Search birdRS Box

Search birdRS blog posts

Browse the Blog Posts

Or scan through the blog archive below for items of interest as only the latest post is shown below, thanks.

Tuesday, 13 December 2016

The Condor Volume 118 Issue 4 November 2016

The Condor
Published by: American Ornithological Society

Table of Contents
Nov 2016 : Volume 118 Issue 4 



Nesting habitat selection influences nest and early offspring survival in Greater Sage-Grouse
Daniel Gibson, Erik J. Blomberg, Michael T. Atamian and James S. Sedinger

Adaptive habitat-selection theory predicts that individuals should use habitats that maximize lifetime fitness. However, trade-offs between life-history stages, environmental variability, and predator–prey dynamics can interact with individual preferences, which may result in individuals selecting suboptimal habitats. Understanding the distinction between adaptive and maladaptive animal use of habitat is central to effective species conservation, because use of maladaptive habitat is counter to conservation objectives. Our objectives were to assess whether habitat characteristics selected by Greater Sage-Grouse (Centrocercus urophasianus) were correlated with increased production of fledged young. We monitored 411 nests and 120 broods from 234 females between 2004 and 2012 in central Nevada, USA. We determined which habitat characteristics were selected as nesting habitat and assessed whether these characteristics influenced nest success and early offspring survival. The relationships between characteristics selected at nest sites and metrics of reproductive success were variable, in that certain characteristics (e.g., forb cover, amount of pinyon–juniper woodlands) were correlated with higher nest survival and chick survival, but other characteristics (e.g., amount of sagebrush, residual grass height) did not improve reproductive success. Despite variability among predictor variables, we found a positive effect of selection of fine-scale habitat characteristics on nest (βNS-Local = 0.14, 85% confidence interval [CI]: 0.04–0.23) and chick survival (βCS-Local = 0.39, 85% CI: 0.27–0.50); however, we did not find that selection of broad-scale habitat characteristics predicted reproductive success (βNS-Landscape = −0.04, 85% CI: −0.15 to 0.06; βCS-Landscape = 0.06, 85% CI: −0.06 to 0.18). Additionally, nest-site selection was more predictive of chick survival than of nest survival, which suggests that females' selection of nesting habitat was based primarily on its qualities as brood-rearing habitat. Together, these findings suggest that nest-site selection may be influenced by more than increased reproductive success, or that there is a landscape-level pattern to local-scale habitat characteristics.

Ancient DNA reveals substantial genetic diversity in the California Condor (Gymnogyps californianus) prior to a population bottleneck
Jesse D'Elia, Susan M. Haig, Thomas D. Mullins and Mark P. Miller

Critically endangered species that have undergone severe population bottlenecks often have little remaining genetic variation, making it difficult to reconstruct population histories to apply in reintroduction and recovery strategies. By using ancient DNA techniques, it is possible to combine genetic evidence from the historical population with contemporary samples to provide a more complete picture of a species' genetic variation across its historical range and through time. Applying this approach, we examined changes in the mitochondrial DNA (mtDNA) control region (526 base pairs) of the endangered California Condor (Gymnogyps californianus). Results showed a >80% reduction in unique haplotypes over the past 2 centuries. We found no spatial sorting of haplotypes in the historical population; the periphery of the range contained haplotypes that were common throughout the historical range. Direct examination of mtDNA from California Condor museum specimens provided a new window into historical population connectivity and genetic diversity showing: (1) a substantial loss of haplotypes, which is consistent with the hypothesis that condors were relatively abundant in the nineteenth century, but declined rapidly as a result of human-caused mortality; and (2) no evidence of historical population segregation, meaning that the available genetic data offer no cause to avoid releasing condors in unoccupied portions of their historical range.

Minimal bias in surveys of grassland birds from roadsides
Christopher M. Lituma and David A. Buehler

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 Meadowlark (Sturnella magna). We used Royle N-Mixture models to estimate species-specific abundance and detection probability. For abundance, distance from the road affected only Henslow's Sparrow; mean Henslow's Sparrow abundance increased by 59% at points 600 m away from a road. For detection probability, distance from the road was not important for any species, suggesting that noise or activity associated with low-traffic roads did not affect bird singing behavior or observer hearing ability enough to affect survey results. Abundance was more strongly related to land-cover covariates than distance from the road. With the exception of Henslow's Sparrow, roadside surveys for 8 high-priority grassland bird species produced estimates of abundance and detection probabilities representative of the broader landscape. Roads can provide opportunities to accurately monitor occupancy, abundance, and density of grassland birds using surveys that account for land-cover variation and seasonal variation in detection probability.

Interactive effects between nest microclimate and nest vegetation structure confirm microclimate thresholds for Lesser Prairie-Chicken nest survival
Blake A. Grisham, Alixandra J. Godar, Clint W. Boal and David A. Haukos

The range of Lesser Prairie-Chickens (Tympanuchus pallidicinctus) spans 4 unique ecoregions along 2 distinct environmental gradients. The Sand Shinnery Oak Prairie ecoregion of the Southern High Plains of New Mexico and Texas is environmentally isolated, warmer, and more arid than the Short-Grass, Sand Sagebrush, and Mixed-Grass Prairie ecoregions in Colorado, Kansas, Oklahoma, and the northeast panhandle of Texas. Weather is known to influence Lesser Prairie-Chicken nest survival in the Sand Shinnery Oak Prairie ecoregion; regional variation may also influence nest microclimate and, ultimately, survival during incubation. To address this question, we placed data loggers adjacent to nests during incubation to quantify temperature and humidity distribution functions in 3 ecoregions. We developed a suite of a priori nest survival models that incorporated derived microclimate parameters and visual obstruction as covariates in Program MARK. We monitored 49 nests in Mixed-Grass, 22 nests in Sand Shinnery Oak, and 30 nests in Short-Grass ecoregions from 2010 to 2014. Our findings indicated that (1) the Sand Shinnery Oak Prairie ecoregion was hotter and drier during incubation than the Mixed- and Short-Grass ecoregions; (2) nest microclimate varied among years within ecoregions; (3) visual obstruction was positively associated with nest survival; but (4) daily nest survival probability decreased by 10% every half-hour when temperature was greater than 34°C and vapor pressure deficit was less than −23 mmHg during the day (about 0600–2100 hours). Our major finding confirmed microclimate thresholds for nest survival under natural conditions across the species' distribution, although Lesser Prairie-Chickens are more likely to experience microclimate conditions that result in nest failures in the Sand Shinnery Oak Prairie ecoregion. The species would benefit from identification of thermal landscapes and management actions that promote cooler, more humid nest microclimates.

Meta-analysis of California Spotted Owl (Strix occidentalis occidentalis) territory occupancy in the Sierra Nevada: Habitat associations and their implications for forest management 
Douglas J. Tempel, John J. Keane, R. J. Gutiérrez, Jared D. Wolfe, Gavin M. Jones, Alexander Koltunov, Carlos M. Ramirez, William J. Berigan, Claire V. Gallagher, Thomas E. Munton, Paula A. Shaklee, Sheila A. Whitmore and M. Zachariah Peery

We assessed the occupancy dynamics of 275 California Spotted Owl (Strix occidentalis occidentalis) territories in 4 study areas in the Sierra Nevada, California, USA, from 1993 to 2011. We used Landsat data to develop maps of canopy cover for each study area, which we then used to quantify annual territory-specific habitat covariates. We modeled the relationships between territory extinction and colonization using predictor variables of habitat, disturbance (logging, fire), climate, and elevation. We found that forests with medium (40–69%) and high (≥70%) canopy cover were the most important predictors of territory occupancy in all study areas, and that both canopy cover categories were positively correlated with occupancy. We used analysis of deviance to estimate the amount of variation explained by the habitat covariates (primarily medium and high canopy cover) and found that these covariates explained from 35% to 67% of the variation in occupancy. Climatic covariates were not correlated with occupancy dynamics and explained little of the variation in occupancy. We also conducted a post hoc analysis in which we partitioned canopy cover into 10% classes, because our original partitioning into 3 classes may have lacked sufficient resolution to identify canopy cover levels where occupancy changed abruptly. In this post hoc analysis, occupancy declined sharply when territories contained more area with <40% canopy cover, and the amount of 50–59% and 60–69% canopy cover had a more positive association with occupancy than did 40–49% canopy cover. Our results suggest that some fuels treatments intended to reduce fire risk and improve forest resilience could be located within Spotted Owl territories without adversely impacting territory occupancy if such treatments do not consistently reduce canopy cover below 50%. We suggest that future work quantify components of forest structure (e.g., large tree density, vertical complexity) known to be selected by owls and relate these characteristics to occupancy and fitness metrics.

Transferability of habitat suitability models for nesting woodpeckers associated with wildfire
Quresh S. Latif, Victoria A. Saab, Jeff P. Hollenbeck and Jonathan G. Dudley

Following wildfire, forest managers are challenged with meeting both socioeconomic demands (e.g., salvage logging) and mandates requiring habitat conservation for disturbance-associated wildlife (e.g., woodpeckers). Habitat suitability models for nesting woodpeckers can be informative, but tests of model transferability are needed to understand how broadly models developed at one location can be applied to inform post-fire forest management at other locations. We developed habitat suitability models and tested their transferability for 2 disturbance-associated woodpecker species, Black-backed (Picoides arcticus) and Lewis's (Melanerpes lewis) woodpecker. Habitat suitability models consisted of weighted logistic regression models comparing environmental conditions at nest versus non-nest sites. We developed models at each of 3 wildfire locations in Washington, Oregon, and Idaho, and then examined predictive performance for each model at alternate (“application”) locations. Models generally discriminated nest from non-nest sites well at locations where they were developed but performance was variable at application locations, indicating limited transferability. Models for Black-backed Woodpecker and those that included field-collected environmental covariates exhibited greater transferability than models for Lewis's Woodpecker and those that only included remotely sensed covariates. Transferability was also generally poor between Oregon and the other 2 locations. Limitations to model transferability observed in this study suggest models developed at any one wildfire location are unlikely to be generally applicable across the entire range of Black-backed and Lewis's woodpeckers. Generally applicable models to inform post-fire forest management will therefore likely require integration of data from multiple wildfire locations.

Density and microhabitat preference of the Southern Bristle-Tyrant (Phylloscartes eximius): Conservation policy implications
Vinicius R. Tonetti and Marco A. Pizo

Demographic and habitat preference studies are of paramount importance for conservation of birds, which is urgent in the mostly devastated Atlantic Forest. Moreover, these studies can indicate microhabitats that deserve more attention for conservation. In this article, we provide the first population density estimate and microhabitat preference assessment for the Southern Bristle-Tyrant (Phylloscartes eximius), a poorly known and threatened insectivorous bird endemic to the Atlantic Forest. The study was conducted at Cantareira State Park between May and December 2014. For density estimates we sampled 600 point counts using distance sampling, and for microhabitat assessment we compared 15 variables in 54 plots where birds were observed foraging with 145 random plots by adjusting generalized linear models and using hierarchical partitioning analysis. The species' density (12.7 [7.3–20.2] individuals per km2) is ∼1/16 the density of a globally threatened congener, the Restinga Tyrannulet (Phylloscartes kronei), and lower than most other small insectivorous passerines in Atlantic Forest. When alone or in pairs, Southern Bristle-Tyrants preferred forests within ∼10m of rivers and lakes; when in mixed-species flocks they preferred valleys (grotas). We suggest that protection of riverine forests would benefit Southern Bristle-Tyrants with positive consequences for the water supply of millions of people living in the Atlantic Forest domain.

Juvenile survival, recruitment, population size, and effects of avian pox virus in Laysan Albatross (Phoebastria immutabilis) on Oahu, Hawaii, USA
Eric A. VanderWerf and Lindsay C. Young

Understanding population dynamics and determining conservation priorities in long-lived species with delayed breeding often is hampered by lack of information about younger age classes. Obtaining accurate estimates of juvenile survival and recruitment can be difficult because young individuals are infrequently observed. We used mark–recapture models to estimate age-specific survival, recruitment, population size, and encounter probability of Laysan Albatrosses (Phoebastria immutabilis) using a 14-yr dataset from Oahu, Hawaii, USA. We also measured the long-term effect of avian pox virus (Poxvirus avium) on the survival and recruitment of albatrosses infected as nestlings. Survival of juvenile albatrosses during the first year after fledging was 0.757 ± 0.042. We were able to estimate juvenile survival, the first such estimate in any long-lived seabird, because our high search effort revealed that some birds began visiting the natal colony at the age of 1 yr. The survival of prebreeders increased rapidly and reached a value in the second year (0.973 ± 0.008) that was similar to the survival of breeding adults (0.973 ± 0.017). The average age of first return to the natal colony was 4.24 ± 0.11 yr. The average age at first breeding was 8.44 ± 0.15 yr, with recruitment probability peaking at ages 9–10 yr and a single bird being recruited into the breeding population at the age of 4 yr. Pox virus decreased survival in the first year by 4%–13% and decreased recruitment probability up to age 12 by 4%–26%, depending on the severity of infection. The total size of the Laysan Albatross population on Oahu in 2015 was 555 birds, consisting of 270 active breeders, 231 prebreeders, and 54 birds that likely skipped breeding that year. The number of prebreeders constituted an average of 44% of the total population. These demographic estimates will be useful for population modeling exercises involving various threat and management scenarios, and for examining environmental factors that influence demography.

Reproductive success of songbirds and waterfowl in native mixed-grass pasture and planted grasslands used for pasture and hay
Stephen K. Davis, Sarah M. Ludlow and D. Glen McMaster

Conservation of grassland birds in agricultural landscapes requires an understanding of the demographic consequences of nesting in native and planted grasslands. Much of the native grassland in agricultural regions has been converted to cropland. Subsequently, seeding cropland to perennial grasslands has become a common strategy to restore habitat for grassland birds, but these grasslands also may be used as hay and pasture forage for livestock. Our objectives were to determine (1) if the abundance of grassland songbirds and the reproductive success of songbirds and waterfowl varied between native pasture and planted grassland, and (2) if the amount of grassland in the surrounding landscape influenced the abundance and reproductive success of songbirds or the nest survival of waterfowl in native and planted grasslands. Our results suggest that planted grasslands used for pasture and hay in our region are likely ecological sinks for grassland specialist songbirds. Sprague's Pipit (Anthus spragueii) nested only in native pasture, and Chestnut-collared Longspur (Calcarius ornatus), Lark Bunting (Calamospiza melanocorys), and Baird's Sparrow (Ammodramus bairdii) were sometimes more abundant in planted pasture or hayfields, but fledged 1.4–4.5 times as many young per nest in native pasture. The reproductive success of waterfowl and grassland songbird generalists was similar in planted grasslands and native pasture. The abundance of all songbirds varied with the amount of grassland or cropland in the surrounding landscape, but landscape composition only weakly influenced the nest survival rates of 1 of 8 songbirds and 4 of 6 waterfowl species. Our results demonstrate that the preservation of native pasture is critical for the conservation of grassland specialists. Other grassland songbirds and waterfowl likely will benefit from the conservation of native and planted grassland and conversion of cropland to perennial grassland used for pasture and hay.

Geolocator tracking of Great Reed-Warblers (Acrocephalus arundinaceus) identifies key regions for migratory wetland specialists in the Middle East and sub-Saharan East Africa
Joshua J. Horns, Evan Buechley, Mark Chynoweth, Lale Aktay, Emrah Çoban, Mehmet Ali Kırpık, Jordan M. Herman, Yakup Şaşmaz and Çağan H. Şekercioğlu

Wetland-dependent migratory songbirds represent one of the most vulnerable groups of birds on the planet, with >67% of wetland-obligate species threatened with extinction. One of the major hurdles for conservation efforts is determining the migration routes, stopover sites, and wintering sites of these species. We describe an annual migration cycle revealed by geolocator tracking of Great Reed-Warblers (Acrocephalus arundinaceus) breeding in the Aras River wetlands of eastern Turkey. Because of its relatively large size and breeding ground fidelity, the Great Reed-Warbler is an excellent candidate for geolocator studies and can serve as an indicator species for other wetland songbirds, many of which are particularly threatened in the Middle East. All birds made use of at least 2 wintering grounds in South Sudan, on the Indian Ocean coast and on the western shores of Lake Malawi, as well as several important stopover sites. We also identified a counterclockwise migration path into and out of Africa. Throughout the year, these birds encountered 277 Important Bird Areas, >40% of which had little or no protection. Many species of wetland songbird, particularly threatened species, may be too rare or too small to be the focus of similar studies. Our results not only allow for comparisons with other Great Reed-Warbler populations, but also reveal previously unknown stopover and wintering locations to target conservation efforts that will help wetland-dependent bird species in the Middle East and East Africa.

Nest survival of Tricolored Blackbirds in California's Central Valley
Kelly Weintraub, T. Luke George and Stephen J. Dinsmore

The Tricolored Blackbird (Agelaius tricolor), almost entirely restricted to California, USA, has recently been proposed for listing under the U.S. Endangered Species Act. Tricolored Blackbirds historically nested in wetlands, but a large proportion of the population now nests in agricultural grain fields where the crop is ready to harvest before the young have fledged. Since 1991, federal agencies have paid farmers to delay harvesting in an effort to increase nesting productivity. However, the relative nesting success of Tricolored Blackbirds breeding in agricultural fields versus wetlands is unknown. Our objectives were to estimate daily survival rate (DSR) of nests, identify habitat covariates that influence nest survival, and estimate the number of young produced per nest. During 2011–2012, we monitored 1,323 Tricolored Blackbird nests in 12 colonies using small temperature data loggers. We modeled DSR using Program RMark with combinations of the following variables: site, habitat type, nest initiation date, nest height, water depth, nest density, colony population size, year, and the proportion of nearby nests that failed. Nest survival varied greatly (range: 0.024–0.719) but was not explained by habitat type. Nest height and nest density were positively associated with DSR. DSR was lowest midway through the breeding season and declined with colony population size. Number of young produced per nest varied by site, was lowest in intermediate-sized colonies of 1,000–5,000 birds, and was highest in 2011. DSR and number of young fledged per nest were similar in agricultural fields and in wetlands. Our results suggest that Tricolored Blackbirds benefit from policies that allow them to complete their nesting cycle in agricultural fields.