Using microphone arrays to examine effects of observers on birds during point count surveys

ABSTRACT Point count surveys are widely used for monitoring songbird populations, but little is known of the effect of the observer on songbird behavior during point counts. We used a novel, wireless array of recorders to determine the location of singing birds with and without the presence of an observer. The array consisted of seven autonomous recording units synchronized to Global Positioning System (GPS) clocks, set around the perimeter of a 50‐m‐radius circle with one in the middle. Units were set to record automatically from half an hour before dawn until 10:00 each morning. We sampled 26 different locations in old fields at the Prince Edward Point National Wildlife Area in eastern Ontario between 1 June and 4 July 2007. Position estimates derived from a time‐lag cross‐correlation algorithm had a mean error of 1.7 m within 50 m and 5.6 m at 100 m from the center of the array. We found no difference in the positions of birds when an observer was present or absent. We also found no difference in the number of individuals or species detected or in the onset of singing. Our results suggest that, at least in the community we studied, observers conducting point counts do not cause significant changes in bird behavior.     

A field test of the distance sampling method using Golden‐cheeked Warblers

ABSTRACT Criteria for delisting Golden‐cheeked Warblers (Dendroica chrysoparia) include protection of sufficient breeding habitat to ensure the continued existence of 1000 to 3000 singing males in each of eight recovery regions for ≥10 consecutive years. Hence, accurate abundance estimation is an integral component in the recovery of this species. I conducted a field test to determine if the distance sampling method provided unbiased abundance estimates for Golden‐cheeked Warblers and develop recommendations to improve the accuracy of estimates by minimizing the effects of violating this method's assumptions. To determine if observers could satisfy the assumptions that birds are detected at the point with certainty and at their initial locations, I compared point‐transect sampling estimates from 2‐, 3‐, 4‐, and 5‐min time intervals to actual abundance determined by intensive territory monitoring. Point‐transect abundance estimates were 15%, 29%, 43%, and 59% greater than actual abundance (N= 156) for the 2‐, 3‐, 4‐, and 5‐min intervals, respectively. Point‐transect sampling produced unbiased estimates of Golden‐cheeked Warbler abundance when counts were limited to 2 min (N= 154–207). Abundance estimates derived from point‐transect sampling were likely greater than actual abundance because observers did not satisfy the assumption that birds were detected at their initial locations due to the frequent movements and large territory sizes of male Golden‐cheeked Warblers. To minimize the effect of movement on abundance estimates, I recommend limiting counts of singing males to 2‐min per point. Counts for other species in similar habitats with similar behavior and movement patterns also should be limited to 2 min when unbiased estimates are important and conducting field tests of the point‐transect distance sampling method is not possible.     

Assessment of census techniques for interspecific comparisons of tropical rainforest bird densities: a field evaluation in the Western Ghats, India

Studies comparing different bird censusing methods are useful for assessing relative biases, synthesizing data across studies, and designing bird population monitoring programmes. A field study was carried out in mid-elevation tropical rainforest in the Western Ghats to compare bird density estimates from line transect, point count and territory spot-mapping methods. Interspecific comparisons were made using data for 13 common resident bird species, including two endemics. Variable-width line transect density estimates were highly correlated with, but slightly (17%) higher than, those produced by territory spot-mapping. Although densities from variable-width point counts and spot-mapping were highly positively correlated, the estimates were 95% higher on average in the former. Higher density estimates relative to spot-mapping were produced mainly for the most abundant species, probably due to their mobility and the inclusion of additional individuals that enter the count area during the count period. Fixed-width strip transects and point counts produced density estimates that were highly correlated with, but significantly lower than, variable-width estimates. Wherever possible, territory spot-mapping and line transects are recommended for density estimates; the former may yield additional information on spatial distribution of birds. Fixed-width transects or point counts, being easier to apply, may be used for large-scale monitoring programmes. Interspecific variation in flocking systems and the poor visibility in dense rainforest vegetation indicate the need for care in collection of data on flock size and its variation, which is necessary for estimating the density of individuals. The variation across methods suggests the need for further research using multiple methods across years and marked individuals to verify territoriality and accuracy.     

Effect of count duration on abundance estimates of Black-capped Vireos

ABSTRACT.   Distance sampling applied to point count surveys (point transects) has become a common method for estimating the absolute abundance of birds. When conducting point transects, detections of focal species are typically recorded during a fixed time interval. However, count duration has varied among studies and the effect of such variation on the resulting abundance estimates is unclear. My objective was to examine the effect of count duration on abundance estimates of male Black-capped Vireos ( Vireo atricapilla ). The abundance of these vireos in a 349-ha area in central Texas was estimated using 3-, 5-, and 6-min point transects and results were then compared to actual number present as determined by banding and territory mapping. The 3-min counts provided an estimate that was 26% greater than the actual number of male Vireos present ( N = 201), but this number was within the corresponding 95% confidence interval ( N = 157–413). Confidence intervals for the 5- and 6-min counts did not include the actual number of vireos present. The shortest count duration may have provided the most accurate abundance estimate because male Black-capped Vireos are typically active, sing intermittently, and sometimes move tens of meters between songs. Thus, shorter-duration counts may also yield the most accurate abundance estimates for other species that exhibit similar behavior. However, because behavior varies among species, I recommend that investigators collect preliminary data to establish an appropriate count duration when accurate estimates of absolute, rather than relative, abundance are important.     

Adjusting count period strategies to improve the accuracy of forest bird abundance estimates from point transect distance sampling surveys

Although point transect distance sampling methods have become widely used in surveys of forest birds, there has been no attempt to tailor field methods to maximize the accuracy of abundance estimates by minimizing the effects of violations of the method's critical assumptions, which are: (1) birds at distance 0 m are detected with certainty, (2) birds are detected at their initial location and (3) distances to objects are measured accurately. We investigate the effects on abundance estimates for Philippine forest birds of varying the count period from 2 to 10 min, and of including and excluding a pre‐count settling down period. Encounter rates were highly sensitive to count period length but density estimates from 10‐min count periods were, on average, only 13% higher than those from 2‐min periods, and in several cases were actually lower than those from periods of 6–8 min. This was because birds tended to be recorded at greater distances from the recorder as the count period went on, thus ‘stretching out’ detection functions while having little effect on detection rates close to the recorder. For some bird groups, including canopy frugivores and upperstorey gleaning insectivores, density estimates were more than twice as high without than with a settling down period. We suggest that movement away from the recorder is more common than attraction to the recorder, and that unless pilot studies show otherwise, similar studies should not use a settling down period for surveying many species. Count periods that maximized probability of bird detection close to the central point while minimizing the unwanted effects of bird movement during the count period were: 4 min for omnivores, 6 min for nectarivores and upperstorey gleaning insectivores, 8 min for understorey insectivores and canopy frugivores, and a full 10 min for sallying insectivores, ground‐dwellers, carnivores and coucals/koels. We use the results to suggest ‘group‐specific’ count period regimes that could help maximize the accuracy of density estimates from similar studies of tropical forest birds.     

Using soundscape recordings to estimate bird species abundance, richness, and composition

ABSTRACT Point counts are the most frequently used technique for sampling bird populations and communities, but have well‐known limitations such as inter‐ and intraobserver errors and limited availability of expert field observers. The use of acoustic recordings to survey birds offers solutions to these limitations. We designed a Soundscape Recording System (SRS) that combines a four‐channel, discrete microphone system with a quadraphonic playback system for surveying bird communities. We compared the effectiveness of SRS and point counts for estimating species abundance, richness, and composition of riparian breeding birds in California by comparing data collected simultaneously using both methods. We used the temporal‐removal method to estimate individual bird detection probabilities and species abundances using the program MARK. Akaike's Information Criterion provided strong evidence that detection probabilities differed between the two survey methods and among the 10 most common species. The probability of detecting birds was higher when listening to SRS recordings in the laboratory than during the field survey. Additionally, SRS data demonstrated a better fit to the temporal‐removal model assumptions and yielded more reliable estimates of detection probability and abundance than point‐count data. Our results demonstrate how the perceptual constraints of observers can affect temporal detection patterns during point counts and thus influence abundance estimates derived from time‐of‐detection approaches. We used a closed‐population capture–recapture approach to calculate jackknife estimates of species richness and average species detection probabilities for SRS and point counts using the program CAPTURE. SRS and point counts had similar species richness and detection probabilities. However, the methods differed in the composition of species detected based on Jaccard's similarity index. Most individuals (83%) detected during point counts vocalized at least once during the survey period and were available for detection using a purely acoustic technique, such as SRS. SRS provides an effective method for surveying bird communities, particularly when most species are detected by sound. SRS can eliminate or minimize observer biases, produce permanent records of surveys, and resolve problems associated with the limited availability of expert field observers.     

Estimation of parrot and hornbill densities using a point count distance sampling method

The suitability of point count distance methods for estimating densities of tropical parrots and hornbills was assessed during surveys in Indonesia. The methods will perform well, so long as the following are considered. (1) Enough bird records must be accumulated to model species' detection curves precisely. For some species, around 2000 point counts may be needed and, in very rare species, the method may not be appropriate. Pooling data across habitats, species or years may increase precision in cases of small sample size. (2) Point counts are likely to be less biased than line transects because bird detection rates close to the recorder may be higher and there may be less chance of double-counting birds. Distances greater than 400 m between census points are unnecessary. (3) Count periods lasting ten minutes may be needed to ensure that most birds close to the recorder are detected. Controlled flushing of concealed birds after the main count period may also be appropriate. (4) The best time of day for census is the period when bird detectability is high but bird mobility low. For many large avian frugivores, this corresponds to the period between one hour after dawn and 10.30 h. (5) Records of flying birds must be excluded from density calculations. In the species studied, between 2% and 20% should be added to density estimates to compensate for the exclusion of flying birds.     

Distance models as a tool for modelling detection probability and density of native bumblebees

Effective monitoring of native bee populations requires accurate estimates of population size and relative abundance among habitats. Current bee survey methods, such as netting or pan trapping, may be adequate for a variety of study objectives but are limited by a failure to account for imperfect detection. Biases due to imperfect detection could result in inaccurate abundance estimates or erroneous insights about the response of bees to different environments. To gauge the potential biases of currently employed survey methods, we compared abundance estimates of bumblebees (Bombus spp.) derived from hierarchical distance sampling models (HDS) to bumblebee counts collected from fixed‐area net surveys (“net counts”) and fixed‐width transect counts (“transect counts”) at 47 early‐successional forest patches in Pennsylvania. Our HDS models indicated that detection probabilities of Bombus spp. were imperfect and varied with survey‐ and site‐covariates. Despite being conspicuous, Bombus spp. were not reliably detected beyond 5 m. Habitat associations of Bombus spp. density were similar across methods, but the strength of association with shrub cover differed between HDS and net counts. Additionally, net counts suggested sites with more grass hosted higher Bombus spp. densities whereas HDS suggested that grass cover was associated with higher detection probability but not Bombus spp. density. Density estimates generated from net counts and transect counts were 80%–89% lower than estimates generated from distance sampling. Our findings suggest that distance modelling provides a reliable method to assess Bombus spp. density and habitat associations, while accounting for imperfect detection caused by distance from observer, vegetation structure, and survey covariates. However, detection/non‐detection data collected via point‐counts, line‐transects and distance sampling for Bombus spp. are unlikely to yield species‐specific density estimates unless individuals can be identified by sight, without capture. Our results will be useful for informing the design of monitoring programs for Bombus spp. and other pollinators.     

Comparison of survey methods for wintering grassland birds

ABSTRACT Although investigators have evaluated the efficacy of survey methods for assessing densities of breeding birds, few comparisons have been made of survey methods for wintering birds, especially in grasslands. In winter, social behavior and spatial distributions often differ from those in the breeding season. We evaluated the degree of correspondence between density estimates based on different survey methods. Surveys were conducted during two winters (2001–2002 and 2002–2003) on 16 grassland sites in southwestern Oklahoma. Line‐transect (using a detection function to account for birds present but not detected) and area‐search (where density was based on the total count within a given area) methods were employed. Observations on line transects were also analyzed as strip transects, where density was based on total count within a given strip width and no detection function was used. Savannah Sparrows (Passerculus sandwichensis), LeConte's Sparrows (Ammodramus leconteii), Song Sparrows (Melospiza melodia), Smith's Longspurs (Calcarius pictus), Chestnut‐collared Longspurs (C. ornatus), and Eastern Meadowlarks (Sturnella magna) were sufficiently abundant to allow comparison. Area‐search density estimates tended to be higher than line‐transect estimates for Savannah Sparrows, Song Sparrows, and Eastern Meadowlarks, suggesting that some individuals initially located close to the transect line were not detected on line transects. The area‐search and line‐transect methods gave similar density estimates for Chestnut‐collared and Smith's longspurs. Area‐search estimates of Eastern Meadowlarks were significantly higher in the second year of the study only. For this species, area‐search estimates did not differ from those of strip transects covering an equal area, so the reason for the differing meadowlark estimates is not clear. Higher density estimates using the area‐search method likely resulted from: (1) birds that might escape detection by hiding were more likely detected (flushed) during area searches because of the repeated passes through the area, and (2) birds close to the line in line transects escape detection by hiding, biasing those estimates low. We also evaluated the correspondence of density rankings for the six species as determined by the different survey methods and for the same species across survey sites. Correlations among the six species of the area‐search results with those of line transects and strip transects generally were high, increasing in 2002–2003 when densities of birds were greater. All three methods provided similar density rankings among species. Density rankings within species across sites for the four non‐longspur species generally were concordant for the three methods, suggesting that any of them will adequately reflect among‐site differences, especially when densities vary greatly across sites. Further research is needed to determine the extent to which grassland birds are missed on line transects. We suggest that workers using line transects to study these species give careful consideration and make additional efforts to satisfy the distance‐sampling assumption that all birds on or near the line are detected. If density is measured as a total count in a fixed area, we recommend that observers pass within <10 m of all points in the area.     

Implications of assumption violation in density estimates of antelope from dung-heap counts: a case study on grey duiker (Sylvicapra grimmia) in Zimbabwe

Dung‐heap counts were used to estimate density of grey duiker (Sylvicapra grimmia Linnaeus 1758) in the Matobo National Park, Zimbabwe. To test assumptions of this method, defecation rate and defecation site selection were investigated under captive and field conditions, and densities were compared with independent estimates derived from territory mapping. Many assumptions were violated: males defecated more frequently than females with mean dry mass per deposit greater in females, but total daily faecal production was similar between sexes. Spatial distribution of faeces was clumped, and 52.8% of locations contained multiple deposits. Duikers exhibited habitat type preferences (i.e. low‐ to medium‐density woodland) with herbaceous layer heights 40–100 cm and visibility >20 m. Calculated grey duiker density from dung‐heap counts in cleared plots was 9.7 ± 1.3 animals km⁻², approximately double the territory‐mapping estimate based on Minimum Convex Polygons (5.13 animals km⁻²) but similar to the 75% Fixed Kernel estimate (10.95 animals km⁻²). Provided that sex ratios approach parity and sampled area is representative of all utilized habitats, violation of basic assumptions of the dung‐heap count method has a minor effect on density estimate accuracy.     

A Field Evaluation of Distance Measurement Error in Auditory Avian Point Count Surveys

ABSTRACT Detection distance is an important and common auxiliary variable measured during avian point count surveys. Distance data are used to determine the area sampled and to model the detection process using distance sampling theory. In densely forested habitats, visual detections of birds are rare, and most estimates of detection distance are based on auditory cues. Distance sampling theory assumes detection distances are measured accurately, but empirical validation of this assumption for auditory detections is lacking. We used a song playback system to simulate avian point counts with known distances in a forested habitat to determine the error structure of distance estimates based on auditory detections. We conducted field evaluations with 6 experienced observers both before and after distance estimation training. We conducted additional studies to determine the effect of height and speaker orientation (toward or away from observers) on distance estimation error. Distance estimation errors for all evaluations were substantial, although training reduced errors and bias in distance estimates by approximately 15%. Measurement errors showed a nonlinear relationship to distance. Our results suggest observers were not able to differentiate distances beyond 65 m. The height from which we played songs had no effect on distance estimation errors in this habitat. The orientation of the song source did have a large effect on distance estimation errors; observers generally doubled their distance estimates for songs played away from them compared with distance estimates for songs played directly toward them. These findings, which we based on realistic field conditions, suggest measures of uncertainty in distance estimates to auditory detections are substantially higher than assumed by most researchers. This means aural point count estimates of avian abundance based on distance methods deserve careful scrutiny because they are likely biased.     

Effects of experimental playbacks on availability for detection during point counts

Point counts are the most commonly used technique for surveying passerines during the breeding season. Several methods for estimating probabilities of detection during point count surveys have been developed. These methods have focused primarily on accounting for the influence of environmental factors (e.g., weather and noise) on detectability, however, the probability that birds are available for detection (e.g., sings or moves) during point counts has received less attention. We used sequential point counts to determine the effect of playback of the mobbing calls of Black‐capped Chickadees (Poecile atricapillus) and the flight calls of Red‐tailed Hawks (Buteo jamaicensis) on availability for detection (e.g., singing or moving) during point‐count surveys. We conducted 180 point counts over a 2‐yr period in central – east central Minnesota to evaluate the possible effect of playbacks on observed density, overall species richness, minute of first detection, and distance of first detection. We also used removal models to quantify the magnitude of changes in detectability and direction of response to playbacks for 10 focal species. Playback of the mobbing calls of Black‐capped Chickadees increased observed density and decreased the average distance of detection and time of first detection, whereas playback of the flight calls of a Red‐tailed Hawk resulted in a decrease in observed density and species richness, and an increased time of first detection. Playback treatment was a covariate in all best performing models for the 10 species analyzed, but the magnitude and direction of response to playbacks were species specific. The importance of playback type in detectability models indicates that the calls of heterospecifics can influence species availability for detection. As such, researchers using playback methods should seek to quantify species‐specific responses in detection probability and consider how component detection probabilities could influence survey outcomes.     

Comparison of methods for estimating density of forest songbirds from point counts

New analytical methods have been promoted for estimating the probability of detection and density of birds from count data but few studies have compared these methods using real data. We compared estimates of detection probability and density from distance and time-removal models and survey protocols based on 5- or 10-min counts and outer radii of 50 or 100 m. We surveyed singing male Acadian flycatchers (Empidonax virescens), cerulean warblers (Dendroica cerulea), Kentucky warblers (Oporornis formosus), Louisiana waterthrushes (Parkesia motacilla), wood thrushes (Hylocichla mustelina), and worm-eating warblers (Helmitheros vermivorum) in bottomland and upland forest across 5 states in the Central Hardwoods Bird Conservation Region during the breeding season in 2007 and 2008. Detection probabilities differed between distance and time-removal models and species detectabilities were affected differently by year, forest type, and state. Density estimates from distance models were generally higher than from time-removal models, resulting from lower detection probabilities estimated by distance models. We found support for individual heterogeneity (modeled as a finite mixture model) in the time-removal models and that 50-m radius counts generated density estimates approximately twice as high as 100-m radius counts. Users should be aware that in addition to estimating different components of detectability, density estimates derived from distance and time-removal models can be affected by survey protocol because some count durations and plot radii may better meet model assumptions than others. The choice of a method may not affect the use of estimates for relative comparisons (e.g., when comparing habitats) but could affect conclusions when used to estimate population size. We recommend careful consideration of assumptions when deciding on point-count protocol and selection of analysis methods. © 2011 The Wildlife Society.     

Evaluation of three methods to estimate density and detectability from roadside point counts

Roadside point counts are often used to estimate trends of bird populations. The use of aural counts of birds without adjustment for detection probability, however, can lead to incorrect population trend estimates. We compared precision of estimates of density and detectability of whistling northern bobwhites (Colinus virginianus) using distance sampling, independent double-observer, and removal methods from roadside surveys. Two observers independently recorded each whistling bird heard, distance from the observer, and time of first detection at 362 call-count stops in Ohio. We examined models that included covariates for year and observer effects for each method and distance from observer effects for the double-observer and removal methods using Akaike's Information Criterion (AIC). The best model of detectability from distance sampling included observer and year effects. The best models from the removal and double-observer techniques included observer and distance effects. All 3 methods provided precise estimates of detection probability (CV = 2.4–4.4%) with a range of detectability of 0.44–0.95 for a 6-min survey. Density estimates from double-observer surveys had the lowest coefficient of variation (2005 = 3.2%, 2006 = 1.7%), but the removal method also provided precise estimates of density (2005 CV = 3.4%, 2006 CV = 4.8%), and density estimates from distance sampling were less precise (2005 CV = 9.6%, 2006 CV = 7.9%). Assumptions of distance sampling were violated in our study because probability of detecting bobwhites near the observer was <1 or the roadside survey points were not randomly distributed with respect to the birds. Distances also were not consistently recorded by individual members of observer pairs. Although double-observer surveys provided more precise estimates, we recommend using the removal method to estimate detectability and abundance of bobwhites. The removal method provided precise estimates of density and detection probability and requires half the personnel time as double-observer surveys. Furthermore, the likelihood of meeting model assumptions is higher for the removal survey than with independent double-observers. © 2011 The Wildlife Society.     

Monitoring populations of Western Sandpipers and Pacific Dunlins during northward migration on the Fraser River Delta,British Columbia, 1991–2013

The Fraser River Delta in British Columbia, Canada, is a globally significant stopover site for shorebirds, but the population status and trends of many species that use the site remain uncertain. We describe an ongoing program to monitor population trends of the two most abundant species, Western Sandpipers (Calidris mauri) and Dunlins (Calidris alpina), during northward migration. Counts of these species were conducted at a mudflat where large flocks assembled at mid‐tide from 15 April to 15 May, 1991–2013, and we estimated species‐specific counts as the product of daily total flock counts and species proportions obtained during supplementary sampling. The median peak count of both species combined was 177,000 birds, and occurred between 24 April and 3 May. Ratios (proportions) of the two species followed a predictable pattern during the migration period, with a low proportion of Western Sandpipers (3%–20%) in flocks before 20 April, followed by a rapid increase to 80%–100% between 25 April and 10 May and a variable decrease to 30%–80% by 15 May. Mean counts of Western Sandpipers showed no significant trend over the study period. Mean counts of Dunlins showed a non‐linear trend, decreasing until 2001 and then increasing to 2013. Bias and random error in field counts were quantified by comparing field counts to counts made from photographs taken during surveys, and analysis revealed that field counts had a downward, but predictable, bias, accounting for >90% of birds present, with a stochastic error rate of 28.0%. Uncertainty in total population estimates was high after accounting for the effect of length of stay and sampling error. Population estimates suggested that 600,000 Western Sandpipers and 200,000 Dunlins typically passed through the site during northward migration. Our estimates indicate the usefulness of daily counts at major stopover sites during northward migration as an effective tool for monitoring shorebird populations, and underscore the need for conserving such sites.     

Density influences accuracy of model‐based estimates for a forest songbird

Golden‐cheeked Warblers (Setophaga chrysoparia) are endangered songbirds that breed exclusively in the Ashe juniper (Juniperus ashei) and oak (Quercus spp.) woodlands of central Texas. Despite being the focus of numerous studies, we still know little about the size of the range‐wide breeding population and how density varies across the spectrum of juniper co‐dominated woodlands. Models that have been tested and shown to be accurate are needed to help develop management and conservation guidelines. We evaluated the accuracy and bias of density estimates from binomial mixture models, the dependent double‐observer method, and distance sampling by comparing them to actual densities determined by intensive territory monitoring on plots in the Balcones Canyonlands Preserve, Austin, Texas. We found that the binomial mixture models consistently overestimated density by 1.1–3.2 times (actual density = 0.07–0.46 males/ha), and the other two models overestimated by 1.1–29.8 times at low density and underestimated by 0.5–0.9 times at high density plots (actual density = 0.01–0.46 males/ha). The magnitude of error for all models was greatest at sites with few or no birds (<0.15 males/ha), with model performance improving as actual density increased. These non‐linear relationships indicate a lack of sensitivity with respect to true changes in density. Until systematic evaluation demonstrates that models such as those we tested provide accurate and unbiased density estimates for a given species over space and time, we recommend additional field tests to validate model‐based estimates. Continued model validation and refinement of point‐count methods are needed until accurate estimates are obtained across the density spectrum for Golden‐cheeked Warblers and other songbird species.     

Habitat associations of papyrus specialist birds at three papyrus swamps in western Kenya

Birds of papyrus swamps have not been adequately studied in Kenya, and little is known about their ecology and habitat associations. Using fixed‐radius point counts and playbacks, we counted papyrus specialist birds and evaluated papyrus physical characteristics and levels of disturbance at a series of sample stations at three papyrus swamps of Dunga, Koguta and Kusa in the Kenyan sector of Lake Victoria. Papyrus height and density were significantly correlated across all sites but negatively correlated with levels of disturbance. Standardized point counts of swamp birds showed the papyrus gonolek Laniarius mufumbiri and Carruthers's cisticola Cisticola carruthersi to be the most abundant papyrus specialists across sites. Only Carruthers's cisticola numbers differed between sites. Overall, papyrus cover was the best predictor of the presence and abundance of all papyrus specialist birds, and significantly predicted the numbers of papyrus gonolek and white‐winged warbler Bradypterus carpalis.     

Quantifying observer heterogeneity in bird counts

An essential pilot study was designed to quantify observer heterogeneity and to compare observation methods for the detectability of forest birds in stands of Eucalyptus and Pinus radiata forest as a basis for a major research project on habitat fragmentation near Tumut, southern New South Wales. Twelve experienced observers participated in the investigation. Point interval counts, zig-zag walks and strip transects were used to count birds in both eucalypt and pine forests. The 65 species of birds recorded in the study were assigned to one of nine groups classified by a set of attributes that characterized bird detection by field observers (e.g. body size, colour and calling patterns). Observer heterogeneity varied between groups of birds and was most apparent for small birds foraging in low shrubs (species such as the white-browed scrub wren, assigned to group 2), frequent calling, active birds (species such as the golden whistler, assigned to group 7), and midstorey, undercanopy foragers with distinctive behaviour (species such as the grey fantail assigned to group 4). For bird groups 2, 4 and 7, additional variability due to observer differences resulted in an average increase of ~ 40% in the width of a 95% confidence interval for the logarithm of bird abundance generated from a 20 minute count. Our analysis shows that taking the average of counts obtained by two or more observers would negate the increase in variance of counts due to observer heterogeneity. Few differences between methods of field observation were found. However, for frequent calling, active birds (group 7) there was evidence that more birds were heard using the point interval count method. Our study clearly demonstrated a need to either control for observer differences or to assign at least two observers to individual sites when designing bird surveys for comparative studies. Failure to do so will result in a decrease in precision of bird counts.     

Effects of point‐count duration on estimated detection probabilities and occupancy of breeding birds

Increasingly, point‐count data are used to estimate occupancy, the probability that a species is present at a given location; occupancy accounts for imperfect detection, the probability that a species is detected given that it is present. To our knowledge, effects of sampling duration on inferences from models of bird occupancy have not been evaluated. Our objective was to determine whether changing count duration from 5 to 8 min affected inferences about the occupancy of birds sampled in the Chesapeake Bay Lowlands (eastern United States) and the central and western Great Basin (western United States) in 2012 and 2013. We examined the proportion of species (two doves, one cuckoo, two swifts, five hummingbirds, 11 woodpeckers, and 122 passerines) for which estimates of detection probability were ≥ 0.3. For species with single‐season detection probabilities ≥ 0.3, we compared occupancy estimates derived from 5‐ and 8‐min counts. We also compared estimates for three species sampled annually for 5 yr in the central Great Basin. Detection probabilities based on both the 5‐ and 8‐min counts were ≥ 0.3 for 40% ± 3% of the species in an ecosystem. Extending the count duration from 5 to 8 min increased the detection probability to ≥ 0.3 for 5% ± 0.5% of the species. We found no difference in occupancy estimates that were based on 5‐ versus 8‐min counts for species sampled over two or five consecutive years. However, for 97% of species sampled over 2 yr, precision of occupancy estimates that were based on 8‐min counts averaged 12% ± 2% higher than those based on 5‐min counts. We suggest that it may be worthwhile to conduct a pilot season to determine the number of locations and surveys needed to achieve detection probabilities that are sufficiently high to estimate occupancy for species of interest.     

Census error and the detection of density dependence

1. Studies aiming to identify the prevalence and nature of density dependence in ecological populations have often used statistical analysis of ecological time-series of population counts. Such time-series are also being used increasingly to parameterize models that may be used in population management. 2. If time-series contain measurement errors, tests that rely on detecting a negative relationship between log population change and population size are biased and prone to spuriously detecting density dependence (Type I error). This is because the measurement error in density for a given year appears in the corresponding change in population density, with equal magnitude but opposite sign. 3. This effect introduces bias that may invalidate comparisons of ecological data with density-independent time-series. Unless census error can be accounted for, time-series may appear to show strongly density-dependent dynamics, even though the density-dependent signal may in reality be weak or absent. 4. We distinguish two forms of census error, both of which have serious consequences for detecting density dependence. 5. First, estimates of population density are based rarely on exact counts, but on samples. Hence there exists sampling error, with the level of error depending on the method employed and the number of replicates on which the population estimate is based. 6. Secondly, the group of organisms measured is often not a truly self-contained population, but part of a wider ecological population, defined in terms of location or behaviour. Consequently, the subpopulation studied may effectively be a sample of the population and spurious density dependence may be detected in the dynamics of a single subpopulation. In this case, density dependence is detected erroneously, even if numbers within the subpopulation are censused without sampling error. 7. In order to illustrate how process variation and measurement error may be distinguished we review data sets (counts of numbers of birds by single observers) for which both census error and long-term variance in population density can be estimated. 8. Tests for density dependence need to obviate the problem that measured population sizes are typically estimates rather than exact counts. It is possible that in some cases it may be possible to test for density dependence in the presence of unknown levels of census error, for example by uncovering nonlinearities in the density response. However, it seems likely that these may lack power compared with analyses that are able to explicitly include census error and we review some recently developed methods.