No abstract

The Vulnerable Pampas meadowlark Sturnella defilippii (Family Icteridae) is a Neotropical grassland bird that suffered a severe population reduction and range contraction during the 20th century. Formerly distributed across most of the pampas grasslands, it is now confined to the southern tip of its original range. There are small groups of wintering birds in southern Brazil, a small reproductive population in eastern Uruguay, and the main reproductive population occurs on the southern pampas grasslands of Argentina. In this paper we report the results of an extensive field survey of these southern pampas grasslands, carried out to estimate the pampas meadowlark's population size and to identify the factors potentially responsible for its range contraction. During the 1999 breeding season we surveyed a total of 296 sample locations (transects, randomly selected points, and sites checked for nesting site reoccupation). We found 66 reproductive groups of pampas meadowlarks. The minimum population size and extent of occurrence were estimated to be 28,000 individuals and 4,810 km2, respectively. This value represents a range contraction of c. 30% compared to that estimated in a study carried out between 1992 and 1996. Pampas meadowlarks reoccupied natural grassland sites for nesting that were used in previous breeding seasons when these sites remained undisturbed. Habitat transformation appears to be the main factor causing the range contraction of the pampas meadowlark. We suggest that long-term monitoring of this population and its preferred habitat, the natural grasslands, is required in order to ensure the conservation of this species.

Appetitive contextual conditioning in rats and ringdoves was investigated in six experiments. In Experiment 1, differential contextual training produced greater anticipatory activity in rats in the presence of a context paired with food than it did in rats in the presence of a different context in which food was never presented. Furthermore, the rats showed a preference for the context associated with food when they were given a simultaneous choice test between contexts. In Experiment 2, rats were more active in and preferred a context associated with a variabletime 30·sec (VT30) schedule as opposed to a VT180 schedule. Experiment 3 was a between-subjects replication ofthe previous experiment. As expected, rats exhibited significantly more anticipatory activity in a context in which food had been presented on a VT30 schedule than they did in a context in which food had been presented on a VT180 schedule. Experiment 4 showed that anticipatory activity was a reflection of context-US associations in ringdoves, and in Experiments 5 and 6, ringdoves also exhibited an inverse relationship between the amount of anticipatory activity and the length of the interreinforcement interval (lRI). These results reveal a relation between IRI and contextual conditioning opposite from that obtained in studies of aversive conditioning.Temporal variables exert a strong influence on associative learning (Gibbon & Balsam, 1981;Gormezano & Kehoe, 1981). Perhaps the best known example is the trial-spacing effect, in which aequisition of an association between a conditioned stimulus (CS) and an unconditioned stimulus (US) is facilitated by lengthening the intertrial intervals (lTIs; see Gormezano & Moore, 1969). Several different underlying mechanisms have been proposed to account for the trial-spacing effect.Variation in ITI may influenee aequisition by affecting US processing or effectiveness. The repeated presentation of the US may result in habituation, and sinee there is greater short-term habituation with shorter intervals between stimuli (Davis, 1970), the US may be relatively less effective when trials are massed rather than more widely spaeed. Yet even as this general predietion follows from aU theories of habituation, more specifie predie- tions ean be derived from partieular theories about how the effectiveness of a stimulus ehanges with repeated presentations. Solomon and Corbit (1974) suggest that reception of a US generates two opponent processes. For exarnple, an electrie shock releases a short-latency, rapid-decay primary process (the a process) of an aversive nature. The primary process gives rise to a long-latency, slow-decay secondary process (the b process) that opposes the aversive state. The effective motivational state of the subject is a function ofthe difference between the two processes. Since the secondary process is slow to decay, when I'I'ls are relatively short, the secondary process indueed on trial n -1 may decrease the motivational properties of the US presented on trial n. Assuming that the rapid-decay ...