Gustavo J. Fernández

EFFECTS OF CLUTCH SIZE AND TIMING OF BREEDING ON REPRODUCTIVE SUCCESS OF GREATER RHEAS

Greater Rheas (Rhea americana) have an unusual breeding system that com- bines harem-defense polygyny, communal egg laying, and exclusive male parental care. We studied survivorship of eggs and nests in relation to clutch size and timing of breeding and estimated the reproductive success of males and females. The number of eggs in a nest varied from 8 to 56, although in most cases it was between 20 and 30. Nests with more than 30 to 35 eggs had longer laying periods, lost more eggs during incubation, and had lower hatch- ability. As a result, the number of chicks hatched per nest increased with clutch size up to 30 to 35 eggs but decreased for larger clutches. More than 65% of the nests were deserted during incubation, and only 5 to 6% of the male population bred successfully each year. Nest- ing success was inversely correlated with rainfall during the breeding season and positively correlated with time of the breeding season. Chick survival during the first months after hatching averaged 60%. We suggest that the proportion of males that are able to attempt breeding is limited by energetic constraints. Received 16 December 1996, accepted 21 August

Identification of cougar and jaguar feces through bile acid chromatography

The use of feces for population size assessment and diet analysis of carnivore species depends on accurate feces identification. Fecal bile acid composition appears to be species specific among carnivores and therefore a useful method to identify fecal samples. To help identify 194 field-collected feces of cougars (Puma concolor) and jaguars (Felis onca), 2 sympatric carnivore species of the Chaco Region (Paraguay), we developed a method to determine feces origin using thin layer chromatography (TLC) bile acid patterns. Bile acids were extracted from 11 fecal samples from cougars and 10 from jaguars from different zoological gardens. Although cougar and jaguar feces contain the same identifiable bile acids, feces can be distinguished by the presence of a single unknown bile acid present consistently only among jaguar feces. The use of this bile acid as an indicator of feces origin, evaluated by means of 2 blind tests of 10 samples each, reached 80%. We also compared our results with a previous identification of the field collected feces based on physical characteristics. We suggest that TLC analysis of fecal bile acids is the best identification technique of cougar and jaguar feces technique available.

Bird–habitat relationship for the declining Pampas meadowlark populations in the southern Pampas grasslands

During the past century, the Pampas meadowlark Sturnella defilippii underwent a severe population drop and now, it is confined mostly to southern Pampas grasslands. We analyzed the habitat and landscape characteristics associated to the presence of reproductive groups of Pampas meadowlarks in this area. During the 1999 breeding season, we surveyed 89 randomly stratified selected points where we noted the presence/absence of Pampas meadowlarks. For each point we estimated seven habitat variables related to vegetation cover and six landscape variables derived from different maps. We found 11 groups of displaying males, nine of them on natural grassland plots. Multivariate analyses indicate that field type and vegetation cover are the main factors associated to the presence of Pampas meadowlark. Reproductive groups were found preferentially at natural grasslands with high vegetation cover. Habitat loss and intensive grazing of fields appear to be the main factors associated to their sharp population decline and distribution retraction.

Alarm calls of the Southern House Wren Troglodytes musculus: variation with nesting stage and predator model

Alarm calls given by parents when risk is detected during nesting may be considered a form of parental defense. We analyzed variations in callings of breeding pairs of the Southern House Wren Troglodytes musculus during the nesting cycle and when faced with different predator models. Nesting birds were exposed to stuffed models at different nesting stages (early and late during incubation, and nests with younger and older nestlings). Nests were also exposed to different predator models where the calling response of breeding adults and acoustic structure variations of the calls were analyzed. The presence of a predator model increased the parents’ alarm calls along the nesting stage. This result supports the hypothesis that the higher the nest reproductive value, the higher the nest defense performed by the Southern House Wren. However, it also supports the notion that alarm calls could be used by parents to silence nestlings and reduce their detectability. Alarm calls also varied according to the predator model presented. We suggest that alarm calls variations of Southern House Wrens could encode information about the kind of predator and the risk envisaged through variations of call rates.

MALE PARENTAL CARE IN GREATER RHEAS (RHEA AMERICANA) IN ARGENTINA

Abstract Greater Rheas (Rhea americana) are precocial birds that show uniparental male care. We evaluated the extent of nest attention by males and estimated its effect on viability and survival of eggs. We assessed whether male absences during incubation were constrained to avoid embryos reaching lethal temperatures or to minimize risk of egg predation. We estimated (1) effect of nest attention on viability of eggs by comparing egg temperature in nests with and without male attention; and (2) effect of male care on egg predation, by comparing survival of eggs in nests with and without male care. Rhea males attended the eggs for 42 days, but effective incubation started 5–7 days after laying of the first egg. The proportion of time that males spent at the nest increased from 64% during egg laying (days 1–10) up to 97.5% during mid and late incubation (days 20–40). Male absences occurred at the warmest hours of the day and their lengths were positively correlated with the temperature of the environment. Male nest attention reduced the rate of egg losses and kept eggs above lower lethal temperatures for embryos during the night. We also estimated relative cost of parental care after hatching by comparing the time allocated to feeding and vigilance by males with chicks, males in nonreproductive groups, and nonreproductive solitary males. Males took care of the chicks for 4–6 months. They allocated less time to feeding and more time to vigilance than males in groups of adults or solitary males. Investment in vigilance decreased as chicks aged. Our results indicate that Greater Rhea males would require high levels of energetic reserves to start a breeding attempt. That high demand could explain why less than only 20% of the males attempt to nest during a breeding season.

Nest-site selection by male Greater Rheas

Abstract Greater Rheas (Rhea americana) are ground-nesting birds that have high rates of nest desertion commonly associated with egg predation. We studied whether male Greater Rheas selected concealed sites to decrease detectability of their nests. We analyzed the spatial distribution of nests and compared the microhabitat characteristics of nest sites vs. sites selected at random, and sites of successful nests vs. sites of deserted nests. We also used experimental nests (nests without male attention) to assess whether egg losses were associated with the microhabitat characteristics of the nest site. The distribution of nest sites differed significantly from a random pattern, and nest sites had a higher percentage of shrub cover, a lower percentage of grass cover, and a higher concealment (low overall visibility) than sites selected at random. However, none of the microhabitat characteristics that we analyzed were associated with nest failure or the rate of egg loss. Experimental nests that were more visible tended to suffer higher and faster egg predation than less visible ones. Our results indicate that Greater Rheas selected concealed sites for nesting, but they did not get any apparent benefit for selecting these sites. We propose that habitat alteration and the type of predation that this species suffers at present may have reduced the benefits of selecting concealed sites for nesting.

Range contraction in the pampas meadowlark Sturnella defilippii in the southern pampas grasslands of Argentina

The Vulnerable Pampas meadowlark Sturn- ella defilippii (Family Icteridae) is a Neotropical grassland bird that suffered a severe population reduction and range contraction during the 20th century. Formerly dis- tributed 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 meadowlarks population size and to iden- tify the factors potentially responsible for its range con- traction. During the 1999 breeding season we surveyed a total of 296 sample locations (transects, randomly selected points, and sites checked for nesting site re-occupation).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 km 2 , respectively. This value repre- sents a range contraction of c. 30% compared to that estimated in a study carried out between 1992 and 1996. Pampas meadowlarks re-occupied 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.

Low frequency of Shiny Cowbird parasitism on Scarlet-headed Blackbirds : anti-parasite adaptations or nonspecific host life-history traits?

Interspecific brood parasitism is a strong selective force that has favoured the evolution of anti-parasite strategies. The Shiny Cowbird Molothrus bonariensis is a generalist parasite that uses more than 200 species as hosts. Scarlet-headed Blackbirds Amblyramphus holosericeus are infrequently parasitized by Shiny Cowbirds in contrast to other sympatric marshbird species that are heavily parasitized. In this paper, we attempted to identify specific anti-parasite defences of the Scarlet-headed Blackbird. Parasitism by Shiny Cowbirds of Scarlet-headed Blackbirds averaged 14.2%, whereas in Brown-and-yellow Marshbirds Pseudoleistes virescens, a sympatric marsh-nesting species, it was 52%. Both host species reared Shiny Cowbird chicks. Scarlet-headed Blackbird did not reject artificially added parasite eggs and did not respond more aggressively toward a female Shiny Cowbrid dummy model than to control species, the Yellow-winged Blackbird Agelaius thilius or the Brown-and-yellow Marshbird. However, high levels of nest attentiveness were recorded during nest-building, egg-laying and incubation stages. About 95-98% of the time at least one parent remained less than 20 m from the nest. Our results show that the low frequency of parasitism on the Scarlet-headed Blackbird may be better explained by life-history traits such as territorial defence, rather than by evolved anti-parasite strategies.

Effect of egg type on the estimation of nest predation in passerines

Abstract We compared predation rates of artificial nests baited with different egg types with the predation rates on nests of the Brown-and-yellow Marshbird (Pseudoleistes virescens), a Neotropical passerine common in the same area, in Argentina. We used deserted natural nests as experimental nests, baited with either Japanese Quail (Coturnix coturnix) or Shiny Cowbird (Molothrus bonariensis) eggs. Each experimental nest was baited with one natural and one paraffin-filled egg. Experimental nests baited with quail eggs suffered lower predation and egg losses than those baited with cowbird eggs. The main cause of egg loss and nest mortality in “cowbird nests” was pecking behavior of cowbirds. However, when we removed the effect of cowbirds, differences still held. In this area, the Shiny Cowbird is common and is the main cause of reduced breeding success of several host species. “Quail nests” had higher nest and egg survival than active Brown-and-yellow Marshbird nests. In contrast, “cowbird nests” and active Brown-and-yellow Marshbird nests had similar nest and egg survival. Experimental nests baited with natural passerine eggs such as those of Shiny Cowbirds seem to approximate the natural nest and egg predation risk for passerines in our study area.

Reduction of nestlings' vocalizations in response to parental alarm calls in the Southern House Wren, Troglodytes musculus

Breeding birds often give alarm calls when a predator is near the nest. These calls have been proposed to serve as distraction displays for the predator, alerts for a mate conveying information about the presence of a threat, or a warning for nestlings about a potential risk. These functions, however, may not be mutually exclusive. In our study, we assessed if alarm calls uttered by breeding Southern House Wrens, Troglodytes musculus, are made to warn nestlings about risk. If so, we expected that nestlings would reduce overall activity in the nest and that the parents’ call rate would be related to the detectability of the young (e.g., vocalizations). We experimentally elicited parents’ alarm calls and compared nestling behavior before and after giving that stimulus. We found that Southern House Wren nestlings reduced their time spent vocalizing and remained inactive for longer when their parents called. Therefore, nestlings reduced their detectability by decreasing their activity inside the nest when their parents produced alarm calls. On the other hand, parental calling rates were not related to the nestling activity registered in any experimental stage. Therefore, we failed to find reliable results supporting the hypothesis that parent calling is uttered to silence nestlings. These results appear to indicate that alarm calling by breeding birds might fulfill other functions besides alerting nestlings. Future studies of this species are necessary to understand if parents are warning nestlings about a threat when they emit alarm calls.ZusammenfassungBrütende Vögel geben häufig Warnrufe ab, wenn sich in der Nähe des Nestes ein Räuber befindet. Es wird angenommen, dass diese Rufe zur Ablenkung für den Räuber, zur Warnung des Brutpartners oder als Warnung der Nestlinge vor einer Gefahrenquelle dienen. Diese Funktionen schließen sich dabei nicht unbedingt gegenseitig aus. In dieser Studie untersuchten wir, ob Warnrufe brütender Südlicher Hauszaunkönige für deren Nestlinge bestimmt waren. In diesem Fall erwarteten wir, dass Nestlinge ihre Aktivität im Nest verringern und dass die Frequenz der elterlichen Warnrufe mit der Erkennbarkeit der Jungen (z. B. durch Lautäußerungen) korreliert. Wir lösten experimentell Warnrufe der Elternvögel aus und verglichen das Verhalten der Nestlinge vor und nach dem Stimulus. Es zeigte sich, dass die Südlichen Zaunkönignestlinge weniger häufig Lautäußerungen von sich gaben und längere Zeit inaktiv waren, wenn die Eltern riefen. Dadurch verringerten die Nestlinge ihre Entdeckungswahrscheinlichkeit im Falle einer Gefahr. Andererseits war die Frequenz der Warnrufe nicht mit der Aktivität der Nestlinge verbunden. Die Ergebnisse unterstützen also nicht eindeutig die Hypothese, dass Eltern rufen, um ihre Jungen zum Schweigen zu bringen. Möglicherweise dienen Warnrufe von Brutvögeln anderen Funktionen außer der Alarmierung von Jungvögeln. Um festzustellen, ob Warnrufe der Eltern die Jungen vor Gefahr warnen sollen, wären weiterführende Studien notwendig.