Affecting Nest Survival Research Articles

Nest-Site Selection and Nest Survival of Early Successional Birds in Central Pennsylvania

Abstract We studied nest-site selection and nest survival of early successional birds at the Barrens Grouse Habitat Management Area in central Pennsylvania in 2006 and 2007. We compared vegetation at nests to that at sites 30 m distant to examine microhabitat characteristics related to nest-site selection and which characteristics were associated with nest survival. Daily nest survival probability was 0.968 ± 0.006 (SE) for Chestnut-sided Warblers (Dendroica pensylvanica), 0.935 ± 0.014 for Field Sparrows (Spizella pusilla), 0.955 ± 0.011 for Indigo Buntings (Passerina cyanea), and 0.960 ± 0.005 for all species combined. Nest-site selection was best characterized by high woody stem density for Chestnut-sided Warblers, Indigo Buntings, and all species combined. Nest survival increased with woody stem density, indicating that nest-site preferences may be adaptive. Ground cover best predicted nest placement for Field Sparrows but no measured variable affected nest survival for this species. We suggest that s...

Factors Influencing Nesting Success of King Eiders on Northern Alaska's Coastal Plain

Abstract: King eider (Somateria spectabilis) populations have declined markedly in recent decades for unknown reasons. Nest survival is one component of recruitment, and a female's chance of reproductive success increases with her ability to choose an appropriate nesting strategy. We estimated variation in daily nest survival of king eiders at 2 sites, Teshekpuk and Kuparuk, Alaska, USA, 2002–2005. We evaluated both a priori and exploratory competing models of nest survival that considered importance of nest concealment, seclusion, and incubation constancy as strategies to avoid 2 primary egg predators, avian (Larus spp., Stercorarius spp., and Corvus corax) and fox (Alopex lagopus). We used generalized nonlinear techniques to examine factors affecting nest survival rates and information‐theoretic approaches to select among competing models. Estimated nest survival, accounting for a nest visitation effect, varied considerably across sites and years (0.21–0.57); however, given our small sample size, much of this variation may be attributable to sampling variation (s̀2process = 0.007, 95% CI: 0.003–0.070). Nest survival was higher at Kuparuk than Teshekpuk in all years; however, due to the correlative nature of our data, we cannot determine the underlying causes with any certainty. We found mixed support for the concealed breeding strategy; females derived no benefit from nesting in areas with more willow (Salix spp.; measure of concealment) except that the observer effect diminished as willow cover increased. We suggest these patterns are due to conflicting predation pressures. Nest survival was not higher on islands (measure of seclusion) or with increased incubation constancy but was higher post‐fox removal, indicating that predator control on breeding grounds could be a viable management option. Nest survival was negatively affected by our nest visitations, most likely by exposing the nest to avian scavengers. We recommend precautions be taken to limit the effects of nest visits in future studies and to consider them as a possible negative bias in estimated nest survival. Future models of the impacts of development within the breeding grounds of king eider should consider the influence of humans in the vicinity of nests.

NEST SUCCESS OF BLACK-BACKED WOODPECKERS IN FORESTS WITH MOUNTAIN PINE BEETLE OUTBREAKS IN THE BLACK HILLS, SOUTH DAKOTA

Abstract Black-backed Woodpeckers (Picoides arcticus) are burned-forest specialists that rely on beetles (Coleoptera) for food. In the Black Hills, South Dakota, standing dead forests resulting from mountain pine beetle (Dendroctonus ponderosae) outbreaks offer food resources for Black-backed Woodpeckers, in addition to providing habitat similar to burned forest. However, data on reproductive rates in these habitats are lacking. We estimated nest success and evaluated factors affecting nest survival of Black-backed Woodpeckers in beetle-killed forests in the Black Hills in 2004 and 2005. Nest success was 78% (n = 12) in 2004 and 44% (n = 31) in 2005. Fledging rates per pair were 2.0 ± 0.3 SE (n = 12) and 1.4 ± 0.3 SE (n = 28) in 2004 and 2005, respectively. Our results showed that temporal effects (e.g., age, date, and year) had the greatest influence on nest survival. Nest age was the most important predictor of daily nest survival; survival was higher in older nests. Nest survival was also lower later i...

GOLDEN-CHEEKED WARBLER NEST MORTALITY AND PREDATORS IN URBAN AND RURAL LANDSCAPES

Abstract Predation is a major cause of nest failure for songbirds, yet information is lacking on the relative importance of predator species in different landscapes. We identified nest predators of Golden-cheeked Warblers (Dendroica chrysoparia) in an urban landscape, compared cause-specific mortality rates between urban and rural landscapes, and evaluated whether monitoring nests with cameras affected nest survival. We monitored 68 nests with cameras in Austin, Texas, during 2005, 2006, and 2008 to identify causes of mortality, including predators, in an urban landscape. The period mortality and predation rates were 0.60 (95% CI: 0.42–0.73) and 0.52 (95% CI: 0.37–0.68), respectively. We identified predators at 20 nests: Texas rat snakes (Elaphe obsoleta lindheimerii) depredated eight nests (40%), Western Scrub-Jays (Aphelocoma californica) depredated six (30%), Cooper's Hawks (Accipter cooperii) depredated two (10%), fox squirrels (Sciurus niger) depredated three (15%), and fire ants (Solenopsis sp.) dep...

Topographic Patterns of Nest Placement and Habitat Quality for Grassland Birds in Tallgrass Prairie

Much of the tallgrass prairie remaining in North America occurs in hilly regions, such as the Flint Hills of Kansas and Oklahoma. Despite the conservation importance of these areas for grassland birds, little is known about how topographic variation in habitat affects the nesting ecology of these species. We examined topographic patterns of nest distribution, daily nest survival and nest-site selection for three species: Dickcissel (Spiza americana), Grasshopper Sparrow (Ammodramus savannarum) and Eastern Meadowlark (Sturnella magna). Dickcissels shifted from nesting more in lowlands to uplands as the season progressed. Grasshopper Sparrows and Eastern Meadowlarks generally nested in midland habitats, but seasonal differences were still evident in the relative proportions of nests found in lowlands vs. uplands. Topography did not affect nest survival of Dickcissels or Grasshopper Sparrows and had only a marginal effect on nest survival for Eastern Meadowlarks. Nest survival for Dickcissels and Grasshopper Sparrows was highly dependent on nest-site vegetation, however. Dickcissels and Eastern Meadowlarks both experienced greater daily nest survival with increasing vertical vegetation structure at nest sites, whereas daily nest survival for Grasshopper Sparrows increased with increasing cover of litter and grass. Although topography may not affect nest survival directly, it may have indirect effects mediated through nest-site vegetation because of selective nest placement. For example, Dickcissels and Eastern Meadowlarks selected sites with greater vertical vegetation structure than generally available, even in upland sites where vegetation structure was reduced. Conservation planning for grassland birds may thus need to consider how topographic variation affects habitat quality within hilly regions where much of the remaining tallgrass prairie occurs.

Factors Affecting Nest Survival of Greater Sage‐Grouse in Northcentral Montana

Abstract: We studied greater sage‐grouse (Centrocercus urophasianus) in northcentral Montana, USA, to examine the relationship between nest success and habitat conditions, environmental variables, and female sage‐grouse characteristics. During 2001‐2003, we radiomarked 243 female greater sage‐grouse, monitored 287 nests, and measured 426 vegetation plots at 4 sites in a 3,200‐km2 landscape. Nest survival varied with year, grass canopy cover, daily precipitation with a 1‐day lag effect, and nesting attempt. In all years, daily survival rate increased on the day of a rain event and decreased the next day. There was temporal variation in nest success both within and among years: success of early (first 28 d of nesting season) nests ranged from 0.238 (SE = 0.080) in 2001 to 0.316 (SE = 0.055) in 2003, whereas survival of late (last 28 d of nesting season) nests ranged from 0.276 (SE = 0.090) in 2001 to 0.418 (SE = 0.055) in 2003. Renests experienced higher survival than first nests. Grass cover was the only important model term that could be managed, but direction and magnitude of the grass effect varied. Site, shrub and forb canopy cover, and Robel pole reading were less useful predictors of nest success; however, temporal and spatial variation in these habitat covariates was low during our study. We note a marked difference between both values and interpretations of apparent nest success, which have been used almost exclusively in the past, and maximum‐likelihood estimates used in our study. Annual apparent nest success (0.46) was, on average, 53% higher than maximum‐likelihood estimates that incorporate individual, environmental, and habitat covariates. The difference between estimates was variable (range = +8% to +91%). Management of habitats for nesting sage‐grouse should focus on increasing grass cover to increase survival of first nests and contribute to favorable conditions for renesting, which should be less likely if survival of first nests increases.

Factors affecting nest survival of Northern Lapwings Vanellus vanellus in arable farmland: an agri-environment scheme prescription can enhance nest survival

Capsule A spring/summer fallow agri-environment prescription improved Lapwing nest survival. Aims To identify the key determinants of nest survival in arable landscapes, and assess the effectiveness of an agri-environment prescription designed to provide nesting habitat for breeding Lapwings. Methods Lapwing nest success was monitored on a number of farms in Shropshire and Staffordshire, UK, during 1999 and 2000. All nests were mapped and a number of habitat variables recorded. Results Compared with previous studies, nest survival was generally quite high, although there were significant differences between crop types. Nests on fields managed under the agri-environment prescription Option 1B had a greater chance of survival than nests on conventionally managed fields. Overall, the main causes of nest loss were predation and agricultural operations, but causes of nest loss were not consistent between crop types. Crop type, nest status and distances to field boundary and predator perches had a significant effect on nest survival. Nests that were more than 50 m from the nearest field boundary had a higher daily survival rate than nests that were closer. Conclusion Lapwing nest survival on arable farmland can be enhanced through the promotion of an agri-environment prescription that promotes spring/summer fallow and consideration of the timing of agricultural operations. To maximize benefits of conservation measures, fields should be managed to have areas within them that are sufficiently distant from field boundaries to reduce predation.

Population-Specific Demographic Estimates Provide Insights into Declines of Lark Buntings (Calamospiza Melanocorys)

Abstract Many North American prairie bird populations have recently declined, and the causes of these declines remain largely unknown. To determine whether population limitation occurs during breeding, we evaluated the stability of a population of prairie birds using population-specific values for fecundity and postfledging survival. During 2001-2003, we radiomarked 67 female Lark Buntings (Calamospiza melanocorys) to determine annual fecundity and evaluate contributing factors such as nest survival and breeding response (number of breeding attempts and dispersal). Collectively, 67 females built 112 nests (1.67 ± 0.07 nests female−1 season−1; range: 1–3); 34 were second nests and 11 were third nests. Daily nest survival estimates were similar for initial and later nests with overall nest survival (DSR19) of 30.7% and 31.7%, respectively. Nest predation was the most common cause of failure (92%). Capture and radiomarking of females did not affect nest survival. Lark Bunting dispersal probabilities increased among females that fledged young from initial nests and females that lost their original nests late in the season. Conservative and liberal estimates of mean annual fecundity were 0.96 ±0.11 and 1.24 ± 0.09 female offspring per female, respectively. Given the fecundity and juvenile-survival estimates for this population, annual adult survival values of 71–77% are necessary to achieve a stable population. Because adult survival of prairie passerines ranges between 55% and 65%, this study area may not be capable of sustaining a stable population in the absence of immigration. We contrast our population assessment with one that assumes indirect values of fecundity and juvenile survival. To elucidate limiting factors, estimation of population-specific demographic parameters is desirable. We present an approach for selecting species and areas for evaluation of population stability. Las Estimaciones Demográficas Poblacionales Específicas Brindan Pistas sobre la Disminución de Calamospiza melanocorys

POPULATION-SPECIFIC DEMOGRAPHIC ESTIMATES PROVIDE INSIGHTS INTO DECLINES OF LARK BUNTINGS (CALAMOSPIZA MELANOCORYS)

Abstract Many North American prairie bird populations have recently declined, and the causes of these declines remain largely unknown. To determine whether population limitation occurs during breeding, we evaluated the stability of a population of prairie birds using population-specific values for fecundity and postfledging survival. During 2001-2003, we radiomarked 67 female Lark Buntings (Calamospiza melanocorys) to determine annual fecundity and evaluate contributing factors such as nest survival and breeding response (number of breeding attempts and dispersal). Collectively, 67 females built 112 nests (1.67 ± 0.07 nests female−1 season−1; range: 1–3); 34 were second nests and 11 were third nests. Daily nest survival estimates were similar for initial and later nests with overall nest survival (DSR19) of 30.7% and 31.7%, respectively. Nest predation was the most common cause of failure (92%). Capture and radiomarking of females did not affect nest survival. Lark Bunting dispersal probabilities increase...

Reassessing the Cowbird Threat

AbstractThe demographic significance of parasitism by Brown-headed Cowbirds (Molothrus ater; hereafter “cowbirds”) has been debated for years, because manipulative studies are few and mathematical models of cowbird-host systems have not led to tests of their predictions. We combine results from a cowbird- removal experiment (Smith et al. 2002, 2003) with a stochastic simulation model that we developed, to reassess the effect of cowbirds on the annual reproductive success (ARS) and nesting success in Song Sparrows (Melospiza melodia). Our model followed the breeding success of individual Song Sparrow nests and extrapolated to output variables including ARS, nest survival, and parasitism rates. We parameterized the model with field data from cowbird control sites (i.e., cowbirds not manipulated) and found that output variables matched those observed. We used the reduced parasitism rates observed on cowbird-removal sites, reran the model, and compared output with observed values. On removal sites, ARS was greater than predicted by the model, which indicates that the model failed to account for some biological phenomenon that occurred when cowbirds were in the system. To assess what this phenomenon might be, we conducted further analyses that indicated that cowbirds may facilitate nest predation by “conventional” predators. Using elasticity analyses, we show that cowbirds have their largest effect on ARS through egg removal and that conventional predators are more important than cowbirds in directly affecting nest survival rates. We report that to assess the effect of cowbirds on nest survival, researchers must avoid using the common technique of comparing survival rates of parasitized and unparasitized nests within populations.Réévaluation de la Menace du Vacher

An inexpensive method for remotely monitoring nest activity

ABSTRACT In studies of avian nest success, investigators often face the difficult task of periodically checking nest status while at the same time limiting observer influence on nest survival. Remotely monitoring nests using temperature data loggers is one method that allows for continuous data capture regarding nest status (i.e., active vs. inactive) without the negative effects associated with repeated nest checks. We used small temperature data loggers (Thermochron iButtons) to remotely monitor nests of Long-billed Curlews (Numenius americanus) in northeastern Nevada. Data loggers programmed to record temperature at 10-min and 20-min intervals were placed in curlew nests. Data loggers were set to collect data throughout the nesting cycle to determine onset of incubation and timing of nest failure. On average, Long-billed Curlews began incubating approximately 3 d after the first egg was laid and onset of incubation coincided with the laying of the third egg. iButtons allowed us to determine when incubation was terminated in 17 of 23 unsuccessful Long-billed Curlew nests, including 13 of 17 depredated nests. The presence of iButtons in Long-billed Curlew nests did not affect daily survival rate, egg hatchability or rate of nest abandonment. iButtons are an efficient and practical means for remotely monitoring nests of large egg-laying birds, such as the Long-billed Curlew. SINOPSIS En estudios de exito de anidamiento, los investigadores muchas veces se encuentran con la dificultad de examinar el nido, tratando de limitar la infuencia del observador en la sobrevivencia de las aves. Uno de los metodos de monitoreo remoto es el uso de bitacoras electronicas de temperatura que permiten tomar datos de forma continua en un nido (e.g., activo vs inactivo) sin los efectos negativos asociados con el examen periodico del mismo. Utilizamos bitacoras electronicas de temperatura (Thermochron iButtons) para monitorear nidos de chorlo (Numenius americanus) en el norte de Nevada. El instrumento se programo para tomar la temperatura dentro del nido a intervalos de 10 a 20 minutos. El instrumento se utilizo para tomar datos a traves del ciclo de anidamiento del ave para determinar el comienzo de la incubacion y el momento en que el nido fuera abandonado. En promedio los chorlos comienzan a incubar aproximadamente tres dias despues de puesto el primer huevo, lo que coincide con la puesta del tercer huevo. iButtons permitio determinar el final del periodo de incubacion de 13 de 17 nidos depredados. La presencia del instrumento en los nidos de chorlo no afecto la tasa diaria de sobrevivencia, tampoco el eclosionamiento o la tasa de abandono. Las bitacoras utilizadas son eficientes y ademas una forma practica de monitorear a remoto los nidos de aves grandes tales como el chorlo estudiado.

Factors Determining Pochard Nest Predation Along a Wetland Gradient

Waterfowl management on breeding grounds focuses on improving nest success, but few studies have compared waterfowl nest success and factors affecting nest survival along a wetland gradient and simultaneously identified nest predators. We monitored nests (n = 195) of common pochards (Aythya ferina) in Trebon Basin Biosphere Reserve, Czech Republic, during 1999–2002. Daily nest survival rates (DSRs, logistic-exposure) declined from island (0.985, 95% confidence interval, 0.978–0.991) to overwater (0.962, 0.950–0.971) and terrestrial (0.844, 0.759–0.904) nests. The most parsimonious model for DSRs included habitat class (DSRs: island > overwater > terrestrial) and nest visibility. Nest survival was improved by reduced nest visibility, increased water depth, and increased distance from the nest to habitat edge in littoral habitats. On islands, nest success increased with advancing date and increased distance to open water. A model of constant nest survival best explained the data for terrestrial nests. There were no observer effects on DSRs in any habitat. In 2003, artificial nests (n = 180; 120 contained a wax-filled egg) were deployed on study plots. The model that best explained variation in DSRs for artificial nests included only 1 variable: habitat class (DSRs: island ≥ overwater > terrestrial). Mammalian predation of artificial nests (by foxes [Vulpes vulpes] and martens [Martes spp.]) was more likely in terrestrial habitats than in littoral habitats or on islands. By contrast, corvids and marsh harriers (Circus aeruginosus) prevailed among predators of overwater and island nests. Our data indicate that artificial islands and wide strips of littoral vegetation may represent secure breeding habitats for waterfowl because those habitats allow nests to be placed in areas that are not accessible to, or that are avoided by, mammalian predators. Management actions should be aimed at preserving these habitats. This, along with creation of new artificial islands, could help to enhance breeding productivity of pochards and possibly other waterfowl species inhabiting man-made ponds.

Songbird Abundance And Parasitism Differ Between Urban And Rural Shrublands

Many studies have examined differences in avian community composition between urban and rural habitats, but few, if any, have looked at nesting success of urban shrubland birds in a replicated fashion while controlling for habitat. We tested factors affecting nest survival, parasitism by the Brown-headed Cowbird (Molothrus ater), and species abundance in shrubland habitat in rural and urban landscapes. We found no support for our hypothesis that nest survival was lower in urban landscapes, but strong support for the hypothesis that survival increased with nest height. We found strong support for our hypothesis that cowbird parasitism was greater in urban than rural landscapes; parasitism in urban sites was at least twice that of rural sites. We found strong support for an urban landscape effect on abundance for several species; Northern Cardinal (Cardinalis cardinalis) and Brown-headed Cowbirds were more abundant in urban landscapes, whereas Field Sparrow (Spizella pusilla) and Blue-winged Warbler (Vermivora pinus) were more abundant in rural sites. There was support for lower abundances of Blue-gray Gnatcatcher (Polioptila caerulea) and Indigo Bunting (Passerina cyanea) with increased housing density. For six other species, edge and trail density or vegetation parameters best explained abundance. Lower abundances and greater parasitism in habitat patches in urban landscapes are evidence that, for some species, these urban landscapes do not fulfill the same role as comparable habitats in rural landscapes. Regional bird conservation planning and local habitat management in urban landscapes may need to consider these effects in efforts to sustain bird populations at regional and local scales.

Nest concealment and parental behaviour interact in affecting nest survival in the blackcap (Sylvia atricapilla): an experimental evaluation of the parental compensation hypothesis

Nest concealment varies strongly within populations of many species. Although some studies have revealed the beneficial effects of concealment in mitigating predation pressure on nests, other studies were unable to find similar effects. One potential reason for the mixed results is that parental behaviour may compensate for the effects of nest cover, and specifically designed experimental studies are needed to reveal this compensation. I studied the effects of concealment on the probability of nest survival in the blackcap (Sylvia atricapilla), by experimentally manipulating the degree of nest-foliage cover. There was a significant effect of the treatment depending on nest type and the phase of nesting. Whereas there was no effect of concealment on nest survival in natural nests, there was a positive effect in real nests baited with plasticine clutches (i.e. without parental activity). Parents probably behaviourally compensated for poor concealment in natural nests (nest guarding, defence). In line with this, there was no effect of concealment on nest survival during incubation, whereas there was probably a positive effect in the nestling phase. Parents spent more time on the nest during incubation (80%) than during the care of nestlings (40%) and, consequently, had more opportunities to compensate for poor cover. In general, we cannot use single measures of behaviours or states (nest concealment) as an indication of predation risk because of the capacity for compensation in other behaviours.

Appearance and vulnerability of artificial duck nests to avian predators

Use of artificial nests in studies of nest success in birds has become increasingly popular, but this practice has been criticised because the appearance and success of artificial nests may differ from natural nests. Typically, artificial nests are presented with few visible eggs throughout the entire exposure period while natural nests during their life have different appearances, possibly producing stage‐dependent predation rates. In ducks, nests may appear with openly visible eggs, eggs covered with nest material and covered by an incubating female. To test how nest appearance may affect nest survival, I simulated such duck nest appearances, and used direct observations to record responses of marsh harriers Circus aeruginosus, the dominant duck nest predator at the Lake Engure, Latvia, during the breeding seasons 2000–2002. I recorded 274 harrier flights over all types of artificial nests, 74 nest discoveries and 54 predation events. Logistic regression analyses showed that nest appearance was a significant predictor of the fate of the nest: nests with openly visible eggs were discovered more often than nests covered with nest material. Nests with dummy females had discovery rates similar to the average of all artificial nest types suggesting that they best reflect the predation risk of artificial nests and could be used in future studies. After discovering nests covered with dummy females, harriers never attacked but tried to scare them off to get access to the eggs. In 52% of cases of nest discovery, harriers gave up before touching the dummy. This shows that duck passive nest defence may prevent clutch predation.

Modeling nest-survival data: a comparison of recently developed methods that can be implemented in MARK and SAS

Estimating nest success and evaluating factors potentially related to the survival rates of nests are key aspects of many studies of avian populations. A strong interest in nest success has led to a rich literature detailing a variety of estimation methods for this vital rate. In recent years, modeling approaches have undergone especially rapid development. Despite these advances, most researchers still employ Mayfield’s ad-hoc method (Mayfield, 1961) or, in some cases, the maximum-likelihood estimator of Johnson (1979) and Bart & Robson (1982). Such methods permit analyses of stratified data but do not allow for more complex and realistic models of nest survival rate that include covariates that vary by individual, nest age, time, etc. and that may be continuous or categorical. Methods that allow researchers to rigorously assess the importance of a variety of biological factors that might affect nest survival rates can now be readily implemented in Program MARK and in SAS’s Proc GENMOD and Proc NLMIXED. Accordingly, use of Mayfield’s estimator without first evaluating the need for more complex models of nest survival rate cannot be justified. With the goal of increasing the use of more flexible methods, we first describe the likelihood used for these models and then consider the question of what the effective sample size is for computation of AICc. Next, we consider the advantages and disadvantages of these different programs in terms of ease of data input and model construction; utility/flexibility of generated estimates and predictions; ease of model selection; and ability to estimate variance components. An example data set is then analyzed using both MARK and SAS to demonstrate implementation of the methods with various models that contain nest-, group- (or block-), and time-specific covariates. Finally, we discuss improvements that would, if they became available, promote a better general understanding of nest survival rates.

ADVANCED TECHNIQUES FOR MODELING AVIAN NEST SURVIVAL

Estimation of avian nest survival has traditionally involved simple measures of apparent nest survival or Mayfield constant-nest-survival models. However, these methods do not allow researchers to build models that rigorously assess the importance of a wide range of biological factors that affect nest survival. Models that incorporate greater detail, such as temporal variation in nest survival and covariates representative of individual nests represent a substantial improvement over traditional estimation methods. In an attempt to improve nest survival estimation procedures, we introduce the nest survival model now available in the program MARK and demonstrate its use on a nesting study of Mountain Plovers (Charadrius montanus Townsend) in Montana, USA. We modeled the daily survival of Mountain Plover nests as a function of the sex of the incubating adult, nest age, year, linear and quadratic time trends, and two weather covariates (maximum daily temperature and daily precipitation) during a six-year stud...

Evidence for Edge Effects on Multiple Levels in Tallgrass Prairie

Abstract We tested how edges affect nest survival and predator distribution in a native tallgrass prairie system in southwestern Missouri using artificial nests, natural nests of Dickcissels (Spiza americana) and Henslow's Sparrows (Ammodramus henslowii), and mammal track stations. Survival of artificial nests was lower within 30 m of forest edge. Nesting success of Dickcissels and Henslow's Sparrows was lower within 50 m to a shrubby edge than at greater distances, whereas fates of nests were not related to distances to roads, agricultural fields, or forests. Evidence from clay eggs placed in artificial nests indicated that mid-sized carnivores were the major predators within 30 m of forest edges. Furthermore, mid-sized carnivores visited track stations most frequently within 50 m of forest edges. Because proximity of woody habitat explained more variation in nest survival and mammal activity than did fragment size, it appears that edge effects were more pronounced than area effects. Edge effects appeared to be caused mainly by greater exposure of nests to mid-sized carnivores. We argue that, based on edge avoidance behavior, “grassland-interior” species such as the Henslow's Sparrow respond to edge effects mainly by a decrease in density, whereas habitat generalists such as the Dickcissel are affected mainly by a decrease in nesting success.

EVIDENCE FOR EDGE EFFECTS ON MULTIPLE LEVELS IN TALLGRASS PRAIRIE

Abstract We tested how edges affect nest survival and predator distribution in a native tallgrass prairie system in southwestern Missouri using artificial nests, natural nests of Dickcissels (Spiza americana) and Henslow's Sparrows (Ammodramus henslowii), and mammal track stations. Survival of artificial nests was lower within 30 m of forest edge. Nesting success of Dickcissels and Henslow's Sparrows was lower within 50 m to a shrubby edge than at greater distances, whereas fates of nests were not related to distances to roads, agricultural fields, or forests. Evidence from clay eggs placed in artificial nests indicated that mid-sized carnivores were the major predators within 30 m of forest edges. Furthermore, mid-sized carnivores visited track stations most frequently within 50 m of forest edges. Because proximity of woody habitat explained more variation in nest survival and mammal activity than did fragment size, it appears that edge effects were more pronounced than area effects. Edge effects appeare...