The Birds of Establecimiento San Ignacio

PEARMAN M; ARETA JI

2009-09-03/2009-10-01

1-39

Biológica

Recursos naturales renovables-Varios

Objectives of the field surveys This study provides the first bird species list for the estancia, and a variety of data regarding their abundance, habitat guilds, migratory status and probable breeding status. Birds are by far the best studied group of vertebrates and an annotated species list such as the one provided here, is an ideal database upon which more detailed studies can be formulated e.g. quantitative surveys which demonstrate the relative abundance of species in different terrains and habitats. Standard methodology for this type of survey can be repeated by ornithologists in different seasons and in different years, to reveal population trends, and to provide an early warning system towards severe habitat modification, misuse or overuse of pesticides and fertilizers. Of special interest is the detection of species which have previously been considered threatened on a global or national scale, with studies of these species forming a particular benchmark. Methods Surveys were conducted in early September and late September/ early October, relating to the start of the austral spring. This was arguably an optimum period to conduct census work because it is the start of the breeding season when species are more easily detected. However, because of the complex nature of bird migration in Argentina, not all species would be present in one single period, although outstanding species could be easily detected during brief visits at other times of year. Direct encounter surveys The initial survey relied upon visual and aural detections by skilled observers with 100% familiarity of all possible species recorded in the province of Buenos Aires (432 bird species; Narosky & Di Giácomo 1993). For this survey, no strict methodology was employed other than direct encounters with species during three intensive fieldwork- days between 3-5 September, and again from 28 September-1 October 2009. We also conducted various playback experiments, associating specific habitat types with certain species which we induced to respond and which may otherwise have gone undetected. Optical equipment included high-end binoculars (Leica, Eagle Optics) and a Swarovski HD 65 telescope with a 25-50 wide angle lens. Playback experiments were carried out from personal i-tune bases on i-pod, while we also made some sound recordings using Marantz PMD 661 and 660 solid state recorders and a variety of long and short Sennheiser microphones, with or without a parabola. All areas of the property were explored, all woodlands were checked and we meticulously sought out all bird species in the natural marsh. There was a strong bias towards time and effort allocated in the marsh since we quickly realized that this area held the greatest bird diversity. Quantitative surveys During a second visit from 28 September to 1 October 2009 we undertook unbiased quantitative surveys. This involved 44 point counts following standard methodology (Ralph et al. 1997) as widely used, e.g. in the American Breeding Bird Survey (BBS) by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service since 1965 (see Robbins et al. 1986). We conducted ten point counts in each of the four main habitats on the property: marsh, pasture, wheat and stubble, and additionally four point counts in the small copses located on the property. No counts were made on arable land, since this was not considered a representative habitat and would in any case rapidly change with the growth of a planted crop. Points were chosen randomly in the four major habitats, and were mostly well spaced between 300-500 meters apart. We rarely conducted more than one count per field, and only when the size of the field merited two counts (e.g., Potrero 16 of 48 hectares). Because of the restricted size of the marsh, the points were necessarily closer to one another in this habitat but were at least 200 m. apart. All point counts were conducted in the first three hours of light from dawn (06:20 - 09:20 am.) when birds were in the height of their daily activity in terms of displaying, singing and/ or foraging (i.e. when they were most easily detectable). Numbers of each species seen and heard were recorded in a 100 meter radius from the point during 10 minutes. For all habitats, species densities were calculated by dividing the total number of individuals counted by the area surveyed through point counts in each habitat.