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.

This paper tests the optimal body mass (OBM) model developed by Lima (1986) for small birds overwintering in a temperate climate. The OBM model views winter body mass as reflecting a trade-off between predation risk and the risk of starvation during periods of inclement weather. Specifically, stochastic computer simulations ofwinter seasons showed that costs (e.g., risk of being preyed upon) and benefits (e.g., fasting capacity) of fat associated with an intermediate mass combine to maximize the probability of overwinter survivorship (Lima 1986). The OBM model predicts that when benefits but not costs of extensive fattening are eliminated by predictable resources, the optimal solution will be for bird species to avoid costs by remaining leaner than species utilizing unpredictable resources. In the latter environment, benefits of fattening are important to surviving suddenly imposed fasting spells. Thus if mass and therefore winter survivorship are optimized among environments, size of the fat reserve should vary inversely with resource predictability.As a test of this prediction, this study compares visible subcutaneous fat among three avian foraging guilds that occupy a gradient of winter resouce predictability. Strictly treeforaging species utilizing tree-borne resources were considered to occupy a predictable environment relative to strictly ground-foraging species. In the latter foraging guild, resources may be unexpectedly and temporarily eliminated by moderate to heavy snowfall. Species with a partial dependence on each of these resource environments were assumed to occupy a resource environment of intermediate predictability. In four separate data sets involving small birds from southern Michigan, central Tennessee, northern Michigan, and southern Indiana, visible fat class varied inversely with resource predictability, as predicted. This trend was also observed in a comparison of species assumed alike in physiological factors that may independently influence size of fat reserve (body size, hypothermia, thermal conductance, basal metabolic rate, nocturnal roost microenvironment). In further concordance with theory, a comparison of dawn fat class between the Dark-eyed Junco and Tree Sparrow (from an unpredictable resource environment) and the Downy Woodpecker and Tufted Titmouse (from a predictable resource environment), which factored out all but a slight effect of basal metabolic rate, indicated significantly greater reserves in the two ground-foraging species. I conclude that aside from possible influences of physiological factors, resource predictability likely affects winter body mass in small birds. This influence takes the form of guild-specific trade-offs between predation risk and fasting capacity (see also Stuebe and Ketterson 1982, Nolan andKetterson 1983). These results support the OBM model (and similar models), and along with previous studies suggest the widespread importance of predation risk as a factor structuring biological systems.

Abstract. Ambient air monitoring as part of the US Environmental Protection Agency's (US EPA's) Clean Air Status and Trends Network (CASTNet) currently uses filter packs to measure weekly integrated concentrations. The US EPA is interested in supplementing CASTNet with semi-continuous monitoring systems at select sites to characterize atmospheric chemistry and deposition of nitrogen and sulfur compounds at higher time resolution than the filter pack. The Monitor for AeRosols and GAses in ambient air (MARGA) measures water-soluble gases and aerosols at an hourly temporal resolution. The performance of the MARGA was assessed under the US EPA Environmental Technology Verification (ETV) program. The assessment was conducted in Research Triangle Park, North Carolina, from 8 September to 8 October 2010 and focused on gaseous SO2, HNO3, and NH3 and aerosol SO42-, NO3-, and NH4+. Precision of the MARGA was evaluated by calculating the median absolute relative percent difference (MARPD) between paired hourly results from duplicate MARGA units (MUs), with a performance goal of ≤ 25%. The accuracy of the MARGA was evaluated by calculating the MARPD for each MU relative to the average of the duplicate denuder/filter pack concentrations, with a performance goal of ≤ 40%. Accuracy was also evaluated by using linear regression, where MU concentrations were plotted against the average of the duplicate denuder/filter pack concentrations. From this, a linear least squares line of best fit was applied. The goal was for the slope of the line of best fit to be between 0.8 and 1.2. The MARGA performed well in comparison to the denuder/filter pack for SO2, SO42−, and NH4+, with all three compounds passing the accuracy and precision goals by a significant margin. The performance of the MARGA in measuring NO3- could not be evaluated due to the different sampling efficiency of coarse NO3- by the MUs and the filter pack. Estimates of "fine" NO3- were calculated for the MUs and the filter pack. Using this and results from a previous study, it is concluded that if the MUs and the filter pack were sampling the same particle size, the MUs would have good agreement in terms of precision and accuracy. The MARGA performed moderately well in measuring HNO3 and NH3, though neither met the linear regression slope goals. However, recommendations for improving the measurement of HNO3 and NH3 are discussed. It is concluded that SO42-, SO2, NO3-, HNO3, NH4+, and NH3 concentrations can be measured with acceptable accuracy and precision when the MARGA is operated in conjunction with the recommendations outlined in the manuscript.

The Clean Air Status and Trends Network (CASTNet) was established by the U.S. EPA in response to the requirements of the 1990 Clean Air Act Amendments. To satisfy these requirements CASTNet was designed to assess and report on geographic patterns and long-term, temporal trends in ambient air pollution and acid deposition in order to gauge the effectiveness of current and future mandated emission reductions. This paper presents an analysis of the spatial patterns of deposition of sulfur and nitrogen pollutants for the period 1990-2000. Estimates of deposition are provided for two 4-yr periods: 1990-1993 and 1997-2000. These two periods were selected to contrast deposition before and after the large decrease in SO2 emissions that occurred in 1995. Estimates of dry deposition were obtained from measurements at CASTNet sites combined with deposition velocities that were modeled using the multilayer model, a 20-layer model that simulates the various atmospheric processes that contribute to dry deposition. Estimates of wet deposition were obtained from measurements at sites operated bythe National Atmospheric Deposition Program. The estimates of dry and wet deposition were combined to calculate total deposition of atmospheric sulfur (dry SO2, dry and wet SO4(2-)) and nitrogen (dry HNO3, dry and wet NO3-, dry and wet NH4+). An analysis of the deposition estimates showed a significant decline in sulfur deposition and no change in nitrogen deposition. The highest rates of sulfur deposition were observed in the Ohio River Valley and downwind states. This region also observed the largest decline in sulfur deposition. The highest rates of nitrogen deposition were observed in the Midwest from Illinois to southern New York State. Sulfur and nitrogen deposition fluxes were significantly higher in the eastern United States as compared to the western sites. Dry deposition contributed approximately 38% of total sulfur deposition and 30% of total nitrogen deposition in the eastern United States. Percentages are similar for the two 4-yr periods. Wet sulfate and dry SO2 depositions were the largest contributors to sulfur deposition. Wet nitrate, wet ammonium, and dry HNO3 depositions were the largest contributors to nitrogen deposition.

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