Symposium Participants
Victor Cueto
Grupo de Investigación en Ecología de Comunidades de Desierto
(Desert Community Ecology Research Group)
Departamento de Ecología, Genética y Evolución
Facultad de Ciencias Exactas y Naturales
Universidad de Buenos Aires
Piso 4, Pab 2, Ciudad Universitaria
C1428EHA Buenos Aires, Argentina
South American Migrant Land Birds: Population Trends and Habitat use in the Central Monte Desert
Víctor R. Cueto and Javier Lopez de Casenave
Migrant land birds of South America could be grouped in two migratory systems (Fig. 1). One group is conformed by birds that breed in temperate zones of South America, and spend the austral winter in the warm humid regions of the continent (e.g., several species of tyrants, swallows, and thrushes). This group is denominated as South American Temperate-Tropical Migratory System (hereafter SATT). The other group is composed by birds that breed in cool-temperate zones of south and central Argentina, and spend the austral winter in northern Argentina, southern Bolivia, Brazil and Paraguay (e.g., several species of tyrants, mockingbirds, seed-eaters, and finches). This group is called as South American Cool-Temperate Migration System (hereafter SACT).
Our research was aimed to study year-to-year density fluctuations and habitat use of migrant land birds in the Biosphere Reserve of Ñacuñán ( Mendoza Province, Argentina). Ñacuñán is located in the central portion of the Monte desert (Fig. 2). This biome is characterised by shrub lands of creosotebush (Larrea spp.), and in several areas there are open woodlands of mesquite tree (Prosopis spp.). In arid and semiarid environments the main factor that regulates ecosystem’s productivity is water availability, and therefore it can indirectly affect the abundance of resources for bird populations. For this reason we also analysed the relationship between the fluctuations in the migratory land bird abundance and previous annual rainfall in Ñacuñán.
During 10 years (1985-1988 and 1993-1998) we were sampling breeding bird abundance with the strip-transect method in the two main habitat types of Ñacuñán reserve: Algarrobal and Jarillal. The first habitat type is an open woodland of Prosopis flexuosa (a thorny tree), with high cover of thorny tall shrubs (Condaliamicrophyla , Capparisatamisquea) and non-thorny tall shrubs (Larreadivaricata) (Fig. 3). The other habitat type is a shrub land dominated by non-thorny tall shrubs (Larrea cuneifolia)(Fig. 4).
Total density of SATT in the Algarrobal was higher than in the Jarillal, but it did not differs among years, although between 1993 and 1995 an increase in the density of this group was observed. SATT was composed by 5 species of Tyrannidae, although dominated by two species, Griseotyrannus aurantioatrocristatus and Elaenia albiceps (34 % and 58 % of the long-term average density, respectively). The other species had lower abundances and sporadic occurrence in the reserve. SACT density also differs between habitats, being higher in the Algarrobal than in the Jarillal, but this group showed significant density fluctuations among years. We recorded lower densities during 1994 in both habitats. In the subsequent years the density of SACT increased, but it did not reach the values of previous years. SACT was composed by 7 species from different families (Furnariidae (1), Tyrannidae (2), Mimidae (1), Phytotomidae (1) and Emberizidae (1)). The most abundant species of this group were Serpophaga griseiceps and Poospiza ornata (43 % and 33 % of the long-term average density, respectively). The other species were less abundant (Mimus triurus 7 %, Synallaxis albencens 6%, Phytotoma rutila 6% and Troglodytes aedon 5 %), or had a very low abundance and a sporadic occurrence (Xolmis coronata less than 1 %).
We considered as wet or dry years to years with values of annual rainfall over or below 1 standard deviation from the average annual rainfall of the 30 year period of climatic data on Ñacuñán Reserve. Thus during this study there were two wet years (1984, 1985) and four dry years (1986, 1988, 1993, 1994). Also, note that two of them were consecutive years of wet or dry conditions. Correlation analyses indicated the SATT density was not associated with previous annual rainfall. Instead, SACT density was positive correlated with previous annual rainfall.
Our results indicate that year-to-year density variations in land bird migrants from the SACT could be responding to local conditions in the breeding areas, associated with long drought periods (at least a 2-years duration). In the case of SATT, density variations could be responding to changes in environmental conditions in the wintering areas or in route.
When we analysed bird habitat use, the species from both migration groups showed the same pattern, being more abundant in the Algarrobal than in the Jarillal habitat. The structural complexity of the Algarrobal could explain the differential habitat use observed. This habitat offers a higher abundance of foraging microhabitats and nest locations than the Jarillal in Ñacuñán.
Main human land use in the Monte desert comprises cattle raising and logging trees and shrubs for firewood and charcoal production (mainly Prosopis spp. trees). Considering the habitat use of land bird migrants and that human activities bring about a modification in both habitat structure and plant composition, these bird species could be facing a complex scenario for conservation.
