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At least 1,135 records · Page 63Linked to original sources

USGS2017

Evaluation of harvest and information needs for North American sea ducks

Wildlife managers routinely seek to establish sustainable limits of sport harvest or other regulated forms of take while confronted with considerable uncertainty. A growing body of ecological research focuses on methods to describe and account for uncertainty in management decision-making and to prioritize research and monitoring investments to reduce the most influential uncertainties. We used simulation methods incorporating measures of demographic uncertainty to evaluate risk of overharvest and prioritize information needs for North American sea ducks (Tribe Mergini ). Sea ducks are popular game birds in North America, yet they are poorly monitored and their population dynamics are poorly understood relative to other North American waterfowl. There have been few attempts to assess the sustainability of harvest of North American sea ducks, and no formal harvest strategy exists in the U.S. or Canada to guide management. The popularity of sea duck hunting, extended hunting opportunity for some populations (i.e., special seasons and/or bag limits), and population declines have led to concern about potential overharvest. We used Monte Carlo simulation to contrast estimates of allowable harvest and observed harvest and assess risk of overharvest for 7 populations of North American sea ducks: the American subspecies of common eider ( Somateria mollissima dresseri ), eastern and western populations of black scoter ( Melanitta americana ) and surf scoter ( M . perspicillata ), and continental populations of white-winged scoter ( M . fusca ) and long-tailed duck ( Clangula hyemalis ). We combined information from empirical studies and the opinions of experts through formal elicitation to create probability distributions reflecting uncertainty in the individual demographic parameters used in this assessment. Estimates of maximum growth ( r max ), and therefore of allowable harvest, were highly uncertain for all populations. Long-tailed duck and American common eider appeared to be at high risk of overharvest (i.e., observed harvest < allowable harvest in 5–7% and 19–26% of simulations, respectively depending on the functional form of density dependence), whereas the other populations appeared to be at moderate risk to low risk (observed harvest < allowable harvest in 22–68% of simulations, again conditional on the form of density dependence). We also evaluated the sensitivity of the difference between allowable and observed harvest estimates to uncertainty in individual demographic parameters to prioritize information needs. We found that uncertainty in overall fecundity had more influence on comparisons of allowable and observed harvest than adult survival or observed harvest for all species except long-tailed duck. Although adult survival was characterized by less uncertainty than individual components of fecundity, it was identified as a high priority information need given the sensitivity of growth rate and allowable harvest to this parameter. Uncertainty about population size was influential in the comparison of observed and allowable harvest for 5 of the 6 populations where it factored into the assessment. While this assessment highlights a high degree of uncertainty in allowable harvest, it provides a framework for integration of improved data from future research and monitoring. It could also serve as the basis for harvest strategy development as management objectives and regulatory alternatives are specified by the management community.

PLoS ONE↗

USGS2000

Geographic signatures of North American West Coast estuaries

West Coast estuaries are geologically young and composed of a variety of geomorphological types. These estuaries range from large fjords to shallow lagoons; from large to low freshwater flows. Natural hazards include E1 Niños, strong Pacific storms, and active tectonic activity. West Coast estuaries support a wide range of living resources: five salmon species, harvestable shellfish, waterfowl and marine birds, marine mammals, and a variety of algae and plants. Although populations of many of these living resources have declined (salmonids), others have increased (marine mammals). West Coast estuaries are also centers of commerce and increasingly large shipping traffic. The West Coast human population is rising faster than most other areas of the U.S. and Canada, and is distributed heavily in southern California, the San Francisco Bay area, around Puget Sound, and the Fraser River estuary. While water pollution is a problem in many of the urbanized estuaries, most estuaries do not suffer from poor water quality. Primary estuarine problems include habitat alterations, degradation, and loss; diverted freshwater flows; marine sediment contamination; and exotic species introductions. The growing West Coast economy and population are in part related to the quality of life, which is dependent on the use and enjoyment of abundant coastal natural resources.

British Columbia;California;Oregon;Washington↗

USGS1972

A new species of warbler (Parulidae) from Puerto Rico

The West Indies are among the best known of the world's tropical regions, and our knowledge of the distribution of indigenous land birds in the Greater Antilles has been thought to be nearly complete (Bond, 1956). The last new species from the Antilles were described in 1927, the results of work in the Zapata Swamp of Cuba (Barbour and Peters, 1927) and the isolated Morne de la Selle in Haiti (Wetmore, 1927). No new species has been found in Puerto Rico in this century, and it would seem unlikely that a bird could escape detection on the island, which has a human population density now approaching 320 per square km, and less than 3,300 ha (0.4 percent of land area) of virgin forest remaining (Wads- worth, 1949). Nevertheless the discovery of extant populations of the Puerto Rican Whip-poor-will ( Caprimulgus noctitherus ) by Reynard (1962) and the Puerto Rican Plain Pigeon ( Columba inornata wetmorei ) by Leopold (1963), both previously presumed extinct (Bond, 1956, 1961), indicated that the inventory of the Puerto Rican avifauna might be incomplete.

The Auk↗

USGS2021

Seasonal and age-related variation in daily travel distances of California Condors

Despite a dramatic recovery from the brink of extinction, California Condors ( Gymnogyps californianus ) still face significant anthropogenic threats. Although condor movement patterns across large temporal scales are understood, less is known about their movements on a fine temporal scale. We used a trajectory-based analysis of GPS telemetry data gathered from condors during 2013 to 2018 to investigate the relationship between the distances condors travel in a day, demographic characteristics (e.g., age and sex), and time of year. Most (>71.4%) daily travel distances by condors were <100 km, and, on average, condors traveled 70.1 ± 60.9 km/d ( x̄ ± SD). On two occasions one condor traveled >400 km in a single day (477 km one day and 415 km the following day). The tendency for condors to travel long distances increased with age, and condors traveled longer distances during the summer and when nesting. Traveling such long distances likely exposes birds to threats across a greater variety of landscapes than would be expected for birds that moved shorter distances. Given anticipated condor range expansion and population increase, this work highlights the importance of coordinating condor conservation across the broad spatial scales at which they move.

California↗

USGS1993

Distribution, abundance, and age ratios of Wrangel Island lesser snow geese Anser caerulescens during autumn migration on the Yukon-Kuskokwim Delta, Alaska

We monitored the distribution, abundance, and productivity of Lesser Snow Geese on the Yukon-Kuskokwim (Y-K) Delta, Alaska during September and October 1991, when the geese were en route from their nesting grounds on Wrangel Island, Russia to wintering areas along the Pacific Coast. Adult geese in brood flocks were captured on Wrangel Island and fitted with either satellite platform transmitting terminal (PTT) transmitters (29 birds) or conventional very high frequency (VHF) radio transmitters (68 birds). All geese with active PTTs used the Y-K Delta. Geese marked with PTTs and VHF transmitters were first detected on the Y-K Delta on 19 and 25 September, respectively, and stayed 8-9 days (range 1 to 25 days) Geese with PTTs used the same areas as unmarked geese and geese with VHF radios, except for the south Delta where only satellite data were obtained. Flocks averaged 1122 birds, and did not vary significantly in size during the study. Population estimates from two independent methods ranged from 58,000 to 88,000 geese during October. Productivity of the Wrangel Island population, as determined from the proportion of young in flocks using the Y-K Delta, has varied from 0.5 to 42.1% with a mean of 29%, since 1975. Age-ratio estimates from the Y-K Delta were highly correlated with, and not significantly different from, those from autumn staging and wintering areas further south, which may indicate that immatures in this population of geese suffer little mortality during the second half of their autumn migration.

Alaska↗

USGS1987

Distribution and effects of acidic deposition on wildlife and ecosystems

Acidic deposition occurs over most of the United States and the deposition patterns and theoretical vulnerabilities of aquatic ecosystems to chemical changes can be delineated, but few data exist on concomitant biological effects. Hypothetical direct effects are limited primarily to toxicity of various heavy metals mobilized at reduced pH. Results of studies in Scandinavia suggest that aluminum interferes with avian reproduction near acidified lakes. Some amphibian populations located on acid-vulnerable substrates may be adversely affected by reduced pH in the vernal pools used for egg laying and larval growth. Indirect effects on populations are difficult to detect because few historical data exist for wildlife populations and trophic relationships in vulnerable areas. Current research in the U.S.A. focuses on measuring habitat characteristics, food availability, and avian use of vulnerable wetland habitats. Results of Scandinavian studies suggest that some species of waterfowl may prefer acidified, I fish-free habitats because invertebrates essential for meeting nutritional requirements are more easily obtained in the absence of competition from fish. However, avian species dependent on fish would be absent from these habitats. Alteration of either the vegetative structure or primary productivity of wetlands might indirectly affect avian populations by causing decreased invertebrate productivity and consequent food limitations for birds.

Book chapter↗

USGS2000

Painted Bunting Breeding Bird Survey trends associated with landscape changes in Georgia and South Carolina

Landscape changes during the first 3 decades of the Breeding Bird Survey (BBS) may account for the Painted Bunting's declining population trend. In the southeastern U.S., it is estimated that this bunting has declined 3.5 % per year since 1966. I collected landscape data centered on identical 5-stop areas (n = 33, 306 ha each) of the BBS during early (1960s - 1970s) and late decades (1980s - 1990s). Peak 30-yr counts for Painted Buntings were found at the center of the 5-stop areas. I used stepwise multiple regression analysis to model the mean number of Painted Buntings (in the area during 3 yr, dependent variable) associated with landscape metrics (independent variables). During the early decades the average amount of edge on developed land (p = 0.10), average patch size of agriculture land (p = 0.01), average size of shrub-scrub and young forest (p = 0.09), and average amount of edge for emergent wetlands (p = 0.03) explained 40% of the variation in Painted Buntings counts. In the late decades average amount of edge on developed land (p = 0.04) and average amount of edge on emergent wetlands (p = 0.005) explained 35% of the variation in Painted Bunting counts. Large losses of agricultural land (proportion = 0.177 to 0.094), which was developed or converted to intensively managed pin plantations, may have reduced potential bunting breeding habitat. Shrub-scrub and young forest habitat was constant (proportion = 0.136 to 0.134) but did not affect mean counts of buntings in the late decades. Protected emergent wetlands remained constnat also from the early to late decades (proportion = 0.056 to 0.06) and may provide habitat to maintain a smaller Painted Bunting population. At this time, it's unclear how develped land, which is increasing (proportion = 0.036 to 0.088), may be affecting the Painted Bunting population in GS and SC.

Book chapter↗

USGS2021

Aerial strip-transect surveys: Indexing autumn–winter waterbird abundance and distribution in South Carolina

Aerial surveys integrating probability-based sample designs have been implemented successfully to estimate relative abundance of wintering ducks in Arkansas, Louisiana, Mississippi, and Missouri, but these approaches have not been evaluated in the Atlantic Flyway except for American black ducks ( Anas rubripes ) along the Atlantic coast. Furthermore, these surveys have not been used to index abundance of other nonbreeding waterbirds. Given elimination or reduction of resources allocated to the Midwinter Waterfowl Survey in the Atlantic Flyway and elsewhere, the South Carolina Department of Natural Resources (SCDNR) expressed a need for reliable surveys to monitor waterfowl and other waterbirds during autumn through winter. We designed stratified aerial strip-transect surveys to estimate population indices for migrating and wintering dabbling ducks (Anatini), diving ducks (Aythini, Mergini, Oxyurini), pelagic and piscivorous waterbirds (Anhingidae, Laridae, Pelicanidae, Phalacrocoracidae), and wading birds (Ardeidae, Ciconiidae, Threskiornithidae) in coastal and inland regions of South Carolina during autumn-winter 2017–2019. We used unequal probability random sampling to estimate population indices with deemed adequate precision (i.e., coefficient of variation [CV] ≤ 20%) and estimated theoretical survey efforts needed to achieve desired precision for future aerial surveys. Indices met our goal for precision in September and January 2018 for wading birds, in February and November 2018 for pelagic waterbirds, and in February 2018 for diving ducks, but never for other ducks during South Carolina waterfowl hunting season. We detected peak abundance of dabbling and diving ducks in January and wading birds and wood storks ( Mycteria americana ) in September. We estimated ~2.5 times greater survey effort was needed across waterbird taxa than was expended to achieve a CV=20%. We also used survey data to depict spatiotemporal variation in waterbird distributions across the study area. Our surveys are applicable for the SCDNR and other agencies seeking to monitor autumn-winter waterbird populations. Although survey refinements are necessary to increase precision in South Carolina, our waterbird indices are useful to assess population trends through time, guide habitat management and restoration efforts, refine local harvest regulations, inform law enforcement to detected illicit activities (e.g., baiting), and monitor possible shifting waterbird distributions in response to land-use and climate change.

South Carolina↗

USGS1984

Effects of egg oiling on larid productivity and population dynamics

Small quantities of petroleum may adhere to the plumage, feet, or nest materials of breeding birds and be transferred to their eggs during incubation. In this study, oil was applied to naturally incubated Great Black-backed Gull ( Larus marinus ) and Herring Gull ( L. argentatus ) eggs, and its effects on reproductive success were assessed. Embryo survival was inversely proportional to the quantity of petroleum applied to eggshell surfaces. Dose responses, however, were dependent on embryonic age at the time of treatment. Eggs of either species, treated with 10-20 μ l of No. 2 fuel oil 4-8 days after laying, experienced significant reductions in hatching success. Embryos oiled past the midpoint of the 28-day incubation period were insensitive to as much as 100 μl of petroleum. Fuel oil weathered outdoors for several weeks was as toxic as fresh oil to larid embryos. Productivity estimates obtained following various oil treatments indicated that only under severe conditions (e.g. large doses of petroleum contaminating young embryos) could egg oiling have a significant impact upon populations of the Herring Gull and species with similar life-history characteristics. Species that are more sensitive to oil, however, those having lower reproductive potentials and higher postfledging mortality rates or those subject to other stresses, may be more adversely affected by oil pollution.

The Auk↗

USGS2002

Exploring the effect of drought extent and interval on the Florida snail kite: Interplay between spatial and temporal scales

The paper aims at exploring the viability of the Florida snail kite population under various drought regimes in its wetland habitat. The population dynamics of snail kites are strongly linked with the hydrology of the system due to the dependence of this bird species on one exclusive prey species, the apple snail, which is negatively affected by a drying out of habitat. Based on empirical evidence, it has been hypothesised that the viability of the snail kite population critically depends not only on the time interval between droughts, but also on the spatial extent of these droughts. A system wide drought is likely to result in reduced reproduction and increased mortality, whereas the birds can respond to local droughts by moving to sites where conditions are still favourable. This paper explores the implications of this hypothesis by means of a spatially-explicit individual-based model. The specific aim of the model is to study in a factorial design the dynamics of the kite population in relation to two scale parameters, the temporal interval between droughts and the spatial correlation between droughts. In the model high drought frequencies led to reduced numbers of kites. Also, habitat degradation due to prolonged periods of inundation led to lower predicted numbers of kites. Another main result was that when the spatial correlation between droughts was low, the model showed little variability in the predicted numbers of kites. But when droughts occurred mostly on a system wide level, environmental stochasticity strongly increased the stochasticity in kite numbers and in the worst case the viability of the kite population was seriously threatened.

Ecological Modelling↗

USGS2013

A hierarchical nest survival model integrating incomplete temporally varying covariates

Nest success is a critical determinant of the dynamics of avian populations, and nest survival modeling has played a key role in advancing avian ecology and management. Beginning with the development of daily nest survival models, and proceeding through subsequent extensions, the capacity for modeling the effects of hypothesized factors on nest survival has expanded greatly. We extend nest survival models further by introducing an approach to deal with incompletely observed, temporally varying covariates using a hierarchical model. Hierarchical modeling offers a way to separate process and observational components of demographic models to obtain estimates of the parameters of primary interest, and to evaluate structural effects of ecological and management interest. We built a hierarchical model for daily nest survival to analyze nest data from reintroduced whooping cranes (Grus americana) in the Eastern Migratory Population. This reintroduction effort has been beset by poor reproduction, apparently due primarily to nest abandonment by breeding birds. We used the model to assess support for the hypothesis that nest abandonment is caused by harassment from biting insects. We obtained indices of blood-feeding insect populations based on the spatially interpolated counts of insects captured in carbon dioxide traps. However, insect trapping was not conducted daily, and so we had incomplete information on a temporally variable covariate of interest. We therefore supplemented our nest survival model with a parallel model for estimating the values of the missing insect covariates. We used Bayesian model selection to identify the best predictors of daily nest survival. Our results suggest that the black fly Simulium annulus may be negatively affecting nest survival of reintroduced whooping cranes, with decreasing nest survival as abundance of S. annulus increases. The modeling framework we have developed will be applied in the future to a larger data set to evaluate the biting-insect hypothesis and other hypotheses for nesting failure in this reintroduced population; resulting inferences will support ongoing efforts to manage this population via an adaptive management approach. Wider application of our approach offers promise for modeling the effects of other temporally varying, but imperfectly observed covariates on nest survival, including the possibility of modeling temporally varying covariates collected from incubating adults.

Wisconsin↗

USGS2019

Facilitated adaptation for conservation – Can gene editing save Hawaii's endangered birds from climate driven avian malaria?

Avian malaria has played a significant role in causing extinctions, population declines, and limiting the elevational distribution of Hawaiian honeycreepers. Most threatened and endangered honeycreepers only exist in high-elevation forests where the risk of malaria infection is limited. Because Culex mosquito vectors and avian malaria dynamics are strongly influenced by temperature and rainfall, future climate change is predicted to expand malaria infection to high-elevation forests and intensify malaria infection at lower elevations, likely resulting in future extinctions and loss of avian biodiversity in Hawaii. Novel, landscape-level mosquito control strategies are promising, but are logistically challenging and require costly long-term efforts. As an alternative or supplemental strategy, we evaluated the potential of releasing a gene-edited malaria-resistant honeycreeper (Iiwi, Drepanis coccinea) in Hawaiian rainforests; a strategy known as facilitated adaptation. While this approach also has significant technical challenges and costs, it may offer a more permanent solution to increasing malaria threats. If malaria-resistant honeycreepers can be developed, facilitated adaptation may provide a practical strategy for the reestablishment of abundant avian populations in Hawaiian forests. A successful strategy could be the release of malaria-resistant Iiwi in mid-elevation forests where development of a resistant population has the best chance of success. Establishment of a resistant Iiwi population could provide a source for dispersal and development of resistant populations in high-elevation forests and a permanent source of resistant individuals for translocation to other vulnerable areas.

Hawaii↗

USGS1995

Geographic patterns in relative abundances and population trends of breeding and wintering loggerhead shrikes in North America

In North America, the Breeding Bird Survey and the Christmas Bird Count provide data that are collected at sites visited each year, and the site-specific data can be placed in a Geographic Information System and smoothed to produce contour maps of relative abundance and trend for the United States and southern Canada. We develop these contour maps for Loggerhead Shrike (Lanius ludovicianus) data from both surveys, and compare the patterns derived from each survey to evaluate the consistency of trends and relative abundances within physiographic strata. Patterns in relative abundance seem to be associated with strata, especially in the breeding season, but trends are often heterogeneous within strata. Because of limitations of the surveys and observed geographic patterns of shrike trends in the survey data, we suggest that there is no natural scale for the analysis of shrike-habitat associations.

Book chapter↗

USGS2025

Stable occupancy of conservation-priority birds amid community shifts across 16 years on Iowa wetland easements

Intensive agriculture in the Prairie Pothole Region of Iowa, USA has resulted in significant wetland drainage and wildlife population declines. However, easement programs are increasingly used to protect and restore wetlands and revitalize biodiversity. Short-term responses (i.e., 1–5 years) of birds to wetland restorations are well-documented, but long-term trends are less understood. We surveyed wetland easements in Iowa during 2007–2009 and 2022–2023 to assess changes in breeding bird communities and occupancy for conservation-priority species. We conducted bird point counts and vegetation surveys at 55 wetland easements. We used species accumulation curves to estimate overall breeding bird richness by guild, and site-occupancy models to estimate occupancy for 20 conservation-priority species. Species richness remained stable between time periods, but community composition shifted, with fewer grassland and more forest bird species despite no change in woody vegetation cover estimates (2007–2009: 3.2%, standard deviation [SD] = 5.9; 2022–2023: 3.1%, SD = 3.8). Occupancy for most species at wetland easements remained stable over time; however, 4 species declined (e.g., Cistothorus stellaris [Sedge Wren]). Six grassland bird species (e.g., Sturnella magna [Eastern Meadowlark]) exhibited positive or stable occupancy trends. Forest-dependent species generally maintained or increased in occupancy (e.g., Icterus galbula [Baltimore Oriole]). Increased forest bird richness despite stable woody vegetation cover may indicate changing structural characteristics as existing forests mature, whereas management to control woody encroachment at wetland easements may explain our observations of stable forest bird occupancy. Wetland easements may be strongholds for some grassland bird species, which are in precipitous decline across North America. Our study highlights the importance of wetland easements in an agricultural landscape to breeding bird communities and emphasizes the value of continued monitoring to track changes over time.

Iowa↗

USGS2010

Occupancy dynamics in a tropical bird community: Unexpectedly high forest use by birds classified as non‐forest species

1. Worldwide loss of biodiversity necessitates a clear understanding of the factors driving population declines as well as informed predictions about which species and populations are at greatest risk. The biggest threat to the long-term persistence of populations is the reduction and changes in configuration of their natural habitat. 2. Inconsistencies have been noted in the responses of populations to the combined effects of habitat loss and fragmentation. These have been widely attributed to the effects of the matrix habitats in which remnant focal habitats are typically embedded. 3. We quantified the potential effects of the inter-patch matrix by estimating occupancy and colonization of forest and surrounding non-forest matrix (NF). We estimated species-specific parameters using a dynamic, multi-species hierarchical model on a bird community in southwestern Costa Rica. 4. Overall, we found higher probabilities of occupancy and colonization of forest relative to the NF across bird species, including those previously categorized as open habitat generalists not needing forest to persist. Forest dependency was a poor predictor of occupancy dynamics in our study region, largely predicting occupancy and colonization of only non-forest habitats. 5. Our results indicate that the protection of remnant forest habitats is key for the long-term persistence of all members of the bird community in this fragmented landscape, including species typically associated with open, non-forest habitats. 6. Synthesis and applications. We identified 39 bird species of conservation concern defined by having high estimates of forest occupancy, and low estimates of occupancy and colonization of non-forest. These species survive in forest but are unlikely to venture out into open, non-forested habitats, therefore, they are vulnerable to the effects of habitat loss and fragmentation. Our hierarchical community-level model can be used to estimate species-specific occupancy dynamics for focal and inter-patch matrix habitats to identify which species within a community are likely to be impacted most by habitat loss and fragmentation. This model can be applied to other taxa (i.e. amphibians, mammals and insects) to estimate species and community occurrence dynamics in response to current environmental conditions and to make predictions in response to future changes in habitat configurations.

Journal of Applied Ecology↗

USGS2003

Managing a subsidized predator population: Reducing common raven predation on desert tortoises

Human communities often are an inadvertent source of food, water, and other resources to native species of wildlife. Because these resources are more stable and predictable than those in a natural environment, animals that subsist on them are able to increase in numbers and expand their range, much to the detriment of their competitors and species they prey upon. In the Mojave Desert, common ravens (Corvus corax) have benefited from human-provided resources to increase in population size precipitously in recent years. This trend has caused concern because ravens prey on juvenile desert tortoises (Gopherus agassizii), a federally threatened species. In this paper, I discuss management strategies to reduce raven predation on desert tortoises. The recommendations fall into three categories: (1) managing raven populations by reducing access to anthropogenic resources; (2) removing offending ravens or other birds in specially targeted tortoise management zones; and (3) continuing research on raven ecology, raven behavior, and methods of reducing raven predation on tortoises. I also recommend approaching the problem within an adaptive management framework: management efforts should first be employed as scientific experiments - with replicates and controls - to yield an unbiased assessment of their effectiveness. Furthermore, these strategies should be implemented in concert with actions that reduce other causes of desert tortoise mortality to aid the long-term recovery of their populations. Overall, the approaches outlined in this paper are widely applicable to the management of subsidized predators, particularly where they present a threat to a declining species of prey.

Environmental Management↗

USGS2011

Effects of harvest and climate on population dynamics of northern bobwhites in south Florida

Context: Hunting-related (hereafter harvest) mortality is assumed to be compensatory in many exploited species. However, when harvest mortality is additive, hunting can lead to population declines, especially on public land where hunting pressure can be intense. Recent studies indicate that excessive hunting may have contributed to the decline of a northern bobwhite ( Colinus virginianus ) population in south Florida. Aims: This study aimed to estimate population growth rates to determine potential and actual contribution of vital rates to annual changes in population growth rates, and to evaluate the role of harvest and climatic variables on bobwhite population decline. Methods: We used demographic parameters estimated from a six-year study to parameterise population matrix models and conduct prospective and retrospective perturbation analyses. Key results: The stochastic population growth rate (λ S = 0.144) was proportionally more sensitive to adult winter survival and survival of fledglings, nests and broods from first nesting attempts; the same variables were primarily responsible for annual changes in population growth rate. Demographic parameters associated with second nesting attempts made virtually no contribution to population growth rate. All harvest scenarios consistently revealed a substantial impact of harvest on bobwhite population dynamics. If the lowest harvest level recorded in the study period (i.e. 0.08 birds harvested per day per km 2 in 2008) was applied, λ S would increase by 32.1%. Winter temperatures and precipitation negatively affected winter survival, and precipitation acted synergistically with harvest in affecting winter survival. Conclusions: Our results suggest that reduction in winter survival due to overharvest has been an important cause of the decline in our study population, but that climatic factors might have also played a role. Thus, for management actions to be effective, assessing the contribution of primary (e.g. harvesting) but also secondary factors (e.g. climate) to population decline may be necessary. Implications: Reducing hunting pressure would be necessary for the recovery of the bobwhite population at our study site. In addition, an adaptive harvest management strategy that considers weather conditions in setting harvest quota would help reverse the population decline further.

Florida↗

USGS1979

Modeling sandhill crane population dynamics

The impact of sport hunting on the Central Flyway population of sandhill cranes (Grus canadensis) has been a subject of controversy for several years. A recent study (Buller 1979) presented new and important information on sandhill crane population dynamics. The present report is intended to incorporate that and other information into a mathematical model for the purpose of assessing the long-range impact of hunting on the population of sandhill cranes.The model is a simple deterministic system that embodies density-dependent rates of survival and recruitment. The model employs four kinds of data: (1) spring population size of sandhill cranes, estimated from aerial surveys to be between 250,000 and 400,000 birds; (2) age composition in fall, estimated for 1974-76 to be 11.3% young; (3) annual harvest of cranes, estimated from a variety of sources to be about 5 to 7% of the spring population; and (4) age composition of harvested cranes, which was difficult to estimate but suggests that immatures were 2 to 4 times as vulnerable to hunting as adults.Because the true nature of sandhill crane population dynamics remains so poorly understood, it was necessary to try numerous (768 in all) combinations of survival and recruitment functions, and focus on the relatively few (37) that yielded population sizes and age structures comparable to those extant in the real population. Hunting was then applied to those simulated populations. In all combinations, hunting resulted in a lower asymptotic crane population, the decline ranging from 5 to 54%. The median decline was 22%, which suggests that a hunted sandhill crane population might be about three-fourths as large as it would be if left unhunted. Results apply to the aggregate of the three subspecies in the Central Flyway; individual subspecies or populations could be affected to a greater or lesser degree.

Special Scientific Report - Wildlife↗