Unsuccessful Nests Research Articles

The influence of nest-site characteristics on the nesting success of the Karoo Prinia (Prinia maculosa)

Choice of nest site has important consequences for nest survival. We examined nest-site characteristics relative to nest success in Karoo Prinias breeding in coastal dwarf shrubland, where high nest predation is the main cause of nest failure. Initially, we compared nests that failed during the building, laying, incubation and nestling stages and those from which young were successfully raised, to test whether nests that survived to progressive stages in the nesting cycle differed in their nest-site characteristics. Subsequently, we compared the characteristics of successful nests with those of unsuccessful nests. The nest-site characteristics considered included nest height, nest-plant height, nest-plant species, distance from lateral foliage edge, nest concealment, nest-patch heterogeneity and vegetation cover at four different heights. We were unable to distinguish between the nest-site characteristics of nests that failed during the various stages of the nesting cycle. Concealment was the main nest-site characteristic that differentiated successful nests from unsuccessful nests, with successful nests being located in more concealed sites. The other variables that contributed to the discrimination between successful and unsuccessful nests by discriminant function analysis included nest-plant type and distance from edge, which are also directly related to concealment. This suggests that nest concealment is the most important variable influencing nesting success at this site, which has a preponderance of visually-oriented predators.

Nest Site Characteristics of Eastern Wild Turkeys in Central Ontario

Abstract We studied nest site selection by Eastern Wild Turkeys (Meleagris gallopavo silvestris) to compare successful and unsuccessful nests at the northern edge of the species' range in central Ontario during 2000. Six of 16 (38%) nests that we studied were successful. Successful nest sites had greater percent vertical obstruction than unsuccessful nest sites. However, both percent vertical and horizontal obstructions were the best predictors of nest success, suggesting that no single set of habitat characteristics may offer protection from a diverse predator community.

BREEDING BIOLOGY OF THE CHIPPING SPARROW IN PONDEROSA PINE FORESTS OF THE COLORADO FRONT RANGE

Abstract We present information on the breeding biology and nest-site characteristics of the western subspecies of the Chipping Sparrow (Spizella passerina arizonae) in montane ponderosa pine (Pinus ponderosa) forests of the Colorado Front Range. We located 83 nests during the summers of 1999–2002. The earliest egg date was 23 May and young fledged as late as 11 August. Nests were typically placed in needle clusters on outer branches of ponderosa pines. Mean canopy cover at nest sites was 50%. We observed no differences in nest placement or nest-site microhabitat between successful and unsuccessful nests. The nesting cycle lasted approximately 30 days, including 4 nest-building days, 3 egg-laying days, 14 incubation days, and 9 brooding days. Nests with known outcomes fledged a mean of 1.67 young from a mean clutch of 3.08 eggs. At least 27 nests (32.5%) were depredated, making predation the primary cause of nest failure. Of 42 successful nests, 5 (11.9%) were parasitized, although at least two Chipping S...

Nest site selection and nesting success of five species of passerines in a South American open Prosopis woodland

I analyzed nest site preferences and whether these preferences were linked to nest success of five bird species breeding in an arid area of southern South America. Most nests (90%) were located in three plant species (Geoffroea decorticans, Capparis atamisquea, and Atriplex lampa). Serpophaga griseiceps, Poospiza ornata, and P. torquata nested in plants with size significantly different from randomly selected plants. At the mesohabitat scale (i.e., habitat patch surrounding the nest), four species showed clear differences from random patches within the general habitat, and the remaining species (Saltatricula multicolor) showed a non-significant trend. The daily survival rate of S. griseiceps nests placed in the preferred plant species (i.e., G. decorticans) was lower than those in the other nest-plants. In the other four bird species, nest survival did not differ between preferred and other plants used for nesting. At the scale of microhabitat (i.e., plant size and location of the nest inside the plant), no significant differences were detected between unsuccessful and successful nests. Similarly, unsuccessful nest patches did not differ significantly in habitat features from those that were successful. High nest predation rates found in this habitat (around 85% for the total assemblage) and a relatively high diversity of predators (and of predation tactics) appear to impede the presence of “safe” sites for nesting (i.e., there are trade-offs between avoidance strategies).

Greater sage‐grouse Centrocercus urophasianus nesting success and habitat use in northeastern California

From mid‐March through mid‐August 1998–2000, we studied greater sage‐grouse Centrocercus urophasianus nesting habitat in northeastern California, USA. We located nest sites of 45 radio‐marked hens, which had an average nest success of 40.2%. The radio‐marked grouse used low sagebrush Artemisia arbuscula cover type less than expected; big sagebrush A. tridentata wyomingensis and mixed shrub cover types were used in proportion to their availability. Grouse used sites with habitat characteristics similar to random sites for nesting. However, successful nests differed from unsuccessful nests in several respects. Mean distance between nest and lek was greater for successful nests ( = 3,588 m, SE = 811 m, N = 20) than for unsuccessful nests ( = 1,964 m, SE = 386 m, N = 20). Rock cover was greater at successful nests ( = 27.7%, SE = 4.6%) than at unsuccessful nests ( = 14.49%, SE = 3.04%). Total shrub height was greater at successful nests ( = 65.5 cm, SE = 4.7) than at unsuccessful nests ( = 49.2 cm, SE = 1.7). The height of visual obstruction was greater at successful nests ( = 40.2 cm, SE = 2.6) than at unsuccessful nests ( = 32.5 cm, SE = 2.0). Our results suggest that sage‐grouse use more diverse vegetation than previously reported, and we conclude that either this represents a natural behaviour for sage‐grouse in this area, or we observed a selection response to a landscape altered by human activity.

Use of nest boxes by Vaux's Swifts

The use of nest boxes by Vaux's Swifts (Chaetura vauxi) was investigated because of the loss of large-diameter hollow trees used for nesting in northwestern North America. Nest attempts and nest success were compared in nest boxes among three habitat types in northeastern Oregon. Air temperatures in the boxes were monitored to determine if boxes overheated and resulted in nestling mortality. Thirty of 103 nest boxes were used by nesting Vaux's Swifts for at least one year during 1999–2002. Of 51 nest attempts, 53% successfully fledged 1–6 young (x̄ = 3.5 nestlings). Over this 4-yr period, 47% of the nests were in late-seral stage grand fir (Abies grandis) stands, 31% were in harvested stands of grand fir, and 22% were in ponderosa pine (Pinus ponderosa) stands. It appeared that temperature did not affect nest success in boxes because there were no significant differences in maximum and minimum temperatures between successful and unsuccessful nest attempts. Nest boxes provided alternative nest sites for Vaux's Swifts in habitats traditionally used by this species as well as in habitats that currently lack natural nest sites.

NEST SUCCESS AND HABITAT SELECTION OF THE SEMIPALMATED PLOVER ON AKIMISKI ISLAND, NUNAVUT

We studied nest site selection by Semipalmated Plovers (Charadrius semipalmatus) to compare microhabitat characteristics at nest and random sites, and to compare successful and unsuccessful nests on the northern shore of Akimiski Island, Nunavut, during 2002. Nesting birds selected sites with more pebbles and less vegetative cover than randomly available in the environment. Nest sites also had smaller percentage of bare mud than random sites. Plovers selected sites within 100 m of Arctic Terns (Sterna paradisaea) more often than expected based on the distribution of random sites in the study area. Twenty-three of 41 (56%) nests hatched successfully. None of the microhabitat features that we measured predicted nest success. All 10 nests near the colony of Arctic Terns hatched, suggesting that interspecific associations are more reliable than habitat features for predicting nest success.

Wood Thrush (Hylocichla Mustelina) Nesting Ecology in Relation to Prescribed Burning of Mixed-Oak Forest in Ohio

Abstract Prescribed burning is increasingly being used to restore and maintain oak-dominated (Quercus spp.) forests in the eastern United States. We assessed effects of prescribed burning on the nesting ecology of the Wood Thrush (Hylocichla mustelina). Recent declines in Wood Thrush populations have prompted concern about their conservation status. Low-intensity surface fires in mixed-oak forests resulted in reductions in midstory vegetation, a documented habitat requirement for Wood Thrushes, but local population levels of Wood Thrushes did not differ between burned and unburned areas. Wood Thrushes inhabiting recently burned areas selected nest sites where leaf litter cover, fern cover, densities of shrubs and saplings, and moisture levels were higher and where fire intensity was lower in comparison to random sites. Wood Thrushes also placed their nests higher off the ground, and in taller and larger-diameter trees and shrubs, in burned than in unburned areas. Reproductive success did not differ between burned and unburned areas. However, successful nests were placed higher off the ground and in areas with lower densities of shrubs and saplings than unsuccessful nests in both burned and unburned areas. Prescribed burning appeared to have minimal effects on nesting ecology of Wood Thrushes, given their flexibility in nest placement, with no adverse consequences in terms of reproductive success. Local variation in fire intensity and moisture levels also maintained availability of suitable nesting habitat within burned areas. Continued monitoring would be appropriate to further assess the response of Wood Thrushes to prescribed burning, particularly in consideration of their conservation status and the uncertainty associated with potential long-term effects of habitat change.

Nest site selection and predation driven despotic distribution of breeding long‐eared owls Asio otus

Long‐eared owls Asio otus do not build their own nests. They occupy old corvid nests in either coniferous or deciduous trees. In my study area owls favoured coniferous over deciduous trees for nesting and occupied nest sites in that order. Comparing overall breeding output, I found no difference between the two kinds of nest sites, but among early nests there was a higher proportion of unsuccessful nests in deciduous trees. This difference was due to differences in predation rate, indicating that the owls' pattern of nest site occupation followed predictions from the ideal despotic distribution model. I also show that, due to a seasonal change in habitat quality, predator driven despotism, although present, can be well masked and hence difficult to detect.

Direct Identification of Northern Sage-grouse, <em>Centrocercus urophasianus</em>, Nest Predators Using Remote Sensing Cameras

The status and apparent decline of Sage-grouse (Centrocercus spp.) has been of increasing concern and lower nesting success could be contributing to population declines. Our objective was to directly identify Sage-grouse nest predators. Following visual confirmation of radio-marked Sage-grouse nest establishment in 1997-1999, we installed automatic 35 mm cameras controlled by an active infrared monitor. Of 26 nests monitored by cameras, 22 successfully hatched and four were unsuccessful. American Elk (Cervus canadensis), Badger (Taxidea taxus), and Black-billed Magpie (Pica hudsonia) destroyed three of the four unsuccessful nests, and domestic cattle caused abandonment of the fourth. Richardson’s (Spermophilus richardsonii) and Thirteen-lined Ground Squirrels (S. tridecemlineatus) were recorded at nests, but were not detected in predation.

The Use of Prickly Pear Cactus as Nesting Cover by Northern Bobwhites

Years of research on the nesting ecology of the northern bobwhile (Colinus virginianus) have documented the use of bunchgrasses as common nesting cover. However, in the Rolling Plains of Texas, recent studies indicate a relatively high use of prickly pear cactus (Opuntia spp.) as suitable nesting cover. To explain this previously unreported use of prickly pear, we evaluated 2 hypotheses: (1) the limited-bunchgrass, and (2) nest-protection hypotheses. We speculated that bobwhites nested in prickly pear cactus because of low availability of bunchgrasses and/or structural protection against predators. We radioinarked 218 bobwhites during spring and summer of 1997 and 1998 on 4 sites in Shackelford County, Texas, USA. Bobwhites nested in prickly pear despite adequate bunchgrass cover (>600 potential nesting sites/ha). Of 81 nests located, 12% were in shrubs (e.g., catclaw [Acacia greggii]), 30% were in prickly pear, and 58% were in bunchgrass. Nest success did not differ among nesting substrates (X 2 = 2.1, df= 2, P= 0.35) but was dependent on the degree of protection provided by the nesting substrate (X 2 = 16, df= 2, P= 0.001). However, successful nests also had a higher density of bunchgrasses in the surrounding arca than unsuccessful nests (P = 0.001). Our data partially supported the nest-protection hypothesis and did not support the limited-bunchgrass hypothesis.

Lark Sparrow (Chondestes Grammacus) Nest-Site Selection and Success in a Mixed-Grass Prairie

Abstract Lark Sparrows (Chondestes grammacus) are declining throughout most of their range. Effective management for this species is hampered because relatively little is known about nesting ecology. We studied habitat characteristics affecting Lark Sparrow nest-site selection and nest success at nine study pastures in a southern mixed-grass prairie in Oklahoma. We used a neural-network technique to discriminate between nest and random locations, and bootstrapping with 95% confidence intervals to compare habitat features of successful and unsuccessful nests. We quantified habitat features at the nest and random points during the breeding seasons of 1999 and 2000 among three grazing treatments (control, moderate, and heavy). We located 40 nests during two years of the study, for which crude nest-success was 26.3%. Most nests were located in either moderately grazed pasture (55%) or heavily grazed pasture (40%). The neural model correctly identified nest and random points 91% of the time. Percentage of structural cover, distance to nearest structural element, bare-ground exposure, and percentage of litter cover were the most important nest-site selection criteria according to the model. Simulation analysis indicated points were classified as nest sites if they were <270 cm from structural elements, <87% bare-ground exposure, <74% litter cover, and >9% structural cover. Successful nests had less bare-ground exposure (x̄ = 6.2 ± 1.9% [SE]) and more litter cover (x̄ = 18.0 ± 4.6%) compared to unsuccessful nests (x̄ = 17.5 ± 3.8% and 10.1 ± 1.6%, respectively). These results suggest that habitat management for Lark Sparrows in mixed-grass prairie should focus on creating abundant structural cover with moderate levels of litter accumulation and bare ground.

Long-Term Productivity of Canvasbacks (Aythya Valisineria) in a Snowpack-Driven Desert Marsh

Abstract Ruby Lake, Nevada, is a large palustrine wetland that hosts the southern-most major breeding population of Canvasbacks (Aythya valisineria). That arid marsh, fed by springs derived from mountain snowpack, differs in climate and hydrology from glaciated potholes of the northern prairies where most Canvasbacks breed. Fourteen years of nesting data on Canvasbacks over a 31 year period (1970–2000) were analyzed to determine factors affecting breeding performance at Ruby Lake and whether they differed from those in the prairies. Long-term Mayfield nest success at Ruby Lake (50% of all nests) was in the range of that in the northern prairies (21–65%). Of all Canvasback nests, 73% were parasitized (mostly by Redheads [Aythya americana]) as compared to 83–97% in a large Manitoba marsh and 57–65% in Manitoba potholes. However, as in the northern prairies, nest parasitism generally had little or no effect on either nest success or percentage of host eggs that hatched. In Manitoba potholes, nest success was unrelated to habitat variables measured; but successful nests at Ruby Lake were over shallower water, farther from shore, in wider bands of emergent vegetation, and surrounded by lower stem densities than unsuccessful nests. Water level is the key factor in breeding performance of Canvasbacks at both Ruby Lake and the northern prairies; however, the source of water differs (mountain snowpack at Ruby Lake, direct precipitation in the prairies) and effects of water-level variations are reversed. In small prairie potholes (mostly <0.4 ha) with many mammalian predators, productivity of Canvasbacks (which build floating nests) is increased by high water that floods the emergent fringe. At Ruby Lake, a very large marsh (2,830 ha) with mostly avian predators, Canvasback productivity is decreased by high water that floods interior emergent stands too deeply. Water level at Ruby Lake was highly correlated (multiple R2 = 0.91) with mountain snowpack up to three years earlier, emphasizing the strong effect of climatic variations on wetland birds in that arid region.

SURVIVAL AND HABITAT OF RUFFED GROUSE NESTS IN NORTHERN MICHIGAN

Abstract Effective management of Ruffed Grouse (Bonasa umbellus) populations requires a full understanding of chick production. Previous reports of nest survival for Ruffed Grouse are biased because they did not account for successful nests being more likely to be found, and the role of habitat quality in determining nest survival is unknown. We determined survival rates of Ruffed Grouse nests in northern lower Michigan using the less biased Mayfield estimator, defined differences between first and second nests, and compared the local habitat characteristics of successful and unsuccessful nests. Median hatching dates were 10 June for first nests (n = 34) and 1 July for second nests (n = 6). First nests had a lower survival rate (0.442, 95% CI = 0.270–0.716), a higher mean clutch size (12.7 eggs ± 0.3 SE), and higher egg hatching rate (0.960, 95% CI = 0.900–0.997) than did second nests (nest survival = 0.788, 95% CI = 0.491–1.00; clutch size = 7.3 eggs ± 0.3 SE; and hatching rate = 0.826, 95% CI = 0.718–0....

LARK SPARROW (CHONDESTES GRAMMACUS) NEST-SITE SELECTION AND SUCCESS IN A MIXED-GRASS PRAIRIE

Abstract Lark Sparrows (Chondestes grammacus) are declining throughout most of their range. Effective management for this species is hampered because relatively little is known about nesting ecology. We studied habitat characteristics affecting Lark Sparrow nest-site selection and nest success at nine study pastures in a southern mixed-grass prairie in Oklahoma. We used a neural-network technique to discriminate between nest and random locations, and bootstrapping with 95% confidence intervals to compare habitat features of successful and unsuccessful nests. We quantified habitat features at the nest and random points during the breeding seasons of 1999 and 2000 among three grazing treatments (control, moderate, and heavy). We located 40 nests during two years of the study, for which crude nest-success was 26.3%. Most nests were located in either moderately grazed pasture (55%) or heavily grazed pasture (40%). The neural model correctly identified nest and random points 91% of the time. Percentage of stru...

Habitat and Nesting Success of Blue Jays (Cyanocitta Cristata): Importance of Scale

Abstract Nesting success of birds often is influenced by habitat features surrounding nests. However, few studies have investigated habitat influences at multiple levels and spatial scales. We examined the relationship between Blue Jay (Cyanocitta cristata) nesting success and habitat at three levels (macrohabitat, microhabitat, and nest site) and at three different scales within the microhabitat level (3.1, 0.3, and 0.07 ha circular plots surrounding nests) in central Florida. We defined successful nests as those fledging at least one offspring. An eight-fold difference in probability of nesting success existed among seven distinct macrohabitats, with Mayfield success estimates ranging from 8 to 63%. However, no difference in number of fledglings per nest existed among macrohabitats when unsuccessful nests were excluded from the analysis. Macrohabitats in which nests were more often successful had more extensive slash pine (Pinus elliottii) forest. Logistic regression analyses indicated that slash pine canopy cover was significantly related to nesting success at all three microhabitat scales, although the tree species in which a nest was placed was unimportant to nesting success. Other microhabitat and nest-site features were unrelated to nesting success. Strong circumstantial evidence indicates predation was the major cause of nest failure, but relative abundance of one potential predator, the gray squirrel (Sciurus carolinensis), was not related to nesting success in macrohabitats or slash pine canopy cover within microhabitats. In this study, logistic regression models using habitat features measured at higher levels and larger scales had better predictive power, indicating that habitat may have influenced predation at scales larger than that of a typical nest patch.

Evaluating the Efficacy of Manipulating Cowbird Parasitism on Host Nesting Success

We studied brown-headed cowbird (Molothrus ater) parasitism on the Arizona Bell's vireo (Vireo bellii arizonae) and the yellow-breasted chat (Icteria virens) to determine if removing adult and nestling cowbirds and addling cowbird eggs resulted in increased host nesting success. Our investigation took place in Arizona along the lower Colorado River at Bill Williams River National Wildlife Refuge. At 3 plots adult cowbirds were removed by trapping (Treatment A); at 3 plots adult cowbirds were removed and cowbird eggs were addled (Treatment B); and at 3 plots no cowbird control was implemented (Reference plots). A significantly greater proportion of vireo nests were parasitized on Reference plots than either treatment type across the study. The proportion of successful vireo nests was not significantly greater across Reference and Treatment plots during 1997, but was significantly greater in 1998. The proportion of successful and unsuccessful nests did not significantly vary across treatment types for the chat in either year. There was a lower proportion of singly and multiply parasitized nests on Treatment plots compared to Reference plots during 1998, but not during 1997. Predation was the leading cause of nest failure for Bell's vireos and yellow-breasted chats in 1997 and 1998.

The effects of selective logging on nest-site selection and productivity of hooded warblers (Wilsonia citrina) in Canada

We examined nest-site selection by hooded warblers (Wilsonia citrina) in two forests differing in composition (extensive pine plantation versus largely deciduous) and management (recent selective logging versus minimal logging). We measured habitat at 52 nests and 66 controls in one forest and 57 nests and 41 controls in the second. Nests had denser ground vegetation, fewer tree stems, less basal area due to small trees and greater basal area due to large trees than controls in both forests. In the managed forest with extensive pine plantations, hooded warblers were in coniferous and logged stands more often than expected by chance, nests had more stumps and greater canopy height than controls, and 91% (52/57) of all nests were found in gaps, 84% of which were created by harvest. Gap age was 6.2 ± 0.9 years (mean ± SE) in 1999, and 7.6 ± 0.6 years in 2000. In the managed forest, 52% of 24 nests were parasitized in 1999 and 39% of 33 nests were parasitized in 2000. The daily survival rate was 0.936 ± 0.019 in 1999 and 0.944 ± 0.014 in 2000. Successful nests did not differ from unsuccessful nests in any habitat variable. To benefit hooded warblers, forest managers should mimic natural gap creation by using selective logging to create gaps no larger than 0.05 ha, and should leave a residual basal area of mature trees (>38 cm diameter at breast height) of at least 12 m2/ha.

NEST SITE SELECTION AND NEST SUCCESS IN A SONG SPARROW POPULATION: THE SIGNIFICANCE OF SPATIAL VARIATION

Abstract I tested the hypothesis that individual Song Sparrows (Melospiza melodia) choose nesting microhabitat that reduces their risk of nest predation, using two years of data (n = 196 nests) from two adjacent sites in California coastal scrub, one grazed and one ungrazed. Nesting habitat was compared between nest patches and random locations, and between successful and unsuccessful nests. In both sites, nest patch habitat differed significantly in structure and plant species composition from habitat available within territories. However, of six habitat characteristics associated with nest patch choice, only two were related to nest success, and that relationship differed between the two study sites. Only in the grazed site was the amount of coyote bush (Baccharis pilularis) in the nest patch positively associated with both nest site selection and nest success. In contrast, coyote bush was unrelated to nest site choice and had a nonlinear relationship with nest success in the ungrazed site, such that ne...

NEST PREDATORS, NEST-SITE SELECTION, AND NESTING SUCCESS OF THE DUSKY FLYCATCHER IN A MANAGED PONDEROSA PINE FOREST

Among hypotheses explaining nest-site selection, the potential-prey-site hypothesis predicts that birds place nests in areas with many potential nest sites, while the nest-concealment hypothesis predicts nest placement in sites with greater surrounding vegetation. We examined these hypotheses by comparing habitat attributes between Dusky Flycatcher (Empidonax oberholseri) nest sites and random sites, successful and unsuccessful nests, and nests depredated by birds versus mammals. Nesting success was 28% (n = 167), and predation was the most important cause of nest failure (96%) during the study (1998–2000) at our site in northeastern California. We identified 28 nest predators using surveillance cameras. Sciurid mammals were the most common predators (17 of 28, 61%), with Cooper's Hawks (Accipiter cooperii; 25%), small owls (11%), and Steller's Jays (Cyanocitta stelleri; 3%) accounting for the rest. Raptors preyed on nestlings more frequently than on eggs, while small mammals depredated nestlings and eggs in proportion to their availability. Dusky Flycatchers nested in larger shrub patches with greater shrub cover than at random sites, supporting the potential-prey-site hypothesis. Successful nests were in larger shrub patches surrounded by fewer seedlings and saplings compared to depredated nests, providing equivocal support for the potential-prey-site hypothesis. Nests depredated by birds were less concealed, located in smaller shrub patches, surrounded by fewer seedlings and saplings, and were closer to the shrub edge than nests depredated by mammals. Our results, strengthened by identification of nest predators, suggest that differences in search strategies among predators may constrain the ability of Dusky Flycatchers to optimize nest-site selection. Depredadores de Nidos, Selección de Sitios de Anidación y Éxito de Anidación de Empidonax oberholseri en un Bosque Manejado de Pino Ponderosa Resumen. Entre las hipótesis que explican la selección de sitios de anidación, la de “presa potencial” predice que las aves ubican sus nidos en lugares con muchos sitios de anidación, mientras que la de “ocultación del nido” predice la ubicación de nidos en sitios con mayor vegetación circundante. Nosotros examinamos estas hipótesis comparando características del hábitat entre sitios de anidación del mosquerito Empidonax oberholseri y sitios aleatorios, entre nidos exitosos y no exitosos, y entre nidos depredados por aves y por mamíferos. Entre 1998 y 2000 en un área del noreste de California, el éxito de anidación fue del 28% (n = 167) y la depredación fue la causa más importante del fracaso de los nidos (96%). Identificamos 28 depredadores de nidos utilizando cámaras. Los mamíferos sciúridos fueron los depredadores más comunes (17 de 28, 61%), mientras que los demás nidos fueron depredados por Accipter cooperii (25%), búhos pequeños (11%) y Cyanocitta stelleri (3%). Las aves rapaces se alimentaron con mayor frecuencia de pichones que de huevos, mientras que los mamíferos pequeños depredaron pichones y huevos en proporción a su disponibilidad. Los mosqueritos anidaron en parches con mayor cobertura de arbustos que sitios aleatorios, lo cual apoya la hipótesis de presa potencial. Los nidos exitosos se ubicaron en parches de arbustos más grandes rodeados por menos plántulas y renovales en comparación con los nidos depredados, lo que apoya equívocamente la hipótesis de presa potencial. Los nidos depredados por aves estuvieron menos ocultos, ubicados en parches de arbustos más pequeños, rodeados por menos plántulas y renovales, y estuvieron más cerca del borde del matorral que los nidos depredados por mamíferos. Nuestros resultados, fortalecidos por la identificación de los depredadores, sugieren que las diferencias en las estrategias de búsqueda entre depredadores pueden limitar la habilidad de los mosqueritos para optimizar la selección de sitios de anidación.