Nest Patch Research Articles

Nest‐site selection by cinnamon teal and other ground‐nesting ducks in Great Salt Lake wetlands

AbstractGround‐nesting ducks can reduce nest depredation by selecting nest sites where local physical structures or vegetation provide olfactory or visual concealment. We evaluated nest‐site selection by comparing duck nests to random sites during 2019 and 2020 in the impounded wetlands of Great Salt Lake (GSL), Utah, USA. During wet springs, most of the wetlands are underwater, leaving only the dams available as nesting substrate. We located the nests of 399 cinnamon teal (Spatula cyanoptera), 137 mallards (Anas platyrhynchos), and 162 gadwalls (Mareca strepera) on the dams. At nest and random sites, we measured overhead concealment, lateral concealment, average height of vegetation in the nest patch, standard deviation of vegetation height in the nest patch, tallest vegetation within 1 m of nest, dam width, and distance from the nest to the road along the center of the dam. We found that mallards and gadwalls did not select for any of these characteristics. Cinnamon teal selected for nest sites that had more overhead concealment. We also found mixed evidence that cinnamon teal selected nest sites where dams were wider. All species also favored heterogeneous patches of vegetation for nesting over homogenous ones. Duck nests had a clumped distribution in wetlands that contained colonies of American avocets (Recurvirostra americana), black‐necked stilts (Himantopus mexicanus), and common terns (Sterna hirundo), with nests located inside the colonies, but showed a random distribution in other wetlands. Prior to 1980, approximately half of the world's population of cinnamon teal nested in GSL wetlands, but since that time the nesting population has declined greatly. Given this decline, management actions should be conducted that will improve overhead cover and widen dikes within the species' nesting habitat.

The role of body size in nest‐site selection by secondary cavity‐nesting birds in a subtropical Chaco forest

Most bird species that nest in tree cavities globally occur in diverse assemblages in little‐studied tropical and subtropical forests which have high rates of habitat loss. Conservation of these communities will require an understanding of how species traits, such as body size, influence nest‐site selection. We examined patterns of nest‐site selection of secondary cavity‐nesting birds at the nest patch, tree and cavity scale, and investigated how these patterns are influenced by body size. Using conditional logistic regression, we compared characteristics of 155 nest tree cavities paired with 155 unused tree cavities in quebracho Schinopsis balansae forests in Chaco National Park, Argentina (2016–2018). The odds of a cavity being used for nesting increased with its depth and height above ground, decreased with entrance size, and were greater for dead trees than live. Small‐bodied (13–90 g) species used floor diameters in proportion to availability, but medium‐ (150–200 g) and large‐bodied (400–700 g) species selected cavities with larger floors. Model selection indicated that characteristics at the nest patch scale (canopy cover, tree density) had little effect on nest‐site selection when cavity‐scale variables were included. Cavity floor diameter, entrance size, cavity height and tree diameter (but not cavity depth) increased with body mass, and larger bird species more often used live trees. Two tree species proved to be key for the community: large and medium‐sized birds used almost exclusively large live Schinopsis balansae, whereas small birds used live and dead Prosopis spp. in a proportion greater than its availability. Small birds could be differentiated according to species‐specific cavity characteristics, but medium and large species overlapped considerably with one another. Although body mass explained much of the overall variation in tree and cavity characteristics between small and medium/large species, several small‐bodied species consistently used cavities outside of the expected characteristics for their body size, suggesting that other natural history traits may play important roles in nest‐site selection by small‐bodied birds. To retain the full suite of secondary cavity‐nesters in species‐rich tropical and subtropical forests, it is necessary to conserve a diversity of trees and cavities that meet the full range of nesting requirements of these trait‐diverse communities.

Stand-level forest management for foraging and nesting of Williamson’s sapsuckers

Williamson’s Sapsuckers, like most woodpeckers, require live and dead standing trees for foraging and nesting. Forest management plans that include Williamson’s Sapsucker habitat conservation guidelines currently focus on nesting trees because little is known about foraging habitat requirements, but foraging habitat in close proximity is also essential for the survival and reproduction of the species. We conducted a study on the selection of stand-level characteristics for foraging and nesting territories to improve knowledge on the habitat requirements of this endangered woodpecker in Canada during the breeding season. We tracked 27 radio-tagged Williamson’s Sapsuckers in managed forest at their northern range limit in two regions of southern British Columbia. We examined the selection of forest composition and configuration characteristics at the foraging patch and stand levels by comparing use and availability, and described foraging trip distances. In the Okanagan region, sapsuckers did not show a selection pattern for foraging patch characteristics. In the Western region, Williamson’s Sapsuckers selected foraging patches with higher densities of large live Douglas-fir trees. Nest patches were usually in small openings and had lower tree densities than foraging patches in the Western region, but not in the Okanagan region. Open areas (e.g., clear-cuts, seed tree cuts, pastures, roads, powerlines) were avoided during foraging trips in both regions. Regarding stand-level configuration, Williamson’s Sapsuckers selected continuous stands for foraging – with ≥30% crown closure in both regions. We used the 50th and 95th percentiles of foraging trip distances to recommend zones for nest reserve (no-harvest; 0–140 m from the nest) and nest management (Okanagan: 340 m, Western: 410 m). For the nest management zone, we recommend only partial harvesting with retained groups of trees extending from 140 to 340 m in the Okanagan region and from 140 to 410 m in the Western region. We suggest that Williamson’s Sapsuckers exhibited stronger selection when foraging in the Western region, because they compensated for longer foraging distances due to higher proportions of open area in their nesting territories.

Assessing the quality of fire refugia for wildlife habitat

Wildfires have important effects on wildlife habitat. Within fire perimeters, ‘fire refugia’ are unburned or less severely burned areas that contain residual pre-fire habitat structures and facilitate species persistence following fire. However, fire refugia may vary in their relative quality. The ability to evaluate the relative quality of fire refugia for wildlife habitat may help efficiently allocate resources for post-fire conservation planning, especially for at-risk species. Using spatial overlays of habitat models and burn severity data, we developed the refugia index, a flexible multi-scale, multi-criteria fuzzy logic model to assess the value of unburned areas for wildlife habitat affected by wildfires and other disturbances. We conducted a case study of the northern spotted owl (Strix occidentalis caurina) and applied our method to 4278 unburned areas in 15 fires that each burned > 400 ha of spotted owl nesting/roosting habitat in the eastern Cascade Mountains, U.S.A. In our study area, unburned areas represented ~ 1.7% of the area within the fire perimeters. Most fire refugia were low quality, because unburned areas were small relative to the desired nest patch size of spotted owls and contained little suitable nesting/roosting habitat. However, each fire contained a few high-quality fire refugia, because they co-occurred with suitable nesting/roosting habitat and were surrounded by desirable habitat characteristics for spotted owls. Our method illustrates how land managers can use readily available spatial data and analysis tools to understand fire effects on species’ habitat (e.g., habitat lost) and prioritize areas for post-fire conservation and management.

Breeding habitat, nest-site characteristics and productivity of the little eagle (Hieraaetus morphnoides) near Armidale, New South Wales

We aimed to elucidate nesting requirements and nest success of the threatened little eagle (Hieraaetus morphnoides). Nest sites (n=12 active and 2–5 recent historical nests) near Armidale, New South Wales, were measured in 2017 at three scales: the nest tree, the nest woodland (≤25m from the nest tree), and (using GIS) the landscape scale (within 200-m and 2-km radii of the nest). The eagles typically nested ≥14m above ground in the canopy of emergent (>20m tall) living eucalypts in sheltered positions (midslope, with a north-easterly to southerly aspect), in woodland patches >5ha (mean 76ha), <200m (mean 78m) from the woodland edge, though ≥11m (mean 190m) from an agricultural edge, ≥38m (mean 485m) from the nearest rural dwelling, >1km from suburbia, and farther from sealed roads (mean 832m) than gravel roads (mean 490m) than minor tracks (mean 291m). Breeding productivity in 2017–18 (n=15 and 18 territories, respectively) was 0.91 young fledged per attempt (clutch laid) and 0.67 young fledged per occupied territory per year. Nest sites were used annually for at least 3–7 years. Nest abandonments or site shifts were associated with human disturbance (e.g. clearing, earthmoving, subdivision and construction in or beside the nest patch), death of the nest tree or nest stand (‘eucalypt dieback’ or rural tree decline), pindone baiting for rabbits (Oryctolagus cuniculus), and displacement by wedge-tailed eagles (Aquila audax) and ravens (Corvus sp.). As most little eagle nests were located on private land, we recommend, inter alia, greater protection of breeding habitat, nest sites and foraging habitat, woodland regeneration (especially riparian), and a buffer around established nests of ≥1km from major developments such as urbanisation.

Does vegetation change over 28 years affect habitat use and reproductive success?

Abstract Individuals should prefer and use habitats that confer high fitness, but habitat use is not always adaptive. Vegetation in natural landscapes changes gradually and the ability of species to adaptively adjust their habitat use to long-term changes is largely unstudied. We studied nest patch and territory use over 28 yr in Orange-crowned Warblers (Oreothlypis celata) in a system that has undergone natural long-term changes in vegetation. Abundance of maple (Acer grandidentatum), its preferred nesting habitat, gradually declined from 1987 to 2015. We examined whether habitat use and its fitness consequences changed as the availability of preferred habitat decreased. We used resource selection function models to determine changes over time in the probability of using a nest patch given available patches, and the probability of using a territory given available territories. We estimated nest survival to evaluate changes over time in the fitness consequences of nest patch use. We also compared habitat use (nest patch and territory) and fitness (nest survival) between areas with naturally reduced abundance of maple and experimentally increased abundance of maple (fenced areas). Nest patch use depended on maple abundance and did not change drastically across 28 yr, even though the availability of preferred maple patches decreased over time. In contrast, nest survival tended to decrease over time. We did not see differences in nest patch use and nest survival between unfenced and fenced areas, unlike territory use, which increased with the abundance of maple in fenced areas and decreased in unfenced areas. Our study depicts one example of relatively unchanged habitat use in the face of decreased availability of preferred vegetation across years, with a resulting decrease in reproductive success. Investigating changes in habitat use and fitness consequences for animals exposed to long-term habitat change is necessary to understand adaptive behavioral responses.

Microscale nest-site selection by the Burrowing Owl (Athene cunicularia) in the pampas of Argentina

Habitat modifications have led many bird species to occupy areas with different characteristics, including human-altered landscapes. In this study, we analyzed how land use influences the nest-site selection at the microscale level by Burrowing Owls (Athene cunicularia) breeding in vegetated sand dunes, periurban areas, and agroecosystems in the Pampas of Argentina. We compared the characteristics of the nest site (percentage of open space) and the nest patch (distance to conspecific nests, tall vegetation and perches and number of perches) within and among the three land-cover types. In addition, we evaluated the breeding performance (nesting success and productivity) of owls nesting in these land-cover types. We found that nest microsite variables did not vary between owl-occupied and owl-unoccupied sites within nest patches, but they differed among land-cover types. Although nest patches differed in their availability of perches at each land-cover type, distance from the nest to the nearest perch did not vary between them. Distances to tall vegetation and to conspecific nests were highly variable and did not differ among land-cover types. Our results indicate that Burrowing Owls that inhabit the Pampas used a variety of land-cover types for nesting and showed little selectivity of nest sites and nest patches, thus reinforcing the idea that they are habitat generalists.

Influence of patch shape on mallard nest survival in northern Iowa

Reproductive success of mallards (Anas platyrhynchos) is influenced by distribution and amount of wetlands and grasslands on the landscape during the breeding season. Most studies of mallard reproductive success have been conducted in areas with high wetland densities and large tracts of grasslands. We investigated nest survival of mallards in intensively cropped northern Iowa, USA, where wetland and grassland habitats were highly fragmented. We radiotracked female mallards nesting during 1998–2000 and located 318 nests in 6 types of land cover. Overall daily survival rate of nests was 0.945 ± 0.003 standard error (SE), corresponding to an estimated nest survival rate of 0.14. Hen success (i.e., the probability that an individual female will hatch a nest in one of her attempts) averaged 0.28 ± 0.03 SE. We used a model selection approach to examine covariates that might affect nest survival. Perimeter-to-area ratio (PAR) of the nest patch was the most important predictor of daily nest survival, with nest survival decreasing with increasing PAR. A greater percentage of nests hatched (18%) in habitats with low perimeter-to-area ratios (e.g., pastures, hayfields, Conservation Reserve Program fields, and managed grasslands) compared with habitats with high PAR (11%) such as drainage ditches, road-side ditches, fencerows, and waterways. Managing habitat in this region to increase mallard nest survival will be challenging, given the propensity of mallards to nest in linear habitats. If the climate change projections materialize in the 21st century, the southeastern portion of the Prairie Pothole Region could become a much more important breeding area for midcontinent mallards. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.

Breeding season microhabitat use by Cerulean Warbler (Setophaga cerulea) in an experimentally-managed forest

Cerulean Warbler (Setophaga cerulea) populations have undergone significant declines over the past four decades, and the species is considered state endangered in Indiana. These migratory songbirds nest in the canopies of mature deciduous forests throughout eastern North America, but local microhabitat preferences vary by geography and scale, and may influence differential nest success rates across the breeding range. Between 2010 and 2015, we searched for nests, mapped out male territories, and monitored nest success in the Hardwood Ecosystem Experiment in central Indiana. We collected data on a variety of microhabitat features at the territory, nest patch, and nest site levels, as well as at non-use points, and evaluated the influence of several characteristics on nest success. We evaluated nest patch and territory attributes using binomial generalized linear models and nest success with a generalized linear mixed-effects model. Cerulean Warblers selected nest patches characterized by taller trees and located on ridges and valleys rather than on mid-slopes, while males selected territories that were located on steeper northeast-facing slopes closer to roadways than random points. Successful nests were placed lower in the nest tree and were found in areas with shorter trees; several features selected by Cerulean Warblers at the nest patch and territory levels were associated with lower nest success rates. As elsewhere in the range, white oak was an important nest tree species. In developing forest management strategies, land managers must strive to balance small-scale microhabitat preferences with the need for regenerating oak-hickory forests to sustain Cerulean Warbler breeding populations.

Microsite selection and survival of greater sage-grouse nests in south-central Wyoming

Greater sage-grouse (Centrocercus urophasianus; hereafter, sage-grouse) populations have declined across North America because of degradation of sagebrush (Artemisia spp.) ecosystems; thus, management for sage-grouse habitat is a high priority. Expansion of some energy development has put additional pressures on sage-grouse populations and their habitats. However, little is known about the effects wind energy development may have on sage-grouse populations. Population dynamics of sage-grouse are partially related to nest success, which may be influenced by the quality of nesting habitat. Consequently, knowledge of vegetation and structural characteristics associated with selection and survival of sage-grouse nests would help inform management decisions to improve nest success. As part of a study quantifying greater sage-grouse demographics prior to construction of a wind energy facility, we monitored female sage-grouse equipped with 30-g solar Argos global positioning system platform transmitter terminals during the breeding season in Carbon County, Wyoming to assess selection and survival of nests at nest area (30-m radius), nest patch (5-m radius), and nest bowl scales. Sage-grouse selected nest sites with increased visual obstruction from 22.9–45.7 cm in height at all scales, less forb cover at the nest area, and more sagebrush cover at the nest patch. Survival of nests to 27 days (i.e., incubation period) was 43.3% (95% CI: 31.8–59.1%) in 2011 (n = 49), 33.2% (95% CI: 21.5–51.3%) in 2012 (n = 40), and 25.1% (95% CI: 15.0–42.0%) in 2013 (n = 39). Grass height had a positive association with survival of nests at the nest area and nest patch, canopy cover of bunchgrasses and sagebrush had an interactive effect on survival of nests at the nest patch, and visual obstruction from 0–22.9 cm in height had a positive relationship with survival of nests at the nest bowl. The standardized precipitation-evapotranspiration index (SPEI), an index of drought, from the previous growing season was also negatively correlated with survival of nests. Our results suggest management for nesting sage-grouse should focus on a heterogeneous compilation of tall bunchgrasses >15 cm and patches of sagebrush cover >30%. Management for grasses >15 cm tall should be especially diligent during drought years, given SPEI was associated with nest survival the following year. Although there could be other factors important to selection or survival of nests that we did not evaluate, our results suggest scale-dependent management strategies at the microsite level may not be necessary. We encourage researchers and managers to use our baseline results in a before–after control-impact design with future, post-construction data to determine the potential effects of wind energy development on selection and survival of sage-grouse nests. © 2016 The Wildlife Society.

Habitat selection, nest survival, and nest predators of Rusty Blackbirds in northern New England, USA

ABSTRACT Rusty Blackbird (Euphagus carolinus) populations have plummeted since the mid-20th century. Recent research in New England, USA, suggests that an ecological trap, created through timber harvesting on the breeding grounds, may be responsible. Red squirrels (Tamiasciurus hudsonicus) were hypothesized to be the primary nest predator, but definitive identification was lacking. The potential for mast cone crops to affect Rusty Blackbird nest predation via trophic interactions also remains unexamined. Our objectives were to identify the mechanisms by which an ecological trap may be operating in New England through a multiscale analysis of Rusty Blackbird habitat selection and nest survival, as well as predator identification and quantification. We located 72 Rusty Blackbird nests in Maine and New Hampshire in 2011 and 2012, and modeled habitat selection and nest survival as a function of habitat characteristics at the nest patch (5 m) and home range (500 m) scale. We placed camera traps at 29 nests to ...

Hierarchical Habitat Selection and Reproductive Success of the Ringed Warbling-Finch in Chaco Woodlands

Abstract. Breeding birds must choose habitat at a multiple scales, from landscapes to nest sites. It is important to understand how birds make these decisions and the relationships between these choices and breeding success. Over 3 years, we studied nest-site selection of the Ringed Warbling-Finch (Poospiza torquata) in the semiarid Chaco of Argentina at the scales of the landscape, nest patch, and nest site. At the landscape scale, Ringed Warbling-Finches established nesting territories in shrub-grassland exclusively. Within shrub-grassland, nest patches and nest sites selected differed in few attributes of vegetation cover from patches and sites that were available, and those differences varied from year to year. Only in the year when brood survival and breeding success were lowest did features of sites of successful and depredated nests differ clearly. The Ringed Warbling-Finch selects features of nesting habitat at the landscape, nest-patch, and nest-site scales. Implications of choices at the scales ...

Nest-site selection and productivity of Vesper Sparrows breeding in grazed habitats

Livestock grazing in the shortgrass steppe of the Intermountain region of British Columbia may have a negative impact on ground-nesting birds, but evidence of such an impact is lacking. We examined nest- site selection and productivity of ground-nesting Vesper Sparrows (Pooecetes gramineus) across sites with different grazing histories. From 2006 to 2008, we monitored Vesper Sparrow nests and measured vegetation characteristics known to be affected by grazing within nest patches. We used an information-theoretic approach to test the relative importance of grazing-affected vegetation variables as predictors of nest-site selection, nest survival, and nestling condition. Vesper Sparrows selected nest sites with greater cover of late-seral grass species that decrease in occurrence in response to grazing (i.e., decreasers) than was available in random patches in the same territories. Daily nest survival was also lower for nests surrounded by shorter vegetation (odds ratio = 1.12). However, decreaser cover wasnotassociatedwitheitherofthetwoindicesofproductivitymeasured(dailynestsurvivalprobabilityandnestling condition). In addition, vegetation height, although an important driver of success, was not linked with nest-site selection, and no vegetation-cover variable was positively associated with productivity, despite nest concealment being central to our predictions. This suggests that predation risk for nests in areas with shorter vegetation was being elevated through some factor unrelated to concealment. Our results show that grazing reduced both the availability of suitable habitat for and nesting success of Vesper Sparrows, indicating that grazing could pose a threat to population persistence at a broader scale and could potentially contribute to observed declines. Additional research is needed to determine if grazing guidelines in the Intermountain region of British Columbia should be amended, better enforced, or both to prevent regional declines in populations of ground-nesting grassland birds.

Nest-Habitat Selection by the Phainopepla: Congruence Across Spatial Scales But Not Habitat Types

Abstract. Animals select habitat to satisfy life-history requirements, yet few habitat-selection studies consider multiple factors, especially food. Because resources vary in abundance and quality across different spatial scales or habitat types, selected habitat features may also vary, making habitat selection incongruent. Using an information-theoretical approach, we assessed whether Phainopeplas (Phainopepla nitens) select breeding habitat for food abundance or vegetation structure at nest-tree and nest-patch scales in two woodland types, acacia and mesquite, in the Mojave Desert. Habitat selection at the tree and patch scales was congruent: models containing both food (mistletoe) and structure variables had stronger support than models with only food or structure variables. In both woodland types, nest trees were larger and had more food, and in nest patches food abundance, tree density, tree height, and cover were greater than in non-nest patches. However, nests in acacia were higher than those in me...

Habitat Characterization of Breeding Territories of the Western Yellow-Breasted Chat in the South Okanagan, British Columbia, Canada

From 2002 to 2007 we measured breeding territory, nest patch, and nest shrub characteristics for Yellow-breasted Chats (Icteria virens auricollis) in the northern edge of their distribution in British Columbia, Canada. Surveys were done at 94 breeding territories, 238 nest patches, 246 nest shrubs, and at random locations for comparison. Habitat selection by chats was not random, and territories were selected for specific vegetation characteristics. Chat breeding territories were located in lower elevation riparian zones consisting mainly of dense shrubs and early successional water-tolerant trees, with the dominant shrub species being wild rose (Rosa spp.). Chats nested in 12 plant species and 72% of nests were within wild rose, which was also the dominant species in 5 m and 11.3 m radius nest patch plots centered at the nest shrub. Based on georeferenced locations of male chat singing or perching locations, chat breeding territories were on average 0.37 ha (s = 0.27 ha) in size (n = 66).

Facultative nest patch shifts in response to nest predation risk in the Brewer’s sparrow: a “win-stay, lose-switch” strategy?

Facultative shifts in nesting habitat selection in response to perceived predation risk may allow animals to increase the survival probability of sessile offspring. Previous studies on this behavioral strategy have primarily focused on single attributes, such as the distance moved or changes in nesting substrate. However, nest site choice often encompasses multiple habitat elements at both the nest site and nest patch scales. We studied the within-season re-nesting strategy of a multi-brooded songbird, the Brewer's sparrow (Spizella breweri), to determine whether pairs utilized a "win-stay, lose-switch" decision rule with respect to inter-nest distance, nest substrate and/or nest patch characteristics in response to previous nest fate. Pairs moved sequential nest sites slightly farther following nest predation versus success. When inter-nest distance was controlled, however, pairs changed nest patch attributes (shrub height, potential nest shrub density) associated with probability of nest predation to a greater extent following nest predation than success. The strategy appeared to be adaptive; daily nest survival probability for previously depredated pairs increased with greater Euclidian habitat distances between attempts, whereas previously successful pairs were more likely to fledge second attempts when nest sites were similar to those of previous attempts. Our results suggest that nesting birds can use prior information and within-season plasticity in response to nest predation to increase re-nesting success, which may be a critical behavioral strategy within complex nest predator environments. Re-nesting site selection strategies also appeared to integrate multiple habitat components and inter-nest distances. The consideration of such proximate, facultative responses to predation risk may clarify often unexplained variation in habitat preferences and requirements.

Nest-Site Selection by Abbott's BabblersMalacocincla abbottiin Northeastern Thailand

Identifying nesting microhabitat variables associated with nest site choice is important for understanding bird-habitat relationships. We studied nest-site selection at two spatial scales along with structural vegetation variables associated with nesting success of the Abbott's Babbler in north-eastern Thailand. Seventy-nine nests were found during the study. Ninety percent of the nests were placed in rattans (Daemonorops sp. or Calamus sp.). The vegetation and vegetation structure around nest sites (0.01 ha circular plots) were compared with those in areas surrounding the nestsite (4 × 0.01 ha circles), defined as a nest patch, and with randomly selected non-use sites. Nest sites had a significantly higher percentage of foliage cover and a greater cover and abundance of rattans, suggesting that Abbott's Babbler sites its nests in areas with a large number of potential nest sites. There were no clear vegetational differences associated with successful versus failed nests, probably reflecting the diversity of local predators and their search patterns.

Factors Affecting Songbird Nest Survival in Northern Mixed‐Grass Prairie

ABSTRACT Factors associated with the nest survival of mixed‐grass prairie passerines are not well known, especially in the context of contemporary grassland management. We documented the nest survival of clay‐colored sparrows (Spizella pallida), savannah sparrows (Passerculus sandwichensis), and bobolinks (Dolichonyx oryzivorus) in managed prairie in northwestern North Dakota, USA. We used logistic exposure models and an information‐theoretic framework to estimate nest survival and evaluate support for mechanisms (grazing, temporal factors, nest parasitism, nest‐site vegetation, and nest‐patch factors) relevant to nest survival. Survival for the entire nesting interval (23–28 days) was low for clay‐colored sparrow (18.2%), savannah sparrow (15.5%), and bobolink (3.5%). We found support for a cubic effect of nest age; survival of savannah and clay‐colored sparrow nests was greatest during mid‐incubation and least during the mid‐nestling period. Parasitized clay‐colored sparrow and bobolink nests had greater survival rates than nonparasitized nests. Nest survival of clay‐colored sparrows increased with increasing vegetation height and density. For savannah sparrows, nest survival was lower when cattle were present than when cattle were absent. Characteristics of the nest patch did not have strong effects based on model coefficients and confidence intervals, though they appeared in many of the most supported models. Positive effects of vegetation height and density on nest survival of clay‐colored sparrows and negative effects of cattle presence on nest survival of savannah sparrows suggest some detrimental effects of grazing. However, the need to restore and maintain intact prairies likely warrants the continuation of cattle grazing on conservation lands.

Nest-Site Selection and Nesting Success of Cavity-Nesting Birds in High Elevation Forest Drainages

ABssAcr.-We identified habitat features that characterized successful and unsuccessful nest sites, nest sites vs. random sites, and differences among coexisting species of cavitynesting birds for 356 nests in central Arizona. Live and, more commonly, dead quaking aspens (Populus tremuloides) were used for 88% of all nest sites, but aspens constituted only 12% of all trees in random plots (n = 152). Nest patches (11.3-m-radius circle centered on the nest tree) of most bird species contained significantly more aspens and conifers, which provided foraging substrates for cavity-nesting species, than random plots, but the patches had fewer deciduous trees (except aspens). Thus, more potential nest sites and foraging substrates existed in chosen nest patches than in random plots. Failed nests generally were more concealed by foliage and were closer to conifers. Nest success was lower for species with lower nest height. Reduced nest success at lower and more concealed nests may occur because predators are more successful. Nonexcavator species had lower nesting success than excavator species possibly because they nested lower, had greater nest concealment, used older cavities, and tended to have smaller body mass. Received 21 May 1990, accepted 26 November 1990.

Habitat structure mediates predation risk for sedentary prey: experimental tests of alternative hypotheses

1. Predation is an important and ubiquitous selective force that can shape habitat preferences of prey species, but tests of alternative mechanistic hypotheses of habitat influences on predation risk are lacking. 2. We studied predation risk at nest sites of a passerine bird and tested two hypotheses based on theories of predator foraging behaviour. The total-foliage hypothesis predicts that predation will decline in areas of greater overall vegetation density by impeding cues for detection by predators. The potential-prey-site hypothesis predicts that predation decreases where predators must search more unoccupied potential nest sites. 3. Both observational data and results from a habitat manipulation provided clear support for the potential-prey-site hypothesis and rejection of the total-foliage hypothesis. Birds chose nest patches containing both greater total foliage and potential nest site density (which were correlated in their abundance) than at random sites, yet only potential nest site density significantly influenced nest predation risk. 4. Our results therefore provided a clear and rare example of adaptive nest site selection that would have been missed had structural complexity or total vegetation density been considered alone. 5. Our results also demonstrated that interactions between predator foraging success and habitat structure can be more complex than simple impedance or occlusion by vegetation.