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At least 505 records · Page 28Linked to original sources
USGS1996

Extremes in ecology: Avoiding the misleading effects of sampling variation in summary analyses

Surveys such as the North American Breeding Bird Survey (BBS) produce large collections of parameter estimates. One's natural inclination when confronted with lists of parameter estimates is to look for the extreme values: in the BBS, these correspond to the species that appear to have the greatest changes in population size through time. Unfortunately, extreme estimates are liable to correspond to the most poorly estimated parameters. Consequently, the most extreme parameters may not match up with the most extreme parameter estimates. The ranking of parameter values on the basis of their estimates is a difficult statistical problem. We use data on 401 species from the BBS and simulations to illustrate the potential misleading effects of sampling variation in rankings of parameters. We describe empirical Bayes and constrained empirical Bayes procedures that provide partial solutions to the problem of ranking in the presence of sampling variation.

Ecology↗
USGS1997

Estimation of population trajectories from count data

Monitoring of changes in animal population size is rarely possible through complete censuses; frequently, the only feasible means of monitoring changes in population size is to use counts of animals obtained by skilled observers as indices to abundance. Analysis of changes in population size can be severely biased if factors related to the acquisition of data are not adequately controlled for. In particular we identify two types of observer effects: these correspond to baseline differences in observer competence, and to changes through time in the ability of individual observers. We present a family of models for count data in which the first of these observer effects is treated as a nuisance parameter. Conditioning on totals of negative binomial counts yields a Dirichlet compound multinomial vector for each observer. Quasi-likelihood is used to estimate parameters related to population trajectory and other parameters of interest; model selection is carried out on the basis of Akaike's information criterion. An example is presented using data on Wood thrush from the North American Breeding Bird Survey.

Biometrics↗
USGS2014

Density and abundance of Wilson's snipe Gallinago delicata in winter in the Lower Mississippi Flyway, USA

Wilson's snipe Gallinago delicata is one of the least studied North American game birds, and information on snipe populations and abundance is mostly unknown. We conducted roadside surveys stratified at the township level in the lower Mississippi Alluvial Valley (LMAV) in Arkansas, Mississippi and Louisiana, as well as the Red River Region, and the Gulf Coastal Plain of Louisiana during winters of 2009 and 2010. We identified observer, vegetation cover, and water cover as important covariates in estimating snipe densities. We detected 2915 snipe along 814 line transects (1450 km) for 2009 and 2010 combined. We estimated snipe densities of 8.05 individuals km -2 (95% CI: 4.57-14.17) in 2009, and 2.13 individuals km -2 (95% CI: 1.47-3.08) in 2010. We used the resulting snipe density estimates within the study area to calculate abundance estimates of 1 026 431 (95% CI: 582 707-1 806 774) in 2009, and 271 590 (95% CI: 187 435-392 722) in 2010 for the LMAV. Our data indicate that a road transect survey method is effective for estimating wintering snipe density and abundance in the lower Mississippi Flyway.

Wildlife Biology↗
USGS2025

Developing research tools for demographic study of Rhynchophanes mccownii (thick-billed longspurs)

Like numerous other North American grassland bird species, Rhynchophanes mccownii (thick-billed longspur) has experienced severe population declines in the last 50 years. Little is known about population-limiting factors, and knowledge gaps limit conservation efforts on the species; however, before research studies aimed at improving conservation and management actions can be developed, other research must resolve notable knowledge gaps that exist in field techniques for efficient and effective large-scale demographic studies. We examined several techniques for the capture, marking (metal, color bands, and transmitters), and reencountering (resights and telemetry) of thick-billed longspurs in croplands and prairies in Valley County, Montana, during the 2022 and 2023 breeding seasons. Our goal was to evaluate the feasibility of obtaining within- and between-season resights of individual thick-billed longspurs using optical equipment and cameras, transmitter receivers, and the Motus automatic receiving station network. This report includes observations and insights that may aid researchers embarking on future demographic studies of thick-billed longspurs, as well as other grassland birds that provide similar research challenges.

Open-File Report↗
USGS2021

American Woodcock singing-ground survey: Comparison of four models for trend in population size

Wildlife biologists monitor the status and trends of American woodcock Scolopax minor populations in the eastern and central United States and Canada via a singing-ground survey, conducted just after sunset along roadsides in spring. Annual analyses of the survey produce estimates of trend and annual indexes of abundance for 25 states and provinces, management regions, and survey-wide. In recent years, researchers have used a log-linear hierarchical model that defines year effects as random effects in the context of a slope parameter (the S model) to model population change. Recently, researchers have proposed alternative models suitable for analysis of singing-ground survey data. Analysis of a similar roadside survey, the North American Breeding Bird Survey, has indicated that alternative models are preferable for almost all species analyzed in the Breeding Bird Survey. Here, we use leave-one-out cross-validation to compare model fit for the present singing-ground survey model to fits of three alternative models, including a model that describes population change as the difference in expected counts between successive years (the D model) and two models that include t -distributed extra-Poisson overdispersion effects (H models) as opposed to normally distributed extra-Poisson overdispersion. Leave-one-out cross-validation results indicate that the Bayesian predictive information criterion favored the D model, but a pairwise t -test indicated that the D model was not significantly better-fitting to singing-ground survey data than the S model. The H models are not preferable to the alternatives with normally distributed overdispersion. All models provided generally similar estimates of trend and annual indexes suggesting that, within this model set, choice of model will not lead to alternative conclusions regarding population change. However, as in Breeding Bird Survey analyses, we note a tendency for S model results to provide slightly more extreme estimates of trend relative to D models. We recommend use of the D model for future singing-ground survey analyses.

Eastern and Central United States and Canada↗
USGS1998

Experimental analysis and simulation modeling of forest management impacts on wood thrushes, Hylocichla mustelina

North American Breeding Bird Survey data show that wood thrush (Hylocichla mustelina) populations in eastern U.S. forests have declined 1.8% per year during 1966-95. The declining quality of breeding forest tracts in North America is one possible cause for the apparent decline of some neotropical migratory birds, such as the wood thrush. In Georgia, however, wood thrush populations have declined during a period of increasing pine forest area and larger patch sizes. We hypothesized that forest management practices such as thinning and prescribed burning might create unsuitable habitat for wood thrushes. We conducted a four-year before/after, treatment/control experiment at the Piedmont National Wildlife Refuge in central Georgia to study to the effects of a treatment of thinning and prescribed burning on wood thrush demographic parameters. We simultaneously monitored wood thrush adults and juveniles with mark-recapture, radio-telemetry, nest searches, and plot-map surveys. Our analyses showed that wood thrushes were less likely to emigrate from the study compartments after the treatment, and wood thrushes exhibited some tendency to increase preference for hardwood habitats and decrease preference for pine habitats following the treatment. However, we observed no effects of treatment on nest success, adult survival, and adult and juvenile dispersal distances. We also found that female wood thrushes had lower survival rates than males during the breeding season, and we documented large-scale, within-year dispersal movements of adult (up to 17 km) and juvenile (up to 7 km) wood thrushes. We conclude that landscape level habitat quantity and quality must be considered during songbird management decisions. The documentation of sex- and age-specific wood thrush survival and movement rates was critical for construction of a set of population models. We used three stochastic models to learn more about wood thrush population dynamics and make predictions about population growth rates, reproductive success, and the effect of habitat changes on wood thrush populations. The simplest source/sink population model suggests that the Piedmont National Wildlife Refuge's wood thrush population is probably stable or increasing, and wood thrush populations in treated areas had higher growth rates than birds in untreated areas. We were able to use the individual-based model of wood thrush productivity to predict fecundity, a parameter that we could not measure directly in the field. Again, females on treated areas had higher fecundity than birds on untreated areas. Our spatially-based model predicted that wood thrush populations should respond positively to predicted changes in the age/size class structure of the Refuge's pine forests. Our model also showed that most wood thrushes leave the Refuge's forest compartments during the breeding season, and these dispersal movements are extremely important to understanding and managing wood thrush populations. The use of prescribed burning and retention shelterwood silviculture at the Piedmont National Wildlife Refuge does not appear to negatively affect the local wood thrush population. Continued use of the current management regime should result in adequate nesting, foraging, and escape habitats for wood thrushes. However, landscape-level habitat availability and quality, including lands outside the Refuge, must be considered when making management decisions that may affect wood thrushes.

Book↗
USGS1993

Population trends of quails in North America

We used North American Breeding Bird Survey data (1966-91) to estimate distribution, relative abundance, and populatiori trends of quails. Population trends in grassland/shrub birds sympatric with northern bobwhite (Colinus virginianus) were also examined. Northern bobwhite and scaled quail (Callipepla squamata) populations have declined since 1966. Rates of decline for these quails have increased during the past decade. California quail (C. californica), Gambel's quail (C. gambeliI), and mountain quail (Oreortyx pictus) populations have been stable over the long-term (1966-91). However, the short-term (1982-91) trend for California quail is positive, whereas Gambel's quail appear to be declining. Patterns in trends indicate similar factors may be negatively affecting breeding populations of grassland/shrub birds throughout the bobwhite's range. We discuss plausible hypotheses to explain population trends and recommend future action.

Book chapter↗
USGS1992

Modelling population change from time series data

Information on change in population size over time is among the most basic inputs for population management. Unfortunately, population changes are generally difficult to identify, and once identified difficult to explain. Sources of variald (patterns) in population data include: changes in environment that affect carrying capaciyy and produce trend, autocorrelative processes, irregular environmentally induced perturbations, and stochasticity arising from population processes. In addition. populations are almost never censused and many surveys (e.g., the North American Breeding Bird Survey) produce multiple, incomplete time series of population indices, providing further sampling complications. We suggest that each source of pattern should be used to address specific hypotheses regarding population change, but that failure to correctly model each source can lead to false conclusions about the dynamics of populations. We consider hypothesis tests based on each source of pattern, and the effects of autocorrelated observations and sampling error. We identify important constraints on analyses of time series that limit their use in identifying underlying relationships.

Book chapter↗
USGS1992

Geographic patterns in population trends of neotropical migrants in North America

We use the route-regression method to estimate the population trends of 100 species of Neotropical migrants using data from the North American Breeding Bird Survey (BBS). We examine long-term (1966-1988) and recent (1978-1988) trends. In the long-term, more species of Neotropical migrants were increasing than were decreasing in the eastern and western parts of the continent, but recent trends indicate that more species decreased than increased in their population index in the east. Recent population declines in the eastern part of the continent were primarily associated with bird species that breed in forested habitat. No association was detected between changes in forest acreages by state and population trends of Neotropical migrants in the United States and Canada.

Book chapter↗
USGS1994

Are more North American species decreasing than increasing?

Population trends for North American species are calculated for 1966-'91 and 1982-'91 using Nonh American Breeding Bird Survey data. Species are grouped into 3 systems of guild classifications representing migration status, breeding habitat and nesting substrate. Trends for these groups are summarized by time period, bioregion and physiographic region. At the continental level, declines approximately equalled increases. There are slightly more declines during the last 10 years. Prairie populations declined more than expected, while western populations increased. Scrub and grassland nesting species declined significantly in several bioregions while woodland species increased. Most guilds did not depart significantly from a ratio of 50:50 increases to decreases or have opposing trends in different bioregions. Guild group changes are usually clustered geographically. Guild and other analyses that search for patterns among population trend estimates offer an excellent means of identifying critical conservation issues.

Book chapter↗
USGS2000

Temporal and geographic patterns in population trends of brown-headed cowbirds

The temporal and geographic patterns in the population trends of Brown-headed Cowbirds are summarized from the North American Breeding Bird Survey. During 1966-1992, the survey-wide population declined significantly, a result of declining populations in the Eastern BBS Region, southern Great Plains, and the Pacific coast states. Increasing populations were most evident in the northern Great Plains. Cowbird populations were generally stable or increasing during 1966-1976, but their trends became more negative after 1976. The trends in cowbird populations were generally directly correlated with the trends of both host and nonhost species, suggesting that large-scale factors such as changing weather patterns, land use practices, or habitat availability were responsible for the observed temporal and geographic patterns in the trends of cowbirds and their hosts.

Book chapter↗
USGS2001

Estimation of species richness and parameters reflecting community dynamics using data from ecological monitoring programs

Although many ecological monitoring programs are now in place, the use of resulting data to draw inferences about changes in biodiversity is problematic. The difficulty arises because of the inability to count all animals present in any sampled area. This inability results not only in underestimation of species richness but also in potentially misleading comparisons of species richness over time and space. We recommend the use of probabilistic estimators for estimating species richness and related parameters (e.g., rate of change in species richness, local extinction probability, local turnover, local colonization) when animal detection probabilities are <1. We illustrate these methods using data from the North American Breeding Bird Survey obtained along survey routes in Maryland. We also introduce software to implement these estimation methods.

Book chapter↗
USGS2008

Long-term decline and short-term crash of the once abundant Rusty Blackbird

The Rusty Blackbird (Euphagus carolinus), a formerly common breeding species of boreal wetlands, has exhibited the most marked decline of any North American landbird. North American Breeding Bird Survey (BBS) trends in abundance are estimated to be -12.5% / yr over the last 40 years, which is tantamount to a >95% cumulative decline. Trends in abundance calculated from Christmas Bird Counts (CBC) for a similar period indicate a range-wide decline of -5.6% / yr. Qualitative analyses of ornithological accounts suggest the species has been declining for over a century before the period covered by the estimated declines. Several studies document range retraction in the southern boreal forest, whereas limited data suggest that abundance may be more stable in more northerly areas. This pattern is both supported and contradicted by winter declines based in CBC data. The lower estimates of decline in the CBC data compared to BBS is consistent with the idea that the coverage of BBS is biased towards the southern boreal whereas CBC covers the entire winter range. However, the CBC declines are similar between the South Atlantic coast (with populations derived from the southeastern boreal) and the Mississippi Valley (populations from the northwest boreal). The major hypotheses for the decline include degradation of boreal habitats from logging and agricultural agricultural development, mercury contamination, and wetland desiccation resulting from global warming. Other likely reasons for decline include loss or degradation of wooded wetlands of the southeastern U.S and mortality associated with abatement efforts targeting nuisance blackbirds. We present a matrix of hypotheses and predictions that test them based on the geography of decline and more detailed indicators of population health which should form the strategic basis of future research.

Book chapter↗