Understanding interactions among site occupancy, reproduction, vegetation, and disturbance for threatened species can improve conservation measures, because important aspects of vegetation and disturbances may be identified and managed. We used 9 yr of survey data collected at 168 sites to investigate dynamic site occupancy and reproduction in a declining population of California Spotted Owls (Strix occidentalis occidentalis) in southern California, USA. We used multistate models to examine the relationship among owl site occupancy, reproduction, high-severity wildland fire, and postfire logging, while accounting for variation in vegetation characteristics and variation in detectability. Both occupancy and reproduction were positively correlated with successful reproduction in the previous year. Tree cover (ha) in a site's 203-ha core area also was positively correlated with both occupancy and reproduction. We detected no effect of disturbance covariates on reproduction, given that a site was occupied. Fire and logging covariates were both negatively correlated with the probability of site occupancy, and the effect sizes of these disturbances were large in sites that were occupied by owls that were nonreproductive the previous year (reduced 0.19 by fire and 0.26 by post-fire logging), but small in sites that were occupied by owls that were reproductive the previous year (reduced 0.02 by fire and 0.03 by postfire logging). This study illustrates the important contribution of consistently occupied and productive breeding sites to this population of Spotted Owls, and demonstrates that both occupancy and reproduction at these productive sites exhibited negligible effects from disturbances. Our results suggest that sites with recent owl reproduction and sites with more tree cover in this study area should receive enhanced protection from management actions that modify vegetation utilized by Spotted Owls.

Human-induced rapid environmental change, such as the introduction of exotic species, can create novel species interactions that might be detrimental to native organisms. For birds, introduced plant species may represent potentially attractive, but dangerous, locations to place a nest. If the environmental cues that birds use to select safe nest sites are unreliable when they apply to nonnative plant species, these plants could act as evolutionary traps: preferred nest substrates that confer the poorest reproductive outcomes. We tested this possibility by assessing reproductive consequences of nest substrate preference in the Veery (Catharus fuscescens). We followed the fates of nests in native plants and in three nonnative plants associated with reduced nest success in previous studies (Amur honeysuckle [Lonicera maackii], multiflora rose [Rosa multiflora], and Japanese barberry [Berberis thunbergii]). Veeries preferred to locate nests in nonnative plants and in denser patches of vegetation more dominated by nonnative plants. Nests placed in nonnative plants were more visually concealed. We found no evidence that these preferences were maladaptive, as there was no difference in the daily survival probability of nests based on nest-site characteristics. Veeries were not victims of an ‘oviposition trap' in this forest system during the period of our study, but rather were facultatively exploiting nonnative plants to their reproductive advantage.

Wetland stopover use by migratory shorebirds is concurrently influenced by habitat characteristics within a stopover site and characteristics related to the broader context surrounding the stopover site. To conserve wetland habitats essential for shorebird migration through the interior of North America, we need to understand how these dual scales influence stopover use. We surveyed >14,000 wetlands within 10 broad-scale landscapes in north-central Oklahoma from 2007 through 2009 to determine how characteristics within (intra) and surrounding (context) wetlands influence the density and species richness of migrating shorebirds at stopover sites. We used zero-inflated modeling and an information theoretic framework to separately examine and then compare the relative importance of intra-habitat variables and habitat context variables in explaining use of stopovers. We observed 38,288 migratory shorebirds of 29 species. Shorebirds were least likely to occur in isolated, small, steep-sided wetland habitats. Among intra-patch variables, shorebird density and richness were best explained by wetland habitat area, slope, and the cover and height of vegetation within a wetland. Shorebird density and richness were more than 2 times higher in large wetland habitats (i.e. >3.3 ha), and both decreased with increased vegetation and slope. The density of wetlands within 1.5 km of a stopover site had the greatest impact on shorebird density and richness among patch context variables. As habitat density increased from 0.07 to 1, shorebird abundance and richness increased by >200%. Shorebirds were positively related to grazing pressure but negatively related to forest/shrubland, urban/suburban development, and grassland land cover contexts. When compared to models with intra-habitat variables, models containing habitat context variables better explained migratory shorebird density and richness. We conclude that characteristics related to the broader context surrounding a wetland stopover sites strongly influence stopover use by migratory shorebirds. Conservation and management of shorebirds migrating through the continental interior should aim to provide large expanses of sparsely vegetated and shallow habitats within stopover sites and focus on open landscapes that contain high densities of stopover habitats.

Steller's Eiders (Polysticta stelleri) that breed in Alaska, USA, are listed as threatened under the U.S. Endangered Species Act (ESA), yet the degree to which these individuals segregate during the nonbreeding period from conspecifics that nest in Russia is unknown. Likewise, very little is known about the timing of use and distribution of autumn migration routes, stopover sites, and molting and wintering areas by the Alaska-breeding population. To address this information need, we implanted 14 Steller's Eiders with satellite transmitters in 2000 and 2001 at their primary Alaskan breeding grounds near Barrow. We found no evidence for segregation of the Alaska-breeding population in midwinter because locations were well-distributed along the Alaska Peninsula, congruent with prevailing knowledge about the wintering distribution of Steller's Eiders that breed in Russia. During the wing molt, from late August to early October, 7 of 13 individuals used Kuskokwim Shoals, corroborating the importance of this area and its designation as critical habitat under the ESA. Steller's Eiders are generally described as preferring shallow waters <10 m deep, but our winter tracking data clearly documented occupancy of deeper offshore waters. Steller's Eiders frequently used up to 30-m deep water almost exclusively at night during winter. We speculate that nighttime occupancy of deeper water habitats may be for resting and/or for consumption of zooplankton species, such as euphausiids, that are abundant and well known for their nocturnal vertical migrations in the water column.

The extensive breeding range of many shorebird species can make integration of survey data problematic at regional spatial scales. We evaluated the effectiveness of standardized repeated count surveys coordinated across 8 agencies to estimate the abundance of American Oystercatcher (Haematopus palliatus) breeding pairs in the southeastern United States. Breeding season surveys were conducted across coastal North Carolina (90 plots) and the Eastern Shore of Virginia (3 plots). Plots were visited on 1–5 occasions during April–June 2013. N-mixture models were used to estimate abundance and detection probability in relation to survey date, tide stage, plot size, and plot location (coastal bay vs. barrier island). The estimated abundance of oystercatchers in the surveyed area was 1,048 individuals (95% credible interval: 851–1,408) and 470 pairs (384–637), substantially higher than estimates that did not account for detection probability (maximum counts of 674 individuals and 316 pairs). Detection probability was influenced by a quadratic function of survey date, and increased from mid-April (~0.60) to mid-May (~0.80), then remained relatively constant through June. Detection probability was also higher during high tide than during low, rising, or falling tides. Abundance estimates from N-mixture models were validated at 13 plots by exhaustive productivity studies (2–5 surveys wk⁻¹). Intensive productivity studies identified 78 breeding pairs across 13 productivity plots while the N-mixture model abundance estimate was 74 pairs (62–119) using only 1–5 replicated surveys season⁻¹. Our results indicate that standardized replicated count surveys coordinated across multiple agencies and conducted during a relatively short time window (closure assumption) provide tremendous potential to meet both agency-level (e.g., state) and regional-level (e.g., flyway) objectives in large-scale shorebird monitoring programs.

Detection probability during point counts and its associated variables are important considerations for bird population monitoring and have implications for conservation planning by influencing population estimates. During 2008–2009, we evaluated variables hypothesized to be associated with detection probability, detection latency, and behavioral responses of male Golden-winged Warblers in pastures in the Monongahela National Forest, West Virginia, USA. This is the first study of male Golden-winged Warbler detection probability, detection latency, or behavioral response based on point-count sampling with known territory locations and identities for all males. During 3-min passive point counts, detection probability decreased as distance to a male's territory and time since sunrise increased. During 3-min point counts with playback, detection probability decreased as distance to a male's territory increased, but remained constant as time since sunrise increased. Detection probability was greater when point counts included type 2 compared with type 1 song playback, particularly during the first 2 min of type 2 song playback. Golden-winged Warblers primarily use type 1 songs (often zee bee bee bee with a higher-pitched first note) in intersexual contexts and type 2 songs (strident, rapid stutter ending with a lower-pitched buzzy note) in intrasexual contexts. Distance to a male's territory, ordinal date, and song playback type were associated with the type of behavioral response to song playback. Overall, ~2 min of type 2 song playback may increase the efficacy of point counts for monitoring populations of Golden-winged Warblers by increasing the conspicuousness of males for visual identification and offsetting the consequences of surveying later in the morning. Because playback may interfere with the ability to detect distant males, it is important to follow playback with a period of passive listening. Our results indicate that even in relatively open pasture vegetation, detection probability of male Golden-winged Warblers is imperfect and highly variable.

Reservoirs often have highly fluctuating water levels. The perimeters of these impoundments, which alternate between being exposed or inundated by water (drawdown zone), are used by nesting birds, but at the risk of nest submergence when water levels rise. For species that nest above the ground in shrubs, foraging and predation may also be affected by flooded habitat. Our objective was to clarify the net impact that habitat flooding has on nest survivorship at Arrow Lakes Reservoir, British Columbia, Canada. This reservoir typically shows a pattern of water management where water is stored during the spring snowmelt (increasing water levels) and released later in the year. Yellow Warblers (Setophaga petechia; n = 272 nests) and Willow Flycatchers (Empidonax traillii; n = 81 nests) nested in similar parts of the drawdown zone, but differences in their nesting behaviors, particularly timing of nesting, caused the flycatchers to experience more nest submergence. Flycatchers also nested on a floating island of bog habitat, offering them some protection from nest submergence. We found little evidence that drawdown zone shrubs functioned as ecological traps. In flooded conditions, 28% of warbler nests failed due to submergence. Warbler nest daily survival rate (DSR) declined with advancing ordinal date, and we concluded that their DSR was not influenced by habitat flooding. For flycatchers, 50% of nest failures were caused by submergence under flooded conditions, but DSR did not differ between flooded (0.960 ± 0.009) and unflooded conditions (0.958 ± 0.012). We speculate that these counterintuitive results may be explained by a reduction in predation levels during flooded conditions, which may compensate for nest submergence. Finally, we found that nest DSR was enhanced in the floating island habitat (0.986 ± 0.005), indicating that floating habitat islands can be highly productive and may hold potential as a management tool for enhancing productivity of reservoir drawdown zones.

We compared the efficacy of standard nest-searching methods with finding nests via radio-tagged birds to assess how search technique influenced our determination of nest-site characteristics and nest success for Golden-winged Warblers (Vermivora chrysoptera). We also evaluated the cost-effectiveness of using radio-tagged birds to find nests. Using standard nest-searching techniques for 3 populations, we found 111 nests in locations with habitat characteristics similar to those described in previous studies: edges between forest and relatively open areas of early successional vegetation or shrubby wetlands, with 43% within 5 m of forest edge. The 83 nests found using telemetry were about half as likely (23%) to be within 5 m of forest edge. We spent little time searching >25 m into forest because published reports state that Golden-winged Warblers do not nest there. However, 14 nests found using telemetry (18%) were >25 m into forest. We modeled nest success using nest-searching method, nest age, and distance to forest edge as explanatory variables. Nest-searching method explained nest success better than nest age alone; we estimated that nests found using telemetry were 10% more likely to fledge young than nests found using standard nest-searching methods. Although radio-telemetry was more expensive than standard nest searching, the cost-effectiveness of both methods differed depending on searcher experience, amount of equipment owned, and bird population density. Our results demonstrate that telemetry can be an effective method for reducing bias in Golden-winged Warbler nest samples, can be cost competitive with standard nest-searching methods in some situations, and is likely to be a useful approach for finding nests of other forest-nesting songbirds.

Successful management of reintroduced populations requires recognizing that ecological conditions may have changed between extirpation and reintroduction. For example, characterizing dietary patterns of generalist apex predators in the past and present can help to define how their functional role may change as translocated populations grow. We identified prey remains collected from Bald Eagle (Haliaeetus leucocephalus) nests and used carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analysis to quantify diet composition of the recently reintroduced Bald Eagle population on the Channel Islands off southern California, USA. We collected >6,000 prey items from recently occupied nests on Santa Catalina, Santa Rosa, Santa Cruz, and Anacapa islands in 2010 and 2011. Prey identification and stable isotope analysis yielded similar results and showed that eagles on Santa Catalina Island consumed a high proportion (~60%) of marine fish and a lower proportion (25–30%) of seabirds, while their counterparts on the Northern Channel Islands consumed equal proportions (~40–45%) of these prey types. Terrestrial resource use was low with the exception of eagles from one nest on Santa Catalina Island, where eagles primarily consumed ground squirrels and freshwater fish. We suggest that a combination of natural and anthropogenic factors is responsible for the interisland differences in Bald Eagle diet. Bald Eagle interactions with a robust recreational fishery off Santa Catalina Island may enhance access to fish species that are not available to eagles on the Northern Channel Islands, where the availability of breeding seabirds is far greater. The proportion of seabirds consumed by eagles on the Northern Channel Islands today is similar to that consumed by eagles from this region historically and prehistorically. This suggests that the restoration of breeding seabirds on the Channel Islands will benefit the long-term viability of eagle populations in the northern archipelago.

Knowledge of the distribution and movements of populations of migratory birds is useful for the effective conservation and management of biodiversity. However, such information is often unavailable because of the difficulty of tracking sufficient numbers of individuals. We used more easily obtained feather stable hydrogen isotope ratios (δ²H) to predict the summer grounds of the small, threatened, and migratory population of Golden Eagles (Aquila chrysaetos) in eastern North America. We then identified summer locations and the extent of migratory connectivity for this population. We collected δ²H (δ²H_f), stable carbon isotope (δ¹³C), and stable nitrogen isotope (δ¹⁵N) data from the body feathers of 47 juvenile, subadult, and adult Golden Eagles. Values of δ¹³C and δ¹⁵N suggested that all but 2 birds obtained food from terrestrial-based food webs and therefore that δ²H data were appropriate for inferring the geographic region of molt for the majority of birds. There was relatively large interfeather variation in the δ²H values of subadults vs. adults, suggesting that these groups molted at different times and places. The most negative δ²H_f values from birds with known summering grounds exhibited (1) a negative correlation with their summering latitude, and (2) a positive correlation with amount-weighted δ²H values of May–August precipitation at the summer location. These data validate the use of δ²H_f values for inferring the summer locations of Golden Eagles of unknown origin. Likelihood-of-origin maps derived from δ²H_f values revealed that (1) the majority of birds spent the breeding season in central Québec and Labrador, and (2) birds that wintered at southern latitudes, from approximately northern Alabama to southwestern Virginia, migrated about twice the distance of birds that wintered at northern latitudes, from Pennsylvania to New York. We observed a positive relationship between δ²H_f values and the latitude of the wintering location, which, along with the likelihood-of-origin maps, revealed moderate patterns of leapfrog migration and migratory connectivity.

Wildfire is an important disturbance regime that can structure wildlife communities and their habitats for many years. Using a before-after-control-impact framework, we evaluated the effect of the Quartz Fire on a mixed broadleaf–conifer forest and associated bird community in southwestern Oregon, USA, over 10 yr. To assess whether fire severity explained changes better than simply whether an area was burned, we used a tiered sampling approach by comparing unburned control points with either all burned points combined (burned) or those same points partitioned by severity level (low, moderate, high). As expected, overall tree cover decreased while cover of shrubs increased in response to greater fire severity. This pattern was most pronounced in high-severity areas, where tree cover declined by 40% and remained depressed, but shrub cover recovered from 10% the year following fire to 75% by year 6. Ordinations of bird species density showed turnover in community composition in all burned areas combined, as well as in moderate-severity areas, shifting to a shrub-associated community 9 yr postfire. For individual species, annual density variations were best explained by fire for 14 of 37 species, with fire severity providing the best-fitting model for 7 species. Of those 7 species, 3 declined and 4 increased with greater severity. When grouped into guilds, flycatching foragers and shrub nesters increased with greater fire severity. Our results illustrate the importance of mixed-severity wildfire in creating diverse vegetation structure and composition that supports distinct bird communities for at least a decade following fire.

Analyzing the genetic diversity of a population can be useful in identifying some of the silent effects of habitat fragmentation. We examined and compared 4 mitochondrial loci of the near threatened Short-tailed Babbler (Pellorneum malaccense), a rainforest understory bird, between a highly fragmented population in Singapore and populations from elsewhere in the Sundaic region. We found that the fragmented Singaporean population is genetically impoverished, with <20% of the intrapopulation genetic diversity present in a comparable sample from Borneo within intact forest. Differences in haplotype diversity among Singaporean sites of occurrence suggest that there may be extremely poor genetic diversity and connectivity among Singaporean subpopulations. These findings highlight the importance of connectivity in maintaining population genetic diversity in urban tropical forest patches. Our results also corroborate previous findings that there are 3 deeply diverged lineages of Short-tailed Babbler across the Sundaic region.

Most abundance estimation methods assume that all sampled individuals are identified correctly. In practice, this assumption may be difficult to meet and can bias abundance estimates, especially when morphologically similar species overlap in range. Over the past 2 decades, Kittlitz's Murrelet (Brachyramphus brevirostris) populations appear to have declined across parts of their Alaskan range, where they co-occur with the Marbled Murrelet (B. marmoratus). Recently, the reliability of Kittlitz's Murrelet declines has been questioned due to variability and uncertainty in species identification between the 2 species. We conducted a field experiment to quantify misidentification and partial identification (identification to genus [Brachyramphus] level only) of Kittlitz's and Marbled murrelets during abundance surveys, and to evaluate the relative impacts of environmental and observational factors on misidentification and partial identification. We applied these results to previously collected survey data to measure the potential bias of abundance estimates resulting from varying identification rates. Overall, the misidentification rate during our field experiment was 0.036 ± 0.004 (SE), with observer experience best explaining the variation. Abundance estimates adjusted for misidentification reflected little bias. The overall partial identification rate was much higher than the misidentification rate (0.211 ± 0.007 SE). Partial identification rates increased in choppy sea states, with greater observation distances, and when murrelets exhibited diving behavior; rates decreased with increased observer experience and when murrelets exhibited flushing behavior. Because observer experience was an important driver of both misidentification and partial identification, we stress the importance of conducting rigorous observer training before and during surveys to increase confidence in species identification and precision in abundance estimates. The methods developed in this study could be modified for any at-sea survey scenario to measure identification rates and the factors influencing these rates. Results may reveal important relationships for adjusting survey protocols to increase confidence in species identification and thereby to increase the precision of abundance estimates.

Conserving threatened and endangered species requires knowledge of breeding productivity and factors that cause variation in reproductive success. We summarized 13 years of lifetime reproductive success (LRS) data for 195 individually marked Snowy Plovers (Charadrius nivosus) breeding in Humboldt County, CA. Reproductive success was highly skewed among individuals with 13% of individuals (n_males = 12, n_females = 14) producing ~50% of fledglings; by contrast, 71% (n = 64) of males and 72% (n = 76) of females produced 2 or fewer during their lifetime. Variance in LRS was best explained by substrate (~100% of Akaike weight), with plovers breeding on gravel having significantly higher LRS compared to those on sandy substrates. Other measures of habitat quality, including use of nest exclosures, as well as corvid and human activity, were not significant predictors of LRS. Results indicated that enhancing the cryptic nature of substrates (for eggs and chicks) may be a productive means of increasing reproductive success in this threatened species.