Project Report

As planned, vegetation sampling and bird counts were conducted through distance sampling in eight 500 m transects within each habitat type. These were conducted using well established methodologies that have been applied extensively in North American studies, enabling comparative analyses between this project's results and those of other North American researchers.

Although the data have not yet been analyzed in detail, some interesting patterns concerning grassland bird communities within the four habitat types have already been identified.

1. There are a few grassland bird species, such as the grassland sparrow and the white-browed blackbird, that inhabit all four types of grassland and grassland-like habitats (croplands, planted pastures, and two types of natural grasslands, one grazed by cattle and sheep and another grazed mainly by Pampas deer). Their densities within each habitat can differ markedly. The white-browed blackbird is the only studied species whose densities are higher in altered habitats (crops and planted pastures) than in less disturbed habitats (natural grasslands).


2. Some species are present in all types of pastures (planted and natural) but are excluded from crops. The upland sandpiper and the short-billed pipit are good examples of this group. The former is a long-distance migrant whose populations seem to have declined in recent decades in Uruguay. This decline may be explained by the fact that upland sandpiper densities are higher in both types of natural grasslands than in planted pastures.


3. There is a suite of species that have been recorded only in natural grasslands, including tawny-throated dotterel, buff-breasted sandpiper, chocolate-vented monjita, ochre-breasted pipit, and Pampas meadowlark. The future of most of these species most likely depends on the effective conservation of suitable areas of natural grasslands within the Pampas.


4. In cropland areas, a high proportion of the local avian diversity seems to be sustained by the presence of relatively small patches of natural grasslands that are maintained among crop patches. These patches cannot be tilled for various reasons. Although it was not previously planned, bird surveys have been conducted within these patches in order to be able to compare, at a local scale, species that actually use crops as habitat from those that manage to live in croplands by virtue of these patches of natural vegetation. All threatened bird species found in croplands are tied to these patches and successfully bred within them, particularly the black-and-white monjita.

Large amounts of data have also been collected on the ecology of grassland birds. The ground-breaking news about the project is the discovery of a new threatened species reported for the first time in the country: the ochre-breasted pipit. This is a poorly known species that is thought to be "genuinely rare and very local". Surprisingly this bird turned out to be common at one of the study sites and happens to be almost exclusively restricted to the less disturbed grassland type under study (Pampas deer grasslands). The population involves at least several dozen individuals. Extensive data were obtained on the pipit's behavior and breeding biology. Until now, very little was known of the nesting biology of this species. Five nests were found and vegetation samples from these nesting sites have already been identified by Dr. Mauricio Bonifacino at the School of Agronomy in Montevideo. These data will be used to identify the species' ecological requirements during the breeding season. Individuals of ochre-breasted pipit were last recorded in January censuses, indicating that the species leaves the area during the non-breeding season. If this finding is confirmed, it will be the first record of a migrating population of this threatened bird.

Additionally, non-breeding groups of the rare chocolate-vented monjita have been recorded for the third consecutive year in the El Tapado region (one of the project study sites). The lack of additional sightings from other parts of the country in the last two decades indicates that this area is most likely the wintering stronghold of the species within Uruguay. According to recent published data, the populations of this austral migrant species may be decreasing overall.