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The Conservation Reserve Program (CRP) is a voluntary program that is available to agricultural producers to help protect environmentally sensitive or highly erodible land. Management disturbances of CRP grasslands generally are not allowed unless authorized to provide relief to livestock producers during severe drought or a similar natural disaster (i.e., emergency haying and grazing) or to improve the quality and performance of the CRP cover (i.e., managed haying and grazing). Although CRP grasslands may not be hayed or grazed during the primary bird-nesting season, these disturbances may have short-term (1 yr after disturbance) and long-term (≥2 yr after disturbance) effects on grassland bird populations. We assessed the effects of haying on 20 grassland bird species in 483 CRP grasslands in 9 counties of 4 states in the northern Great Plains, USA between 1993 and 2008. We compared breeding bird densities (as determined by total-area counts) in idle and hayed fields to evaluate changes 1, 2, 3, and 4 years after haying. Haying of CRP grasslands had either positive or negative effects on grassland birds, depending on the species, the county, and the number of years after the initial disturbance. Some species (e.g., horned lark [Eremophila alpestris], bobolink [Dolichonyx oryzivorus]) responded positively after haying, and others (e.g., song sparrow [Melospiza melodia]) responded negatively. The responses of some species changed direction as the fields recovered from haying. For example, densities for common yellowthroat (Geothlypis trichas), sedge wren (Cistothorus platensis), and clay-colored sparrow (Spizella pallida) declined the first year after haying but increased in the subsequent 3 years. Ten species showed treatment × county interactions, indicating that the effects of haying varied geographically. This long-term evaluation on the effects of haying on breeding birds provides important information on the strength and direction of changes in bird populations following a disturbance. Results from this study can help guide management of CRP and other grasslands and inform future agricultural programs that address biomass energy production. © 2016 This article is a U.S. Government work and is in the public domain in the USA.

We consider a variety of regression modeling strategies for analyzing observational data associated with typical wildlife studies, including all subsets and stepwise regression, a single full model, and Akaike's Information Criterion (AIC)-based multimodel inference. Although there are advantages and disadvantages to each approach, we suggest that there is no unique best way to analyze data. Further, we argue that, although multimodel inference can be useful in natural resource management, the importance of considering causality and accurately estimating effect sizes is greater than simply considering a variety of models. Determining causation is far more valuable than simply indicating how the response variable and explanatory variables covaried within a data set, especially when the data set did not arise from a controlled experiment. Understanding the causal mechanism will provide much better predictions beyond the range of data observed. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Numerous metrics describing landscape patterns have been used to explain landscape-scale habitat selection by birds. The myriad metrics, their complexity, and inconsistent responses to them by birds have led to a lack of clear recommendations for managing land for desired species. The amount of a target land cover type in the landscape (percentage cover) often has been a useful indicator of the likelihood of species occurrence or of habitat selection; is it also a more adequate and parsimonious measure for explaining species distributions than patch size or more complex measures of landscape configuration? We examined responses of 6 woodland-interior bird species to the percentage tree cover within prescribed areas and to patch size, edge density, and other metrics. We examined responses in 2 landscapes: a mixed woodland-savanna and an eastern deciduous forest. For these 6 species, percentage tree cover explained bird occurrence as well as or better than other measures in both study areas. We the...

Scent stations usually are deployed in clusters to expedite data collection and increase the number of stations that can be operated for a given cost. Presumed benefits of cluster sampling may not be realized, however, unless cluster sizes are chosen with respect to sampling variation within and among clusters. To encourage and facilitate the use of efficient designs and reporting standards, we used data collected in Minnesota, USA, during 1986-1991 to (1) compare the performance of survey designs with various numbers of stations/cluster; (2) estimate relations between required sample sizes and visitation rates, changes in visitation rates, and error rates; and (3) compare 2 measures of carnivore response: proportions of scent stations (station index) and proportions of clusters (line index) visited by red foxes (Vulpes vulpes) and striped skunks (Mephitis mephitis). Despite broad ecological differences between the species, results were similar for foxes and skunks. Foxes visited 2-21% of stations and 15-84% of fines. Skunks visited 1-16% of stations and 3-54% of lines. Station and line indices were closely related (r 2 > 0.86) and were similarly sensitive indicators of change in visitation rates. Low visitation rates greatly limited the potential usefulness of scent-station surveys because required minimum sample sizes increased exponentially as visitation rates decreased. For visitation rates below 5-10%, required minimum sample sizes were very large and difficult to anticipate. Relative to single-stage sampling, cluster sampling with 10 stations/cluster inflated sample variances, hence sample sizes required to achieve a fixed level of precision, by a factor of 1.6-2.2. Cluster sampling is advantageous only when cost savings permit increases in sample sizes that outweigh concomitant increases in sampling variability. Costs and sampling variation both should be considered when choosing survey designs, and designs should be evaluated and refined as data accumulate.

Grassland bird populations are declining. Invasive plant species may be contributing to these declines by altering habitat quality. However, the effects of invasive plants on grassland birds are largely unknown. Leafy spurge (Euphorbia esula) is an exotic, invasive weed in the northern Great Plains. We examined the effects of leafy spurge infestation on densities of breeding birds, nest-site selection, and nest success in grasslands on the Sheyenne National Grassland (SNG), North Dakota, USA, 1999-2000. We categorized spurge-infested grasslands into 3 groups (low, medium, high), based on the area covered by spurge patches. We surveyed 75 100-m-radius circular points (25 in each group), and searched for nests in 6 16-ha plots (2 in each group). Grasshopper sparrow (Ammodramus savannarum) and savannah sparrow (Passerculus sandwichensis) densities were lower on high-spurge points than on low-and medium-spurge points. Bobolink (Dolichonyx oryzivorus) and western meadowlark (Sturnella neglecta) densities were not significantly different among spurge cover groups. Spurge cover did not appear to be an important factor in nest-site selection. However, western meadowlark nest success was positively associated with spurge cover. Vegetation structure is an important indicator of habitat quality and resource availability for grassland birds. Changes in vegetation structure caused by introduced plant species, such as spurge, can alter resource availability and hence affect bird community composition. Managers of spurge-infested grasslands should continue current spurge control measures to help prevent further declines in grassland habitat quality and grassland bird populations.

Wildlife ecology and management studies have been widely criticized for deficiencies in design or analysis. Manipulative experiments-with controls, randomization, and replication in space and time-provide powerful ways of learning about natural systems and establishing causal relationships, but such studies are rare in our field. Observational studies and sample surveys are more common; they also require appropriate design and analysis. More important than the design and analysis of individual studies is metareplication: replication of entire studies. Similar conclusions obtained from studies of the same phenomenon conducted under widely differing conditions will give us greater confidence in the generality of those findings than would any single study, however well designed and executed.

Statistical testing of null hypotheses recently has come under fire in wildlife sciences (Cherry 1998; Johnson 1999; Anderson et al. 2000, 2001). In response to this criticism, Robinson and Wainer (2002) provide some further background information on significance testing; they argue that significance testing in fact is useful in certain situations. I counter by suggesting that such situations rarely arise in our field. I agree with Robinson and Wainer that replication is the key to scientific advancement. I believe, however, that significance testing and resulting P-values frequently are confused with issues of replication. Any single study can yield a P-value, but only consistent results from truly replicated studies will advance our understanding of the natural world.

We give suggestions for the presentation of research results from frequentist, information-theoretic, and Bayesian analysis paradigms, followed by several general suggestions. The information-theoretic and Bayesian methods offer alternative approaches to data analysis and inference compared to traditionally used methods. Guidance is lacking on the presentation of results under these alternative procedures and on nontesting aspects of classical frequentist methods of statistical analysis. Null hypothesis testing has come under intense criticism. We recommend less reporting of the results of statistical tests of null hypotheses in cases where the null is surely false anyway, or where the null hypothesis is of little interest to science or management.

Despite their wide use in scientific journals such as The Journal of Wildlife Management, statistical hypothesis tests add very little value to the products of research. Indeed, they frequently confuse the interpretation of data. This paper describes how statistical hypothesis tests are often viewed, and then contrasts that interpretation with the correct one. I discuss the arbitrariness of P-values, conclusions that the null hypothesis is true, power analysis, and distinctions between statistical and biological significance. Statistical hypothesis testing, in which the null hypothesis about the properties of a population is almost always known a priori to be false, is contrasted with scientific hypothesis testing, which examines a credible null hypothesis about phenomena in nature. More meaningful alternatives are briefly outlined, including estimation and confidence intervals for determining the importance of factors, decision theory for guiding actions in the face of uncertainty, and Bayesian approaches to hypothesis testing and other statistical practices.

The scent-station survey method has been widely used to estimate trends in carnivore abundance. However, statistical properties of scent-station data are poorly understood, and the relation between scent-station indices and carnivore abundance has not been adequately evaluated, We assessed properties of scent-station indices by analyzing data collected in Minnesota during 1986-93. Visits to stations separated by

Identification and assessment of the relative importance of factors affecting duckling growth and survival are essential for effective management of mallards on breeding areas. For each of 3 years (1993-95), we placed F 1 -generation wild mallard (Anas platyrhynchos) females on experimental wetlands and allowed them to mate, nest, and rear broods for 17 days. We manipulated invertebrate densities by introducing fathead minnows (Pimephales promelas) at high densities in half of the wetlands on which broods were confined. Day-17 body mass of surviving ducklings (n = 183) was greater for ducklings that were heavier at hatch; the difference averaged 1.7 g at day 17 for each 1.0 g at hatch (P = 0.047). Growth ratio (the proportion of body mass attained by ducklings when they were last measured relative to that predicted for wild female mallard ducklings) also was positively related to body mass at hatch (P = 0.004). Mean day-17 body mass and mean growth ratio of ducklings per brood (each adjusted for body mass at hatch) were positively related to numbers of aquatic invertebrates (Ps < 0.001) and negatively related to variance in the daily minimum air temperature during the exposure period (Ps < 0.020). Early growth of mallards was more sensitive to variation in numbers of invertebrates than to air temperature or biomass of invertebrates. Duckling survival was positively related to growth ratio (P < 0.001). Our study provides parameter estimates that are essential for modeling growth and survival of mallard ducklings. We emphasize the need for conserving brood-rearing wetlands in the Prairie Pothole Region that are capable of supporting high densities of aquatic invertebrates.

Increased predation on nests of ducks in prairie uplands, as a result of habitat alteration, has been hypothesized to cause decreased nest success and population sizes. We tested whether, and by how much, nest success declined using data compiled from 37 studies conducted between 1935 and 1992 at 67 sites in the Prairie Pothole Region of Canada and the United States. Nest success declined (P = 0.0002) over time, but time explained only 10% of the variation ; precipitation (P = 0.79) did not account for additional variation in nest success. Nest success declined at similar (P = 0.13) rates among 5 species, but late nesters (gadwall [Anas strepera], blue-winged teal [A. discors], and northern shoveler [A. clypeata]) had higher success (P = 0.004) than early nesters (mallard [A. platyrhynchos], and northern pintail [A. acuta]). Populations of gadwalls and northern shovelers, however, have not declined, indicating that declines in nest success may not be related causally to population change. Long-term population declines in blue-winged teal, northern pintails, and mallards coincide with large-scale temporal declines in nest success. Declines in nest success were parallel in parkland and grassland regions, suggesting a causal agent (or agents) that act(s) at a broad scale, despite inherent differences in the composition of the predator communities and habitats between regions.

Duckling survival, an important factor affecting annual recruitment, has not been determined adequately for canvasbacks (Aythya valisineria). We investigated the magnitude, timing, and causes of mortality of canvasback ducklings from hatch to fledging at the Agassiz National Wildlife Refuge (NWR) in northwestern Minnesota during 1987-90. During the 4 years, 217 day-old ducklings were radiomarked and released in 52 broods. Another 141 ducklings were radiomarked at ≥ 4 weeks of age. Survival was estimated with the Kaplan-Meier nonparametric estimator and the Weibull parametric model. Most mortalities occurred within 10 days after hatch. Total brood loss occurred in 18 (35%) of 52 broods released. The primary sources of mortality were predation, principally by mink (Mustela vison), and exposure to precipitation and cold temperature. For combined years, females had lower survival than males (P = 0.03). If the disparate survival between sexes of canvasbacks observed in this study is representative of canvasbacks in their breeding range, this phenomenon contributes to reduced reproductive potential and the male-biased sex ratio of the species.

Although sampling plans and estimators of abundance have good theoretical properties, their performance in real situations is rarely assessed because true population sizes are unknown. We evaluated widely used sampling plans and estimators of population size on 3 known clustered distributions of pronghorn (Antilocapra americana). Our criteria were accuracy of the estimate, coverage of 95% confidence intervals, and cost. Sampling plans were combinations of sampling intensities (16, 33, and 50%), sample selection (simple random sampling without replacement, systematic sampling, and probability proportional to size sampling with replacement), and stratification. We paired sampling plans with suitable estimators (simple, ratio, and probability proportional to size). We used area of the sampling unit as the auxiliary variable for the ratio and probability proportional to size estimators. All estimators were nearly unbiased, but precision was generally low (overall x coefficient of variation [CV] = 29). Coverage of 95% confidence intervals was only 89% because of the highly skewed distribution of the pronghorn counts and small sample sizes, especially with stratification. Stratification combined with accurate estimates of optimal stratum sample sizes increased precision, reducing the mean CV from 33 without stratification to 25 with stratification; costs increased 23%. Precise results (x CV = 13) but poor confidence interval coverage (83%) were obtained with simple and ratio estimators when the allocation scheme included all sampling units in the stratum containing most pronghorn. Although areas of the sampling units varied, ratio estimators and probability proportional to size sampling did not increase precision, possibly because of the clumped distribution of pronghorn. Managers should be cautious in using sampling plans and estimators to estimate abundance of aggregated populations

Body mass of adult female and male sandhill cranes (Grus canadensis) increased an average of 17 and 20%, respectively, from early September to late October on staging areas in central North Dakota and varied by year. Increases in body mass averaged 550 and 681 g among female and male G. c. canadensis, respectively, and 616 and 836 g among female and male G. c. rowani. Adult and juvenile G. c. rowani were lean at arrival, averaging 177 and 83 g of fat, respectively, and fat reserves increased to 677 and 482 g by mid-October. Fat-free dry mass increased by 12% among juveniles, reflecting substantial growth, but remained constant among adults. The importance of fall staging areas as conditioning sites for sandhill cranes, annual variation in body mass, and vulnerability of cranes to habitat loss underscore the need to monitor status of fall staging habitat in the northern plains region and to take steps to maintain suitable habitat where necessary. J. WILDL. MANAGE. 54(2):234-238 During fall migration, midcontinent populations of 3 subspecies of sandhill crane (G. c. canadensis, G. c. rowani, and G. c. tabida) stop for extended periods on traditional staging areas in the northern plains region of North America before continuing southward to wintering grounds located primarily in Texas and New Mexico (Johnson and Stewart 1973, Lewis 1977, Tacha et al. 1984). Cranes that nest in central and arctic Canada, Alaska, and eastern Siberia spend most of September and October on staging areas in eastern Alberta, Saskatchewan, Manitoba, North Dakota, South Dakota, and northwestern Minnesota (Lewis 1977, Melvin and Temple 1983). In North Dakota, subspecies composition varies by site. Cranes staging in the westcentral region (McLean County) are primarily G. c. canadensis, whereas cranes in the This content downloaded from 207.46.13.189 on Thu, 04 Aug 2016 05:14:12 UTC All use subject to http://about.jstor.org/terms J. Wildl. Manage. 54(2):1990 CRANE BODY COMPOSITION * Krapu and Johnson 235 central part (Pierce and Kidder counties) are primarily G. c. rowani (Johnson and Stewart 1973). The fall distribution of the midcontinent population of G. c. tabida is centered in northwestern Minnesota, and their distribution in North Dakota is limited primarily to a small population in Kidder County (Johnson and Stewart 1973). The subpopulations of sandhill cranes that stage in Kidder and McLean counties spend winter primarily along the Texas Gulf Coast (50,000-70,000 cranes, Tacha et al. 1984) and western Texas (450,000 cranes, Iverson et al. 1985), respectively. The significance of fall staging areas in the northern plains region as conditioning sites for the midcontinent sandhill crane population is poorly understood. The prolonged stay on these staging areas probably prepares birds physiologically for the continuation of fall migration (Melvin and Temple 1983) and possibly for winter, much as spring staging areas along the Platte and North Platte rivers physiologically prepare cranes for spring migration and reproduction (Krapu et al. 1985). However, fat levels among migrant sandhill cranes collected in Oklahoma in October 1979 were much lower than observed late in the spring staging period of the same year in Nebraska (Iverson 1981). This difference suggests, at least during some years, that cranes acquire less fat during autumn staging intervals in the Great Plains region than during spring, possibly because of less suitable foraging conditions on fall staging areas. We address (1) patterns of body mass change among adult G. c. canadensis and G. c. rowani on staging areas in North Dakota during the fall stopover and (2) the magnitude of change in body nutrient composition of G. c. rowani from early to late in the fall staging period. The North Dakota Game and Fish Department and cooperators provided measurements on hunter-shot cranes; B. A. Hanson and R. O. Woodward assisted in collecting and processing cranes, and C. R. Luna prepared the figures. D. P. Fellows gave constructive comments on an earlier draft of the manuscript.

COWAN, W. F. 1973. Ecology and life history of the raccoon (Procyon lotor hirtus Nelson and Goldman) in the northern part of its range. Ph.D. Thesis. Univ. North Dakota, Grand Forks. 161pp. FRITZELL, E. K. 1978a. Habitat use by prairie raccoons during the waterfowl breeding season. J. Wildl. Manage. 42:118-127. . 1978b. Aspects of raccoon (Procyon lotor) social organization. Can. J. Zool. 56:260-271. GREENWOOD, R. J. 1979. Relating residue in raccoon feces to food consumed. Am. Midi. Nat. 102:191-193.

Nesting studies are used to assess the production of birds and to evaluate nesting habitats. Most such studies involve finding nests in a given area and subsequently determining the proportion that hatched. Unfortunately, the results are often biased by unrecognized differences in the probabilities of finding successful and unsuccessful nests. The observed hatch rates of 1,900 nests of blue-winged teal (Anas discors) are presented to illustrate the relationship of hatch rates to time remaining until the nests should hatch. The Mayfield method of correcting for these biases is illustrated. Other examples demonstrate the possible effects of sampling procedures on observed hatch rates and nest density. J. WILDL. MANAGE. 42(3):471-476 Nesting studies are common in investigations of waterfowl and other birds. Most are undertaken to assess the production of breeding birds and to evaluate nesting habitats and the techniques of managing such habitats. The objectives are to determine hatch rates and density of nests in selected habitats. The procedures commonly used are searching selected areas to find nests and subsequently checking those nests to ascertain whether or not the eggs were hatched. Unfortunately, the nests of most species are initiated over periods of at least several days during which some of the nests may be destroyed. If nests are destroyed, many females will renest 1 or more times; consequently, it is common to find newly initiated nests after others of the same species have hatched. Continuous searches over such prolonged periods generally are impractical; hence, most studies involve periodic searches. If some nests were missed because they were initiated and destroyed between searches, the observed nest density clearly would be biased downward. Less obviously, but more importantly, the observed nesting success would be biased upward. We have restricted our discussion to these biases. These potential biases, among several commonly occurring in nesting studies, were recognized previously by Hammond and Forward (1956) and Mayfield (1960, 1961). Mayfield (1961) elaborated the method of estimating nesting success from nests observed during all or any portion of the period between initiation and hatch. The method has not been widely adopted. Mayfield therefore published the method again 14 yr later and noted correctly (Mayfield 1975:456) that "not every published report shows awareness of the problem." At least 2 other investigators (Townsend 1966, Reed 1975) acknowledged the problem in waterfowl studies; Townsend used Mayfield's method in his analysis. Our purpose is to bring the potential biases associated with periodic searching to the attention of investigators who may consider undertaking nesting studies. Our studies will exemplify the magnitude of these biases. We will also illustrate how the inconsistency of the biases invalidates many comparisons commonly made within and among nesting studies. We will demonstrate Mayfield's method for obtaining better estimates of the success and density of nests. ' Present address: U.S. Fish and Wildlife Service, 1978 South Garrison Street, Denver, Colorado 80227. J. Wildl. Manage. 42(3):1978 471 This content downloaded from 157.55.39.102 on Sat, 24 Dec 2016 05:19:18 UTC All use subject to http://about.jstor.org/terms 472 BIAS IN NESTING STUDIES Miller and Johnson We hereby acknowledge the support and guidance of H. K. Nelson, former Director, and W. R. Goforth, Director of the Northern Prairie Wildlife Research Center, in the studies which led to this paper. We are especially grateful for the use of the nest records provided by, and for the constructive comments of, H. F. Duebbert, K. F. Higgins, L. M. Kirsch, A. T. Klett, and J. T. Lokemoen. Our thanks to D. A. Davenport for assistance with computer programing, to C. W. Shaiffer for drafting the figures, and to H. F. Mayfield for reviewing an earlier draft of this report. Our special thanks to L. M. Cowardin for perceptive technical and editorial comments on the manuscript.

A technique for distinguishing adult from yearling wild mallards (Anas platyrhynchos), from late winter through the nesting season, was developed by applying discriminant analysis procedures to selected wing feather characters of 126 yearlings and 76 adults (2-year-olds) hand-reared from wild eggs during 1974, 1975, and 1977. Average values for feather characters generally increased as the birds advanced from yearlings to adults. Black-white surface area of greater secondary covert 2 was the single most reliable aging character identified during the study. The error rate was lowest in females (3%) when discriminant functions were used with measurements of primary 1 weight and black-white area cf greater secondary covert 2 and in males (9%) when the functions were used with black-white area of greater secondary coverts 1, 2, and 3. Methodology precludes aging of birds in the field during capture operations. J. WILDL. MANAGE. 43(2):384-393 Although the mallard has been studied widely, only limited information has been published on age-related aspects of its life cycle, especially age-related productivity. This paucity of information can be attributed, in part, to the lack of a reliable aging technique applicable during the nesting season. Although keys have been developed on the basis of certain wing feather characters to determine age of mallards during the fall and winter (Carney and Geis 1960, Carney 1964), the reliability of these characters during the breeding season is diminished because of feather replacement and wear before and during the nesting season. The present study was undertaken to develop a technique for reliably separating yearling from older (adult) mallards from late winter to the onset of wing molt in late summer. Several feather measurements were chosen for establishing discriminant functions for each group. Wing feathers were selected on the basis of observed patterns of variation in feather characters, with knowledge of age-related feather variation based on published 1 Present address: Division of Wildlife Research, U.S. Fish and Wildlife Service, Washington, DC 20240. literature, and on the basis of pattern and timing of molt. Variation in primary feather lengths of yearlings and adults was recognized in blue-winged teal, Anas discors (Dane 1968), and redhead, Aythya americana (Dane and Johnson 1975). Wing covert markings were used to age yearlings of certain species, including gadwall, Anas strepera (Oring 1968); common merganser, Mergus merganser (Anderson and Timken 1971); redhead (Smart 1962, Dane and Johnson 1975); and blue-winged teal (Dane 1968). We thank C. W. Shaiffer, who made most of the measurements; B. A. Hanson for assistance in data collection; T. J. Dwyer for his encouragement and help during development of the study; D. C. McGlauchlin, Manager, Audubon National Wildlife Refuge, for support in obtaining mallard eggs for aging studies; F. B. Lee for supervising the rearing of known-age mallards; and R. J. Greenwood for critically reviewing the manuscript.

The principal spring staging areas of the midcontinent population of sandhill cranes (Grus can- adensis) are along the Platte and North Platte rivers in south-central Nebraska. Most of these lands are privately owned and managed for corn and cattle production. Diurnal habitat use by radio-tagged cranes was primarily in cropland (55%), native grassland (28%), and tame hayland (15%). Ninety-nine percent of the cropland use was in cornfields; 55% as grazed stubble, 36% as disced, cultivated, and plowed stubble, 7% as ungrazed stubble, and 1% unclassified. Grazed pastures accounted for 93% of the grassland locations and mowed alfalfa fields 77% of the tame hayland locations. Other habitats were seldom used. Time budget analyses indicated that cranes, while in croplands, grasslands, and haylands, spent 35, 36, and 50% of the time foraging, respectively Cranes roosted in the shallows and on nearby sandbars of about 111 km of river channel. Cranes usuallv roosted where the channel was at least 150 m wide and avoided stretches narrower than 50 m. Height of woody vegetation along shorelines and on islands influenced where cranes roosted when unobstructed channel width was less than 150 m; bridges or roads adjacent to the channel also reduced use by about half. Management recommendations are made for maintaining suitable habitat for sandhill cranes on their staging areas in Nebraska. J. WILDL. MANAGE. 48(2):407-4 17 Approximately one-half million sand- hill cranes (four-fifths of the continental population) gather annually along the Platte and North Platte rivers in Nebraska during March and early April while en route to their breeding grounds in central and arctic Canada, Alaska, and Siberia (US. Fish and Wildl. Serv., unpubl. data). Water developments in the upper Platte River Basin during this century have dras- tically reduced flows, causing major changes in channel width in the Big Bend reach (Williams 1978) where most of the cranes gather. Extensive encroachment by woody vegetation has accompanied chan- nel shrinkage (Currier 1982), and pro- posed developments would cause addi- tional habitat degradation and loss (Krapu et al. 1982), raising concern for the well

The mark-recapture method for estimating striped skunk (Mephitis mephitis) abundance was evaluated by systematically livetrapping a radio-equipped population on a 31.4-km2 study area in North Dakota during late April of 1977 and 1978. The study population was 10 females and 13 males in 1977 and 20 females and 8 males in 1978. Skunks were almost exclusively nocturnal. Males traveled greater nightly distances than females (3.3 vs. 2.6 km, P < 0.05) and had larger home ranges (308 vs. 242 ha) although not significantly so. Increased windchill reduced night-time activity. The population was demographically but not geographically closed. Frequency of capture was positively correlated with time skunks spent on the study area. Little variation in capture probabilities was found among trap-nights. Skunks exhibited neither trap-proneness nor shyness. Capture rates in 1977 were higher for males than for females; the reverse occurred in 1978. Variation in individual capture rates was indicated among males in 1977 and among females in 1978. Ten estimators produced generally similar results, but all underestimated true population size. Underestimation was a function of the number of untrapped skunks, primarily those that spent limited time on the study area. The jackknife method produced the best estimates of skunk abundance. J. WILDL MANAGE. 49(2):332-340 The estimation of animal abundance is often difficult. Mark-recapture is used commonly to estimate the abundance of secretive mammals, including striped skunks (Verts 1967, Bailey 1971, Lynch 1972). Although there are many estimators available (Cormack 1968, 1979; Otis et al. 1978; Pollock 1981; Seber 1982), there are few data on their application to populations of known size. This paper assesses the applicability of several estimators with data for populations of striped skunks of known size in 2 years and discusses aspects of skunk biology that influence capture probability. Although captures in this study are fewer than desired for most markrecapture evaluations, the approach represents a practical application of the mark-recapture method. W. B. Bicknell and J. L. Piehl provided field assistance. Radiotelemetry equipment was obtained from the James Ford Bell Mus., Cedar Creek Natl. Hist. Area, Univ. of Minnesota, St. Paul. D. R. Anderson and G. C. White provided the CAPTURE computer program. Appreciation is extended to D. W. Sparling, who verified J. Wildl. Manage. 49(2):1985 STRIPED SKUNK MARK-RECAPTURE * Greenwood et al. 333 some of the calculations and reviewed the manuscript, and to D. R. Anderson, K. P. Burnham, R. M. Cormack, L. L. Eberhardt, D. L. Otis, K. H. Pollock, and G. C. White, who provided constructive comments on the manu

Waterfowl managers can effectively use simple population models to aid in making management decisions. We present a basic model of the change in population size as related to survival and recruitment. A management technique designed to increase survival of mallards (Anas platyrhynchos) by limiting harvest on the Chippewa National Forest, Minnesota, is used to illustrate the application of models in decision making. The analysis suggests that the management technique would be of limited effectiveness. In a 2nd example, the change in mallard population in central North Dakota is related to implementing programs to create dense nesting cover with or without supplementary predator control. The analysis suggests that large tracts of land would be required to achieve a hypothetical management objective of increasing harvest by 50% while maintaining a stable population. Less land would be required if predator reduction were used in combination with cover management, but questions about effectiveness and ecological implications of large scale predator reduction remain unresolved. The use of models as a guide to planning research responsive to the needs of management is illustrated. J. WILDL. MANAGE. 43(1):18-35 Management of a wildlife population is a complicated task that often requires decisions based on limited data. To make logical decisions we need (1) a management policy or objective defined in terms of some goal, (2) a relationship between that goal and parameters of the population to be managed, and (3) estimates of these parameters. Such a relationship is often termed a "model," and can be either quite rudimentary and qualitative or complicated and quantitative. Inadequate information often forces us to use models that grossly oversimplify the behavior of the real population and to employ estimates that are frequently little better than informed guesses. The manager cannot postpone a decision until the dynamics of the population are fully understood and precise estimates of all parameters are available. Inaction is in fact a decision by default. The development of simple models, imperfect though they may be, and the graphic representation of these models can facilitate logical and orderly development of management strategy. Because of the large quantity of pertinent banding and survey data available, the mallard has been the subject of several published models (Walters et al. 1974, Anderson 1975a,b, Brown et al. 1976). These models use estimates of production based on survey data for the continental population. In contrast, our model relates recruitment to various components that are frequently measured in local studies and that are subject to manipulation by the waterfowl manager. The exercise of presenting a model has a number of benefits: (1) inspection of the model may focus attention on critical parameters; (2) the model provides a frame of reference for evaluating policy and assessing the feasibility of objectives; (3) insight into possible alternatives for reaching an objective is gained; and (4) we are forced to face the reality of the management problem and to answer important questions such as cost effectiveness and political feasibility. The purpose of this paper is to demonstrate a simple model that can facilitate decisions concerning management of mallards in the north central

Eighty-seven fall-collected wings from female redhead ducks (Aythya americana) were assigned to the adult or juvenile group based on "tertial" and "tertial covert" shape and wear. To obtain spring age-related characters from these fall-collected groupings, we considered parameters of flight feathers retained until after the first breeding season. Parameters measured included: markings on and width of greater secondary coverts, and length, weight, and diameter of primary feathers. The best age categorization was obtained with discriminant analysis based on a combination of the most accurately measured parameters. This analysis, applied to 81 wings with complete measurements, resulted in only 1 being incorrectly aged and 3 placed in a questionable category. Discriminant functions used with covert markings and the three 5th primary parameters were applied to 30 known-age juvenile, handreared redhead females; 28 were correctly aged, none was incorrectly aged, and only 2 were placed in the questionable category. J. WILDL. MANAGE. 39(2):256-263 To estimate the production of a given waterfowl breeding population, information is needed on the expected productivity of various age cohorts. Of particular importance is the productivity of yearling birds compared with that of older females. Most of the techniques used to distinguish firstyear birds from adults are not valid for most ducks in the spring, because adult growth and sexual development have been attained and identifying feathers have been replaced. Even with those external characters of waterfowl that are age-related in in the spring (e.g., eye color [Trauger 1974] and bill spots [Kuroda 1937, Dane 1968]), the overlap between adult and first-year birds diminishes the usefulness of these

A Lipid Index, the ratio of fat to fat-free dry weight, is proposed as a measure of fat stores in birds. The estimation of the index from field measurements of live birds is illustrated with data on the sandhill crane (Grus canadensis) and greater white-fronted goose (Anser albifrons). Of the various methods of assessing fat stores, lipid extraction is the most accurate but also the most involved. Water extraction is a simpler laboratory method that provides a good index to fat and can be calibrated to serve as an estimator. Body weight itself is often inadequate as a condition index, but scaling by morphological measurements can markedly improve its value. J. WILDL. MANAGE. 49(3):569-575 Body weight and nutrient reserves, which are often used to characterize "condition," have been related to both survival (Lack 1966:276277) and breeding performance (Jones and Ward 1976) of birds (but see King and Murphy 1984). Assessing the condition of birds is therefore important in the study and management of bird populations (Bennett and Bolen 1978). Birds are capable of storing several nutrients for mobilization during critical periods of their life cycle. Although fat, protein, and Ca have each been identified as potentially limiting for breeding females (Ankney and Maclnnes 1978), we believe that fat is the most frequent limiting nutrient during the year because of its numerous functions, including lipid source for egg synthesis (Raveling 1979), energy source during migration (Odum et al. 1964, Blem 1980) and food deprivation (Hanson 1962), and as insulation (Evans and Smith 1975). Protein and Ca requirements are relatively small except during egg production, when a larger turnover of these nutrients occurs (Robbins 1981). Our purpose is to recommend a Lipid Index that represents fat stores of birds of various sizes and to indicate how the index can be estimated from measurements taken in the field on live birds. We also evaluate the performance of several published condition indices. The methods are illustrated with data on the sandhill crane and greater white-fronted goose. 1 Present address: Patuxent Wildlife Research Center, Room 509, 820 South Street, Vicksburg, MS 39180. 2 Present address: School of Forest Resources, Nutting Hall, University of Maine, Orono, ME 04469. We thank R. Atkins, C. M. Boise, C. R. Frith, B. A. Hanson, C. Jorgenson, T. C. Tacha, and P. A. Vohs for their assistance in obtaining specimens from various study areas. R. R. Campbell, J. R. King, J. Longmuir, and D. W. Sparling provided valuable comments on an earlier draft of the report. STUDY AREAS AND METHODS Study Areas All sandhill crane specimens were collected in 1978-79. From late February to mid-April, 119 were taken in the Platte River Valley of Nebraska. Additional samples included 28 taken in late April to early May near Last Mountain Lake in Saskatchewan, 20 taken during May or early June at Clarence Rhode National Wildlife Range in Alaska, 14 taken in late August or mid-October in central North Dakota, and 15 taken in mid-February near Muleshoe National Wildlife Refuge in Texas. Most greater white-fronted geese (49) were collected from late February to early April in 1979-80 near the Platte River or in the Rainwater Basin of Nebraska. Six others were taken in late April or early May of 1979 in the Last Mountain Lake area. Field and Laboratory Methods Specimens were weighed and measured at field laboratories. Measurements included (flattened) Wing, (diagonal) Tarsus, and Culmen (post nares). External features and gonads were examined to ascertain the sex and age (young of the year and older; Lewis 1979) of each bird. Subspecies were identified according to Johnson and Stewart (1973).

The relative importance of various foods occurring in the diet of blue-winged teal (Anas discors), pintail (A. acuta), and gadwall (A. strepera) breeding in south-central North Dakota and lesser scaup (Aythya affinis) breeding in the vicinity of Great Slave Lake, Northwest Territories, are compared by the aggregate volume and aggregate percent methods. Advantages of the aggregate percent method are discussed in relation to the information presented. J. WILDL. MANAGE. 38(2):302-307 Recent investigations of the foods consumed by breeding and immature ducks inhabiting prairie and subarctic wetlands of North America have emphasized the value of using the esophageal contents rather than the gizzard for this purpose (Perret 1962; Bartonek and Hickey 1969a, 1969b; Dirschl 1969; Sugden 1969; Bartonek and Murdy 1970; Swanson and Bartonek 1970; Swanson and Nelson 1970; Krapu 1972; Swanson and Sargeant 1972). This change was implemented primarily through improved sampling procedures which provided birds containing substantial amounts of food in their esophagi. The trend toward utilizing the esophagus of waterfowl somewhat paralleled an earlier and similar change that occurred in food habit studies of upland game birds (Martin et al. 1946; Martin et al. 1951). The purpose of this paper is to reevaluate two existing methods of presenting either volumetric or weight data in light of the current use of the esophagus as a source of information. The data that form the basis for these comparisons were gathered to support feeding ecology studies of blue-winged teals, pintails, and gadwalls in south-central North Dakota and lesser scaups in the Northwest Territories. Appreciation is extended to P. F. Springer for critically reviewing the manuscript.

We estimated nest success of mallard (Anas platyrhynchos), gadwall (A. strepera), blue-winged teal (A. discors), northern shoveler (A. clypeata), and northern pintail (A. acuta) for 5 regions in North Dakota, South Dakota, and Minnesota, for 1-3 periods between 1966 and 1984, and for 8 habitat classes. We obtained composite estimates of nest success for regions and periods by weighting each habitat proportional to the number of nest initiations. The distribution of nest initiations was derived from estimates of breeding populations, preferences of species for nesting habitats, and availability of habitats. Nest success rates ranged from 15,000 records of nests found in the United States portion of the Prairie Pothole Region (Fig. 1). Supplemental information on breeding population levels, nesting habitat availability, and habitat use by nesting 432 DUCK NEST SUCCESS * Klett et al.