Bell's Vireo Research Articles

The Prado Wetlands: 24 years of river diversion treatment wetlands reducing nitrate and phosphate while increasing an endangered bird

Nitrate pollution of surface water from farms and urban runoff is widespread – impairing drinking water supplies, recreation, and wildlife habitat. The scale of the problem in rivers has overwhelmed most watershed-based best management practices. A new paradigm is needed. In this paper we highlight the 119 ha (294 acres) Prado Wetlands in southern California, USA as an example of a gravity-enabled, free surface flow vegetated wetlands to treat a river – the nitrate-rich, effluent-dominated Santa Anna River. The wetland treats ∼1–2 m3/s of the 7.1 m3/s (250 cfs) flow of the river in 28 wetland cells in two parallel trains (residence time 2–4 d, hydraulic loading rate 10–20 cm/d). The cleaner water is returned to the river and infiltrated downstream into a managed groundwater basin supplying 2.4 million customers. Since 1998, ∼1200 t of nitrate-N have been permanently removed from the river and released as N2-gas into the air. Over 24 years, in the warm summers an average of 6.1 mg-N/L entered the wetland and 3.7 mg-N/L flowed out, a removal efficiency of 57%. In the cooler winters, an average of 6.4 mg NO3-N/L entered and 2.6 mg NO3-N/L flowed out, a removal efficiency of 45%. Individual months varied widely in concentration removal efficiency from 7% (winter) to 98% (high summer). Areal summer removal averaged 137 mg NO3-N/m2/d and the apparent “k” was around 46 m/y. All-year rates were 129 mg NO3-N/m2/d or 47 g/m2/y, a reasonable performance considering the size of the system and its configuration for both nitrate removal and duck hunting (i.e., more open water). Annual average nitrate removal efficiency increased substantially over time (45 to 72% for 1998–2004 versus 2016–22) and is attributed to an increase in hydraulic residence time, upgraded internal plumbing, and increased winter temperatures (+3C°). Prado Wetlands also removes phosphorus, though not as well as nitrate; average warm season removal efficiency was 23% (inflow 1.20 mg PO4-P/L, outflow 0.93 mg PO4-P/L). The aerial removal rate was high at 23 mg P/m2/d or 8.5 g/y, probably due to the high inflowing P-concentrations and the hard water. Wildlife was also enhanced. Least Bell's Vireo (Vireo belli pusillus), an endangered bird in the Prado Basin, increased from 19 pairs (1998) to a recent average of 444 pairs due to habitat improvement and reduction of cowbird parasitism. Results from the Prado Wetlands show that cleaning up rivers via diversion through wetlands benefits both water quality and wildlife.

An efficient acoustic classifier for high‐priority avian species in the southern Great Plains using convolutional neural networks

AbstractPassive acoustic monitoring is a valuable ecological and conservation tool that allows researchers to collect data from vocal species across large geographic areas and temporal spans. Grassland bird populations, many of which are indicators of ecosystem health, have experienced precipitous declines over the past several decades. Acoustic monitoring of grassland bird populations provides opportunities to monitor declines and focus conservation practices, yet the ability to identify species efficiently and accurately from acoustic data is challenging. Therefore, development of automated classifiers such as convolutional neural networks (CNNs) are at the forefront of streamlining detection and identification of individual species. Here, we present a CNN classifier for 5 key grassland bird species across southcentral Oklahoma, a part of the southern Great Plains: northern bobwhite (Colinus virginianus), painted bunting (Passerina ciris), dickcissel (Spiza americana), eastern meadowlark (Sturnella magna), and Bell's vireo (Vireo bellii). We compiled a high‐quality training dataset consisting of 6,933 calls, built semiautonomously using template matching that can be expanded easily to any bird species of interest. Our trained multilabel CNN achieved a high level of classification accuracy (≥98%) for the 5 species using the library of test calls and field recordings played using a programmable game caller. The ability to conduct acoustic wildlife surveys across large spatial extents will allow for more efficient monitoring of wildlife to determine key population parameters and trends and effects of biotic and abiotic factors (e.g., vegetation, disturbance, weather) on these key species.

Developing an adaptive management approach to cowbird control on the Santa Clara River, California

AbstractBrood parasitism by the brown‐headed cowbird (Molothrus ater) has caused population declines for conservation‐reliant songbird species such as the federally and state endangered subspecies least Bell's vireo (Vireo bellii pusillus). Although cowbird trapping has increased vireo populations, it is costly, and may prevent the development of parasitism‐avoidance behaviors and result in high rates of non‐target species captures. Therefore, wildlife managers have a compelling interest in evaluating multiple means of cowbird control. We conducted a study between 2016 and 2018 on the Santa Clara River (SCR) in Ventura County, California, USA, to test how the removal of cowbird traps would affect cowbird parasitism of the vireo. At our control site, cowbird trapping occurred in spring using modified Australian crow (Corvus spp.) traps. At our treatment site, we stopped this method of trapping and instead used target‐netting of individual cowbirds and a very short duration trap to capture specific individual cowbirds (i.e., contingency trapping). We also tested the impact of shortening the cowbird trapping period on nest parasitism rates, and used point counts to compare cowbird densities between sites. Additionally, we compared the costs of these techniques. A shorter trapping period still resulted in a large number of cowbird captures on the control site (n = 452 captures) with no observed brood parasitism (n = 67 nests). On the treatment site, cowbird parasitism rates varied from 0% to 14% (n = 75 broods). The target‐netting method was not effective for capturing female cowbirds, probably because of their low densities. The short‐term contingency trapping method effectively removed female cowbirds in the vicinity of vulnerable vireo nests. Cowbird traps captured >300 individuals of 9 different non‐target species each year on the control site, whereas target‐netting and contingency trapping resulted in only 8 captures of 4 different non‐target species. Costs of continuous trapping for 2 months versus point counts, nest monitoring, and contingency trapping were $21,100.00 versus $16,996.00, respectively. Based on these findings, we recommend that an adaptive cowbird management program to benefit songbird populations should consider using regular point counts and nest monitoring to provide data on cowbird impacts, in lieu of continued intensive trapping without monitoring. Thresholds for the density of cowbird females and the allowable level of parasitism for host species should also be established to guide an adaptive cowbird management program. Short‐term contingency trapping based on the thresholds should be considered, but restoration of native vegetation, especially in southwestern riparian areas, is also crucial and effective at limiting parasitism, and so should strongly be considered. Lastly, large numbers of non‐target species captures in cowbird traps are probably having consequential impacts to these species, and need to be monitored and minimized.

Does Conservation Practice and Site Age Influence Vegetation Structure and Avian Abundance in Restored Fields?

ABSTRACTFarmland set‐aside programs provide important habitat for many wildlife species, yet little information exists regarding how vegetation structure and species respond to conservation practice and site age. This information could provide managers with a guide for how to implement, enhance, and maintain wildlife benefits of conservation programs. We describe how vegetation structure and avian species respond to conservation practice and time since restoration at 172 sites enrolled in the Conservation Reserve Enhancement Program (CREP) in Illinois, USA. We surveyed 172 sites enrolled in 4 different conservation practices (CP) within CREP during the breeding seasons of 2012–2015 using avian point counts and vegetation surveys. Vegetation structure and composition varied among CPs, with hardwood tree plantings (CP3A) having the greatest amount of understory vegetation and tree cover. Conversely, permanent wildlife habitat (CP4D) had the greatest grass cover and least tree cover. Many bird species were found in similar numbers across CP types; however, only dickcissel (Spiza americana) density was greater in sites enrolled as permanent wildlife habitat. Bell's vireo (Vireo bellii) and yellow‐billed cuckoo (Coccyzus americanus) were more numerous in hardwood tree plantings. Dickcissel density decreased and field sparrow (Spizella pusilla) density increased as fields aged, but the relationships were not consistent among CP types. Differences among CPs largely resulted from differences in dominance in woody vegetation due to initial habitat management and differential succession rates. Interestingly, many of our focal species had wider successional tolerances than previously suggested. Overall, our results demonstrate that conservation benefits change over time depending on the starting CP, that there can be considerable similarity in vegetation structure and bird communities among what appear to be very different conservation practices. This information can be used to target conservation benefits toward conservation priority species. © 2020 The Wildlife Society.

Cost-share conservation practices have mixed effects on priority grassland and shrubland breeding bird occupancy in the Central Hardwoods Bird Conservation Region, USA

Globally, agricultural ecosystems continue to be the most imperiled for biodiversity. Subsequently, countries focused on cost-share conservation programs whereby private landowners are paid to conserve or restore their land. In the United States, populations of grassland and shrubland breeding birds continue to decline, despite state, federal, and private cost-share conservation programs. We related avian occupancy to cost-share conservation programs across the Central Hardwoods Bird Conservation Region using spatially explicit patch-scale survey data. Once per year from 2008 to 2012, we surveyed 5303 discreet points for 9 grassland and shrubland breeding bird species using roadside 5-min point-count surveys in 37 counties across 7 states. We modeled point-specific land cover and heterogeneity in conjunction with cost-share conservation program information using a multi-season robust-design occupancy model. Land-cover covariates were most important in models for species occupancy. For every species except Bell's vireo (Vireo bellii), Field sparrow (Spizella pusilla), and Prairie warbler (Setophaga discolor), addition of conservation covariates to best-supported land-cover models improved occupancy model fit, though effects of conservation programs on occupancy were mixed. Eastern meadowlark occupancy was negatively correlated to the presence of conservation. Alternatively, Northern bobwhite (Colinus virginianus) occupancy was 200% greater when a conservation practice was at a point rather than being located 4 km away. If cost-share conservation is going to reverse biodiversity declines in agricultural ecosystems, then scale-appropriate models will need to be used to prioritize future implementation because the status quo provides limited mixed effects on grassland and shrubland bird species occupancy.

Avian fitness consequences match habitat selection at the nest-site and landscape scale in agriculturally fragmented landscapes.

Habitat selection theory suggests that when choosing breeding sites, animals should choose the best available habitat; however, studies show that individuals fail to choose habitats that maximize their fitness especially in drastically altered landscapes. Many studies have focused on selection at single scales, often using a single measure of fitness. However, links between habitat selection and fitness may vary depending on the spatial scale and measure of fitness, especially in situations where agricultural land use has altered the surrounding landscape.We examined multiscale habitat selection and fitness measures of the Bell's Vireo (Vireo bellii) and Willow Flycatcher (Empidonax traillii) using data collected in agriculturally fragmented landscapes.We found evidence for selection of nest sites with dense understory, larger patches, and increasing restored habitat cover and decreasing forest cover in the surrounding landscape.For both focal species, selection for dense understory significantly increased nest survival; however, there appeared to be no concurrent benefit for fledgling production. Selection for broader scale features positively influenced nest survival for the Willow Flycatcher with no concurrent benefit for fledgling production. The observed mismatches may be due to anthropogenic habitat fragmentation at broader scales or may represent reproductive trade‐offs for a fitness benefit not measured in this study.Fine‐scale habitat selection decisions by our focal species appear to match fitness outcomes, whereas habitat selection at broader scales only provided fitness benefits for the Willow Flycatcher. Though providing no fledgling production benefit, when combined with suitably dense nesting habitat, larger patches in landscapes with greater amounts of restored habitat cover for Bell's Vireo and lower amounts of forest cover for Willow Flycatcher will produce more fledglings per unit area than smaller patches in landscapes with less restored habitat and more forest cover, respectively, which could help inform conservation decisions for these at‐risk species.

Are avian population targets achievable through programs that restore habitat on private‐lands?

AbstractAgricultural intensification has negatively affected biodiversity throughout the world. In the United States, population declines of many early successional bird species have been linked with habitat loss due to agriculture. In light of this, recent efforts have attempted to prioritize conservation actions at continental scales. Importantly, these efforts have used existing data to estimate the global population size for declining species and set future population goals. Though these efforts consider broad scales for conservation efforts and provide population targets, they lack explicit plans to achieve established goals. One way to achieve these population goals is through the process of habitat restoration, such as voluntary private‐land conservation programs that restore habitat in agricultural landscapes and are implemented at a sufficiently large scale to achieve population goals. Our goal was to understand how private‐land conservation programs can be used to achieve population goals for declining species. To do this, we examined the population‐level impacts of the Conservation Reserve Enhancement Program, a program that has created more than 57,000 ha of restored habitat in Illinois, for four declining conservation priority species, the Bell's Vireo (Vireo bellii bellii), Field Sparrow (Spizella pusilla), Northern Bobwhite (Colinus virginianus), and Willow Flycatcher (Empidonax trailli trailli). We surveyed 172 randomly chosen restored fields in 10 counties in central and west‐central Illinois during the 2012–2015 breeding seasons. We found that current private‐land conservation efforts in Illinois are probably effective in achieving state population goals for rare species with restricted habitat preferences such as the Bell's Vireo and may be close to achieving the goal for the Willow Flycatcher. However, more than 10 times the current amount of restored habitat (~1% of land used for agriculture in Illinois) would be required to achieve the population goals set for all of our focal species. Our results suggest that achieving population goals for early successional bird species using land restored through private‐land programs may be possible for some rare species, but the large amount of habitat needed for many species makes achieving these goals only possible with significant increases in private‐land habitat.

Minimal bias in surveys of grassland birds from roadsides

ABSTRACT Edges, including roads, can have unintended deleterious impacts on wildlife. However, roads also present opportunities for replicable, and spatially and temporally consistent, wildlife monitoring. Assessing sources of variability associated with roadside-based surveys could improve the accuracy and extend the inferences of surveys, thus strengthening their applicability to management. We assessed roadside effects on abundance (λ) and detection probability (p) of high-priority grassland and shrubland songbirds in western Kentucky and Tennessee, USA. We delineated transects 300 m apart perpendicular to secondary roads and, along each transect, positioned point counts at 0 m, 300 m, and 600 m off the road. We surveyed 8 species: Northern Bobwhite (Colinus virginianus), Bell's Vireo (Vireo bellii), Prairie Warbler (Setophaga discolor), Field Sparrow (Spizella pusilla), Grasshopper Sparrow (Ammodramus savannarum), Henslow's Sparrow (Ammodramus henslowii), Dickcissel (Spiza americana), and Eastern Mead...

Irrigation and avifaunal change in coastal Northwest Mexico: has irrigated habit attracted threatened migratory species?

Irrigation in desert ecosystems can either reduce or increase species diversity. Groundwater pumping often lowers water tables and reduces natural wetlands, whereas canal irrigation often creates mesic habitat, resulting in great increases in avian diversity from irrigation. Here we compare a dataset of potential natural vegetation to recent datasets from areal and satellite imagery to show that 60% of the land in the coastal plain of southern Sonora and northern Sinaloa lying below 200 m elevation has been converted by irrigation to more mesic habitats. We then use the record of bird specimens in the world’s museums from this same region of Mexico to examine the avian community before and after the development of extensive irrigation. In general these museum records show an increase in the abundance and diversity of breeding birds associated with mesic habitats. Although thorn forest birds have likely decreased in total numbers, most are common enough in the remaining thorn forest that collection records did not indicate their probable decline. Four migrants having most of their breeding ranges in the US or Canada, Yellow-billed Cuckoo, Cliff Swallow, Bell’s Vireo, and Orchard Oriole, apparently have increased dramatically as breeders in irrigated habitats of NW Mexico. Because these species have decreased or even largely disappeared as breeding birds in parts of the US or Canada, further research should assess whether their increases in new mesic habitats of NW Mexico are linked to their declines as breeding birds in Canada and the US For Bell’s Vireo recent specimens from Sinaloa suggest its new breeding population in NW Mexico may be composed partly of the endangered Least Bell’s Vireo.

The role of territory settlement, individual quality, and nesting initiation on productivity of Bell's vireos Vireo bellii bellii

Variation in habitat quality among territories within a heterogeneous patch should influence reproductive success of territory owners. Further, territory settlement order following an ideal despotic distribution (IDD) should predict the fitness of occupants if territory selection is adaptive. We recorded settlement order and monitored nests in territories occupied by individually marked Bell's vireos Vireo bellii bellii across a range of shrubland habitats in central Missouri, USA. We used an information theoretic approach to evaluate multiple hypotheses regarding the relationship between territory settlement order and seasonal territory productivity (productivity), which we define as the number of young fledged from all nest attempts in a territory. Territory settlement order and arrival date were not analogous and later arriving males displaced early settlers in 13 of 49 territories. Settlement order and lay date together were the best predictors of a territory's productivity; productivity decreased 2.08 young from earliest to latest settlement rank and lay date. Males that defended the same territory in successive years occupied territories with earlier settlement dates, but we found little evidence that age or prior ownership influenced productivity. Territory selection by male Bell's vireos was adaptive because males preferred to settle in territories that had high seasonal offspring production, but even though settlement rank was linked to territory quality, high productivity was only realized on high quality territory when also linked to early nest initiation date. While settlement rank was related to territory quality, obtaining a high quality territory had to be combined with early nest initiation to maximize productivity. We found support for the IDD hypothesis because the highest quality territories, (i.e. most productive), were settled earlier. Research that identifies high quality habitat by linking individual fitness with habitat characteristics may elucidate the importance of habitat quality, individual experience and temporal factors to productivity of Bell's vireos.

Frequent Vocalizing Is Negatively Associated with Brood Parasitism in a Host of the Brown-headed Cowbird

Abstract. Brood parasitism by the Brown-headed Cowbird (Molothrus ater) can substantially affect host species' reproductive success. The “host-activity” hypothesis suggests that parasites eavesdrop on conspicuous behaviors to locate and parasitize hosts, and several studies of cowbird hosts support this hypothesis. In contrast, a recent study of the Least Bell's Vireo (Vireo bellii pusillus) reported a negative association between the host's vocalization rate near the nest and brood parasitism. This contradictory pattern is intriguing because Bell's Vireo is a common cowbird host and vocalizes near and on its nests. We tested a key assumption of the host-activity hypothesis in a different subspecies (V. b. arizonae) to determine whether the contradictory pattern reported in V. b. pusillus is an anomaly or could be generalized to other subspecies. Unparasitized vireos vocalized more frequently than parasitized birds, confirming that the pattern in Bell's Vireos is the opposite of that reported for other co...

Seasonal Fecundity and Source-Sink Status of Shrub-Nesting Birds in a Southwestern Riparian Corridor

Abstract Saltcedar (Tamarix spp.) has increasingly dominated riparian floodplains relative to native forests in the southwestern U.S., but little is known about its impacts on avian productivity or population status. We monitored 86 Arizona Bell's Vireo (Vireo bellii arizonae), 147 Abert's Towhee (Melozone aberti), and 154 Yellow-breasted Chat (Icteria virens) nests to assess reproductive parameters in cottonwood-willow (Populus-Salix), saltcedar, and mesquite (Prosopis spp.) stands along the San Pedro River, Arizona during 1999–2001. We also assessed source-sink status for each species in each vegetation type using field data combined with data from the literature. There were no significant differences in reproductive parameters between vegetation types for Abert's Towhee or Yellow-breasted Chat, although seasonal fecundity was quite low across vegetation types for the latter (0.75 ± 0.14; mean ± SE). Bell's Vireo had extremely low seasonal fecundity in saltcedar (0.10 ± 0.09) and significantly fewer fle...

First Record of Interspecific Breeding of Least Bell's Vireo and White-eyed Vireo

Abstract We provide the first known documentation of a male Least Bell's Vireo (Vireo bellii pusillus) breeding with a female White-eyed Vireo (V. griseus) and the first report of a White-eyed Vireo breeding in California at the San Luis Rey River, Oceanside, San Diego County. We discovered the pair building a nest on 12 May 2010. The female laid four eggs, and the pair successfully raised and fledged four nestlings. We collected DNA samples from each nestling and the female during the nestling stage and banded them with a numbered federal leg band for future identification. We obtained detailed nest measurements after fledging and monitored the territory for further nesting attempts. No additional nesting attempts were detected.

Bird Nesting in Chickasaw Plum Related to Age of Plum in Oklahoma

Abstract We described passerine nesting in Chickasaw plum (Prunus angustifolia) as related to plum age in north-central Oklahoma during the 2007 and 2008 nesting seasons to better understand the role of plum in bird ecology. We measured properties of nests (height aboveground), nest-supporting stems (height, diameter, age) and nesting stands (age, area). We observed Bell's vireos (Vireo bellii), brown thrashers (Toxostoma rufum), field sparrows (Spizella pusilla), northern mockingbirds (Mimus polyglottos) and painted buntings (Passerina ciris) nesting in plum stands and inferred nesting by blue grosbeaks (P. caerulea), greater roadrunners (Geococcyx californianus), mourning doves (Zenaida macroura) and northern cardinals (Cardinalis cardinalis). Nest height was consistent among species at averages ranging from 0.7 m (se = 0.13, n = 5) for field sparrows to 1.1 m (se = 0.07, n = 30) for northern mockingbirds. Bell's vireos and field sparrows nested on younger stems (average age = 7 y) than the ot...

Avian Density and Nest Survival on the San Pedro River: Importance of Vegetation Type and Hydrologic Regime

Abstract: Lowland riparian vegetation in the southwestern United States is critically important for maintaining a high richness and density of breeding birds. Further investigation is needed within riparian corridors, however, to evaluate the relative importance of vegetation type and hydrologic regime for avian density and nest survival as targets for regional conservation or restoration efforts. We estimated the densities of 40 bird species and for species grouped on the basis of nest height and dependence on surface water in gallery cottonwood–willow (Populus spp.–Salix spp.) forests, saltcedar (Tamarix spp.) shrub lands, and terrace vegetation types along a gradient in the hydrologic regime of the San Pedro River, Arizona, USA. We also assessed nest survival for shrub‐nesting insectivores and herbivores. Canopy‐nesting birds as a group and 14 individual bird species reached their greatest densities in cottonwood forests regardless of the hydrologic regime. Water‐dependent birds as a group reached their highest density in both intermittent‐ and perennial‐flow cottonwood stands, but certain species occurred almost exclusively in perennial‐flow sites. Two shrub‐nesting species and the brown‐headed cowbird (Molothrus ater) were most abundant in saltcedar shrub lands, and the brown‐headed cowbird was most abundant in saltcedar stands with intermittent flows. Mesquite (Prosopis spp.) and big sacaton (Sporobolus wrightii) grassland each maintained the highest densities of certain species within ≥1 hydrologic regime. Shrub‐nesting insectivores had the greatest nest survival in cottonwood, including Arizona Bell's vireo (Vireo bellii arizonae), and also had lower proportions of nests parasitized and preyed upon, although 95% confidence intervals among vegetation types overlapped. Nest survival for both shrub‐nesting insectivores and herbivores was lowest in intermittent‐flow saltcedar, although, again, confidence intervals overlapped. Nest survival was lower in parasitized than nonparasitized nests in mesquite and across vegetation types for Arizona Bell's vireo and in cottonwood for Abert's towhee (Pipilo aberti). Riparian management that maintains heterogeneous riparian vegetation types, including floodplain vegetation comprising cottonwood–willow gallery riparian forests with some stretches of perennial flow, are important for maintaining the high diversity and abundance of breeding birds on the San Pedro River and probably across the region. Cottonwood stands also appear to maintain highest nest survival for some shrub‐nesting birds.

Least Bell's Vireo Breeding Records in the Central Valley Following Decades of Extirpation

The Least Bell's Vireo (Vireo bellii pusillus) was listed as state endangered in 1980 and federally endan- gered in 1986 in response to a sharp population decline and range reduction. This vireo commonly bred in riparian forests throughout the Central Valley of California, but prior to 2005, no nesting pairs had been confirmed in the region in over 50 years. On 29 June 2005, a Least Bell's Vireo nest was located in a 3-year-old riparian restoration site at the San Joaquin River National Wildlife Refuge in Stanislaus County, California. In 2006, a Least Bell's Vireo pair returned to the refuge to successfully breed, followed by an unsuccessful attempt in 2007 by an unpaired female. These records are approximately 350 km from the nearest known breeding population and appear to be part of a growing number of sightings outside of the species' current southern California breeding range. These nesting attempts lend credence to the idea that extirpated species can recolonize restored habitat by long-distance dispersal.

Community-level Patterns of Host Use by the Brown-headed Cowbird (Molothrus ater), a Generalist Brood Parasite

Quantifying community-level host use by generalist brood parasites is important because it provides a measure of the resources that parasites need for reproduction. During the 2002–2007 breeding seasons, we quantified host use by the Brown-headed Cowbird (Molothrus ater) at Konza Prairie Biological Station in northeast Kansas. Overall, 54.4% of nests (n = 2,621) of 22 passerine host species that typically accept parasitic eggs were parasitized, and parasitized nests contained an average of 1.9 ± 1.03 (SD) Brown-headed Cowbird eggs, nestlings, or both. Multiple parasitism was common on the study site in all years: 55.7% of parasitized nests (n = 1,425) received ≥2 Brown-headed Cowbird eggs. The Bell's Vireo (Vireo bellii; 70.5%) and the Dickcissel (Spiza americana; 69.6%) were parasitized at a significantly greater rate than the Red-winged Blackbird (Agelaius phoeniceus, 21.9%); collectively, these three species represented 85.8% of potential host nests found in all years. The host preference of Brown-headed Cowbirds did not appear to shift among the three species and instead they exhibited a consistent preference for Dickcissel nests over the course of the breeding season and among years. Both the rate and the intensity of parasitism on Dickcissels were significantly lower during the present study than in one conducted several decades earlier at the same site. Our results indicate that cowbirds in northeast Kansas differentially parasitize hosts, that most cowbird eggs are laid in the nests of a small number of host species, and that the Dickcissel appears to be preferred over other hosts.

Foraging Strategy of A California Kingsnake in Searching for Fledglings of the Least Bell's Vireo

Nest predation involving a California kingsnake (Lampropeltis getula californiae) and two fledglings of least Bell's vireo (Vireo bellii pusillus) was observed along the San Luis Rey River in northern San Diego County, California. The kingsnake was discovered consuming a fledgling when the adults were observed mobbing the snake. The kingsnake then proceeded to systematically search for the second fledgling by traveling out and back from the nest multiple times over the next 15 min, each time in a new direction, traveling ca. ≤6–10 m before returning to the nest. The snake then found the original roosting branch of the surviving fledgling and proceeded quickly toward the fledgling that had moved ca. 15 m away. The kingsnake traveled this distance in ca. 2 min and killed and consumed the last fledgling. While California kingsnakes have been reported to feed on birds and bird eggs, this systematic searching behavior has not been described previously.

COWBIRD REMOVALS UNEXPECTEDLY INCREASE PRODUCTIVITY OF A BROOD PARASITE AND THE SONGBIRD HOST

Generalist brood parasites reduce productivity and population growth of avian hosts and have been implicated in population declines of several songbirds of conservation concern. To estimate the demographic effects of brood parasitism on Bell's Vireos (Vireo bellii), we removed Brown-headed Cowbirds (Molothrus ater) in a replicated switchback experimental design. Cowbird removals decreased parasitism frequency from 77% and 85% at unmanipulated plots to 58% and 47% at removal plots in 2004 and 2005, respectively. Vireo productivity per pair was higher at cowbird removal plots when years were pooled (mean = 2.6 +/- 0.2 [SE] young per pair) compared to unmanipulated plots (1.2 +/- 0.1). Nest desertion frequency was lower at cowbird removal plots (35% of parasitized nests) compared to unmanipulated plots (69%) because removal of host eggs was the proximate cue for nest desertion, and vireos experienced lower rates of egg loss at cowbird removal plots. Nest success was higher among unparasitized than parasitized nests, and parasitized nests at cowbird removal plots had a higher probability of success than parasitized nests at unmanipulated plots. Unexpectedly, cowbird productivity from vireo pairs was higher at cowbird removal plots (mean = 0.3 +/- 0.06 young per pair) than at unmanipulated plots (0.1 +/- 0.03) because fewer parasitized nests were deserted and the probability of nest success was higher. Our study provides the first evidence that increases in cowbird productivity may be an unintended consequence of cowbird control programs, especially during the initial years of trapping when parasitism may only be moderately reduced. Thus, understanding the demographic impacts of cowbird removals requires an informed understanding of the behavioral ecology of host-parasite interactions.

A MULTISCALE ANALYSIS OF NEST PREDATION ON LEAST BELL'S VIREOS (VIREO BELLII PUSILLUS)

Abstract We examined variables influencing nest predation on the endangered Least Bell's Vireo (Vireo bellii pusillus) at three spatial scales to determine what nest-site, habitat, or landscape characteristics affect the likelihood of nest predation and to determine the spatial distribution of predation risk and the variables influencing it. We used MARK to calculate daily survival rates of Least Bell's Vireo nests and applied an information-theoretic approach to evaluate support for logistic regression models of the effect of habitat variables on predation risk. Analysis of data for 195 nests collected during 1999 and 2000 at the San Luis Rey River and Pilgrim Creek in southern California revealed no effect of fine-scale factors, including nest height, supporting plant species, and three measures of nest concealment, on the likelihood of predation. At the intermediate scale, distances to the riparian-habitat edge and to internal gaps in the canopy were unrelated to nest survival. Surrounding land-use typ...