ISSN:0012-9658    

DOI:10.2307/1940151

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940152

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940153

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940154

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940155

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The purpose of this study was to identify and quantify the demographic and genetic factors that favor long—term persistence of resident bird populations on small islands. Two species of Darwin's Ground Finches, Geospiza scandens (Cactus Finch) and G. fortis (Medium Ground Finch), were studied on Isla Daphne Major, Galapagos, from 1975 to 1991. Four cohorts born in the years 1975—1978 were followed to the point where almost every individual had died. Life tables were constructed from survival and reproductive data, and used to calculate genetically effective population sizes. Annual rainfall was highly variable and erratic. Extremes were 0 and 1359 mm. As a consequence the finch populations fluctuated in all demographic parameters. In years of little or no rainfall breeding did not occur, in years of abundant rainfall as many as eight breeding attempts were made by individual pairs. Maximum ages were 15 yr for G. scandens and 14 yr for G. fortis. Males of both species tended to live longer than females and to breed later; most females bred for the first time at ages 1—3 yr, whereas most males bred for the first time at ages 2—6 yr. An unusual feature of the survival and reproductive schedules is an increase in reproductive value sometimes occurring moderately late in life more than once, associated with occasional extremely favorable conditions for reproduction. Harmonic mean breeding population Sizes (N) were 94 G. scandens and 197 G. fortis. Effective population sizes (Ne) were much lower, principally as a result of a large variance in the production of recruits per parent, especially by G. fortis. Average effective sizes were 38 G. scandens and 60 G. fortis by one method of calculation, and slightly larger by another. The proportional rate of loss of selectivity neutral heterozygosity or additive genetic variance in quantitative traits potentially caused by random genetic drift (1/2Ne) in these populations is °0.003—0.005, or 0.3—0.5%/yr, and between 0.8 and 1.4% per generation. We suggest that a general estimate for the effective size of terrestrial bird populations is about one—quarter of average breeding numbers. These results are discussed in relation to the long—term viability of the populations and the maintenance of genetic variation. The demographic features that enable the finch species to persist in the face of extreme environmental stochasticity are a high maximum life—span, a generally high adult survival under the stressful conditions of drought, a flexible period of maturity, and a high reproductive rate. Despite their relatively small effective population sizes and the likelihood of genetic impoverishment through random drift they remain genetically variable through gene flow, principally hybridization. This study highlights fluctuating (unstable) age structures as a methodological constraint on some of the theoretical calculations. An expanded demographic and population—genetic theory is needed to overcome this constraint.

ISSN:0012-9658    

DOI:10.2307/1940156

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

On beaches along the Manu River of southeastern Peru, four bird species (Black Skimmer, Rynchops niger; Large—billed Tern, Phaetusa simplex; Yellow—billed Tern, Sterna superciliaris; Sand—colored Nighthawk, Chordeiles rupestris) nest in mixed—species associations. Costs and benefits of the association were distributed asymmetrically among the species. Nighthawks never actively defended their nests against predators. Terns and skimmers defended their nests by aggressively mobbing predators, and indirectly defended nearby or adjacent nests of nighthawks. As a result, nighthawks had greater hatching success when nesting near terns and skimmers. Terns and skimmers spent more time engaged in antipredator behaviors and being vigilant, but less time in parental care, and experienced lowered hatching success when among large nighthawk groups. In addition, both tern species fledged significantly fewer young per successful nest when nesting with many nighthawks. Results of this study demonstrate the first known parasitic relationship among members of a mixed—species nesting group.

ISSN:0012-9658    

DOI:10.2307/1940157

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The abundance and distribution of corvids (Common Raven, Corvus corax; Hooded Crow, C. corone; Jackdaw, C. monedula; Black—billed Magpie, Pica pica; and European Jay, Garrulus glandarius) were studied across a gradient from a landscape dominated by agricultural land to a landscape dominated by forest in south—central Sweden. The total density and the predation rate on dummy nests increased as the proportion of agricultural land increased. This supports the suggestion that the density of corvids increases as forest becomes fragmented and intermixed with agricultural land, causing an increase in nest predation in small forest fragments. However, the different species responded differently to the proportion of forest in the landscape. The two forest—living corvids, the Jay and Raven, were absent from small forest fragments in a matrix of agricultural land and mainly preyed upon dummy nests inside the large forest fragments. Thus, both of these species were habitat specialists restricted to forests. Three corvids basically inhabit agricultural land, the Hooded Crow, Jackdaw, and Magpie. The Jackdaw and the Magpie showed a strong preference for agricultural land and they mainly preyed upon dummy nests in agricultural land. On the other hand, the density of Hooded Crows was higher in landscapes with a mixture of agricultural land and forest than in landscapes dominated by either agricultural land or forest, indicating a use of both agricultural land and forest. Further, Hooded Crows regularly preyed upon nests in both forest habitat and agricultural land; it is a habitat generalist. Therefore, the Hooded Crow was the most important species in the corvid family, causing increased predation pressure close to forest—farmland edges and in small forest fragments surrounded by agricultural land.

ISSN:0012-9658    

DOI:10.2307/1940158

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The population dynamics of the Song Sparrow, Melospiza melodia, were studied for 15 yr on Mandarte Island, in southwestern British Columbia, Canada. This population exhibited a high median density (7.8 females/ha) and fluctuated strongly (10—fold or more) from year to year. The population received few successful immigrants, even at low densities, and its dynamics were thus driven by local events. Two strong density—dependent regulating mechanisms were detected. First, reproductive output was strongly depressed at high densities because of an increased rate of nest failure and a decline in mean clutch size. Greater nest failure at high density was due to increased predation on eggs and nestlings. Nest failure increased with the rate of nest parasitism by Brown—headed Cowbirds (Molothrus ater) and was lowest when cowbirds were absent from Mandarte, suggesting that cowbirds either cause or facilitate nest failure. Second, the rate of juvenile recruitment was inversely related to the density of adults, because of the interference competition for breeding territories between adults and yearlings. The survival rate of adults was independent of population density. Survival rates of adults and juveniles were negatively affected by weather in February 1989, when an annually cold spell coincided with the loss of over 90% of the population. A second severe population crash occurred during the winter of 1979—1980, but the cause of this crash is not known. On two other occasions, periods of severe winter weather occurred at the same time as a substantial fraction of the population disappeared. However, not all periods of poor weather were accompanied by marked disappearances of birds. Adult females survived better than adult males during the breeding season but equally well outside it. We found no correlations between the survival rates of juveniles and adult males and females in years without a population crash suggesting that there is no common cause underlying the observed variation in survival among age and sex classes in most years. Simple models of the population indicate that either density—dependent reproductive success or density—dependent recruitment of locally hatched juveniles can regulate population size. These two regulatory factors operate in a sequential and independent manner. Despite the presence of strong regulating factors, population crashes caused marked instability in population size, nearly leading to extinction in 1989. The rate of juvenile recruitment was the only factor closely related to the net rate of population growth in noncrash years and in all years combined, indicating that juvenile recruitment is the central factor affecting the abundance of Song Sparrows on Mandarte Island.

ISSN:0012-9658    

DOI:10.2307/1940159

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Leach's Storm—Petrels (Oceanodroma leucorhoa), the smallest and most abundant seabirds that breed in the northwest Atlantic, are not usually included in avian energetics models. To round out an energy analysis of seabirds spanning the full range of body sizes in the northwest Atlantic, the field metabolic rates (FMRs) of breeding Leach's Storm—Petrels were measured using doubly labeled water techniques (DLW). Metabolic, dietary and, demographic data were used as inputs in a populations energetics model to estimate the energy requirements and prey harvests of the Leach's Storm—Petrel population of eastern North America. FMRs of storm—petrels that remained in burrows averaged 83.3 ± 19.4 KJ/d. FMRs increased significantly with time (i.e., x in hours per day) at sea (y = [85.8 ± 6.5] + [3.13 ± 0.48]x), and metabolism at sea was estimated to be 161 ± 18 KJ/d. FMRs integrated over activity on land and activity at sea averaged 124 KJ/d during incubation and 142 KJ/d during chick rearing. Incubating adults weighed significantly more than adults rearing chicks, although adult mass did not vary with age of chick. Basal metabolic rate (BMR, measured as O2flux) averaged 2.02 ± 1.01 cm3°g—1°h—1, equivalent to 45.4 ± 30.4 KJ/d, very similar to values obtained for breeding conspecifics in the Bay of Fundy and in the Bering Sea. Thermal conductance averaged 0.124 ± 0.065 cm3°g—1°h—1°°C—1. FMR/BMR ratios averaged 2.73 for incubating birds and 3.13 for birds rearing young. Population energetics modelling indicated that during a 7—mo breeding period Leach's Storm—Petrels at colonies in the northwest Atlantic Ocean consume>74 000 Mg of prey, mostly myctophids and amphipods (mostly Hyperia galba), as wall as euphausiids (mostly Meganyctiphanes norvegica) and other prey (decapods, copepods, isopods). Leach's Storm—Petrels make up °80% of the breeding seabird population in eastern Canada, but owing to small body size and metabolic efficiency, they account for comparatively little of the energy that flows through the avian assemblage of the northwest Atlantic. An intra—specific, inter—colony difference in FMR is suggested: FMRs measured in a Newfoundland colony were significantly higher than those determined with the same DLW procedure in the Bay of Fundy near the southern limit of the species' breeding range in the northwest Atlantic. Differences in ambient temperatures and wind speed associated with different oceanographic regimes may generate energetic differences. Like inter—colony differences in feeding ecology, inter—colony differences in FMR need to be considered in population and community energetics models and in meso— and mega—scale extrapolations.

ISSN:0012-9658    

DOI:10.2307/1940160

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY