four subareas (Villa Iris, Chasicb, Meadowlarks were very common near the city Felipe Sol& and Napostb) based on maps (scale 1: of Buenos Aires until 1925, but now they are 100,000) from the Instituto Geogr&ico Militar. Inside each subarea we counted the number of potential sites found almost exclusively on the margins of their (NPS) judged to be suitable habitat for Pampas Mead- former distribution, mainly in the southwestern owlarks. We then multiplied the NPS by the proportion part of the province of Buenos Aires. of sites occupied by the species and by the mean num- Although the range contraction of this species ber of Pampas Meadowlarks per site. has been rapid and extensive, the pace of the For the study of habitat preferences, we considered contraction has not been steady; most of the lo- three types of habitats differing in species composition cal extinction occurred between 1890 and 1950, and structure: natural grasslands, planted pastures, and coincident with the period of extensive agricul- croplands. Planted pastures and croplands revert to nat- tural transformation of natural grasslands (Soria- ural grasslands when abandoned for periods ranging no et al. 1991). Since 1950, Pampas Meadow- from 5 to 2 15 yr. It is important to note that natural grasslands are not natural in a strict sense because all larks have been seen regularly in the marginal of them have been cultivated or grazed at some point areas of the southwestern Pampas (Short 1968; (Soriano et al.
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