It counts who counts: an experimental evaluation of the importance of observer effects on spotlight count estimates

Spotlight surveys conducted by volunteers is a promising method to assess the abundance of nocturnally active mammals, but estimates are subject to bias if different observer groups differ in their ability to detect animals in the dark. We quantified the variation amongst volunteer spotlight observers with respect to their ability to detect and estimate distance to realistic animal silhouettes at different distances. Detection probabilities were higher for observers experienced in spotlighting mammals than for inexperienced observers, higher for observers with a hunting background compared with non-hunters and decreased as function of age but were independent of sex or educational background. If observer-specific detection probabilities were applied to real counting routes, point count estimates from inexperienced observers without a hunting background would only be 43 % (95 % CI, 39–48) of what inexperienced hunters with a hunting background would obtain and 29 % (25–33) of what experienced spotlight observers would detect. Mean estimated distances to objects did not deviate from true distances (no bias) but were highly imprecise. Female non-hunters estimated distances less precisely than other observers and precision increased with age. The study shows that observer effects may influence abundance estimates and underlines the importance of testing and accounting for observer effects when designing citizen science-based population survey programmes.     

Strong observer effect on tree microhabitats inventories: A case study in a French lowland forest

Validating biodiversity indicators requires an analysis of their applicability, their range of validity and their degree of correlation with the biodiversity they are supposed to represent. In this process, assessing the magnitude of observer effect is an essential step, especially if non-specialist observers are involved. Tree microhabitats – woodpecker cavities, cracks and bark characteristics – are reputed to be easily detected by non-specialists as microhabitat observation does not require prior forestry or ecology knowledge. We therefore quantified the probabilities of true and false positive detections made by observers during inventories.Within two 0.5 ha plots in a forest reserve that has not been harvested for at least 150 years, 14 observers with various backgrounds visually inventoried microhabitats on 106 oak (Quercus petraea and Quercus robur) and beech (Fagus sylvatica) trees. We used parametric and Bayesian statistics to compare these observers’ recorded observations with results from an independent census.The mean number of microhabitats per tree varied widely among observers – from 1.4 to over 3. Only five observers reported a mean number of microhabitats per tree that was statistically equivalent to the reference census. The probability of true detection also varied among observers for each microhabitat (from to 0 to 1) as did the probability of false positive detection (from 0 to 0.7). These results show that microhabitat inventories are particularly prone to observer effects.Such strong observer effects weaken the usefulness of microhabitats as biodiversity indicators. If microhabitat inventories are to be developed, we recommend controlling for observer effects by (i) defining standard operating procedures and multiplying the number of observer training sessions and of consensual standardization censuses; (ii) using pairs of observers to record microhabitats whenever possible (though the efficiency of this method remains to be tested); (iii) planning fieldwork so that the factors of interest are not confused with observer effects; and (iv) integrating observer profiles into the statistical models used to analyze the data.     

Citizen science and observer variability during American pika surveys

Within- and between-group observer variability can confound scientific discovery. If observer variability can be quantified and is addressed, data collected by participants with wide ranges of experience and training can yield more reliable inferences. The American pika (Ochotona princeps) is a mammalian sentinel of climate change that has received consideration for listing under the United States Endangered Species Act. As a result, numerous pika monitoring initiatives have been started throughout the mountains in western North America. Some initiatives employ research teams of biological science technicians (professionals), whereas many rely on networks of citizen scientists, or volunteers, for data collection. To date, few studies have quantified observer variability during pika surveys; none have explored the reliability of professional crews or volunteers. We conducted pika surveys in Glacier National Park, Montana, to quantify observer variability. We investigated observer variability 1) among a crew of professionals, 2) among volunteers, and 3) between professionals and volunteers. Professionals were more consistent at identifying pika signs and estimating potential home ranges and consistently found more pika signs than did the volunteers, with the exception of pika sightings. Estimates of pika occupancy were consistent at each site among volunteers conducting sitting surveys. We suggest that sitting surveys conducted by volunteers can reliably detect pika site occupancy. However, data on population dynamics of pikas (e.g., density) should be collected by professionals. Observer variability analyses of this nature should be common practice for wildlife-resource managers and scientists, especially with observers of varying levels of experience and motivation. © 2012 The Wildlife Society.     

Embryo recovery from superovulated mouflons (Ovis gmelini musimon) and viability after transfer into domestic sheep

In this study multiple ovulation and embryo transfer (MOET) technology was tested as a method for increasing the number of offspring obtained from superovulated mouflons and then using Sardinian ewes as recipients. Two experiments were carried out over consecutive years. In Experiment 1, female mouflons received a standard superovulatory treatment during both breeding and anoestrous seasons. Sarda sheep, used as controls, received the same treatment. Mean superovulatory response (corpora lutea and large follicles) was higher in the domestic sheep than in the mouflons (4.8 vs. 10.1 and 4.2 vs. 8.8 in breeding and anoestrous seasons, respectively; P < 0.05). A high percentage of mouflons showed early luteal regression which negatively affected recovery rate (35% and 30% in mouflons vs. 69% and 71% in sheep) and the yield of embryos suitable for transfer (37% and 25% in mouflons vs 74% and 69% in sheep; P < 0.05). In Experiment 2, ten mouflons were treated by the same superovulatory protocol and divided into two groups. In the first (Group 5), embryos were recovered earlier by oviductal flushing and cultured in vitro with oviductal cells in CZB medium until the morula/blastocyst stage; in the second (Group 6), the usual embryo recovery time was followed. Recovery rate was higher in the former (89% vs. 31%; P < 0.01) than in the latter. After 4 days of culture, 53% of embryos reached compact morula or early blastocyst stage (16/30). Lambing rate was 57% for mouflon embryos transferred immediately and 56% for those cultured in vitro for 4 days; the lambing rate in the sheep control group was 71%. The length of gestation was longer in ewes carrying mouflons than in those carrying lambs (155 vs. 148 days).     

Detection probability in aerial surveys of feral horses

Observation bias pervades data collected during aerial surveys of large animals, and although some sources can be mitigated with informed planning, others must be addressed using valid sampling techniques that carefully model detection probability. Nonetheless, aerial surveys are frequently employed to count large mammals without applying such methods to account for heterogeneity in visibility of animal groups on the landscape. This often leaves managers and interest groups at odds over decisions that are not adequately informed. I analyzed detection of feral horse (Equus caballus) groups by dual independent observers from 24 fixed-wing and 16 helicopter flights using mixed-effect logistic regression models to investigate potential sources of observation bias. I accounted for observer skill, population location, and aircraft type in the model structure and analyzed the effects of group size, sun effect (position related to observer), vegetation type, topography, cloud cover, percent snow cover, and observer fatigue on detection of horse groups. The most important model-averaged effects for both fixed-wing and helicopter surveys included group size (fixed-wing: odds ratio = 0.891, 95% CI = 0.850–0.935; helicopter: odds ratio = 0.640, 95% CI = 0.587–0.698) and sun effect (fixed-wing: odds ratio = 0.632, 95% CI = 0.350–1.141; helicopter: odds ratio = 0.194, 95% CI = 0.080–0.470). Observer fatigue was also an important effect in the best model for helicopter surveys, with detection probability declining after 3 hr of survey time (odds ratio = 0.278, 95% CI = 0.144–0.537). Biases arising from sun effect and observer fatigue can be mitigated by pre-flight survey design. Other sources of bias, such as those arising from group size, topography, and vegetation can only be addressed by employing valid sampling techniques such as double sampling, mark–resight (batch-marked animals), mark–recapture (uniquely marked and identifiable animals), sightability bias correction models, and line transect distance sampling; however, some of these techniques may still only partially correct for negative observation biases. © 2011 The Wildlife Society.     

Observer aging and long‐term avian survey data quality

Long‐term wildlife monitoring involves collecting time series data, often using the same observers over multiple years. Aging‐related changes to these observers may be an important, under‐recognized source of error that can bias management decisions. In this study, we used data from two large, independent bird surveys, the Atlas of the Breeding Birds of Ontario (“OBBA”) and the North American Breeding Bird Survey (“BBS”), to test for age‐related observer effects in long‐term time series of avian presence and abundance. We then considered the effect of such aging phenomena on current population trend estimates. We found significantly fewer detections among older versus younger observers for 13 of 43 OBBA species, and declines in detection as an observer ages for 4 of 6 vocalization groups comprising 59 of 64 BBS species. Consistent with hearing loss influencing this pattern, we also found evidence for increasingly severe detection declines with increasing call frequency among nine high‐pitched bird species (OBBA); however, there were also detection declines at other frequencies, suggesting important additional effects of aging, independent of hearing loss. We lastly found subtle, significant relationships between some species' published population trend estimates and (1) their corresponding vocalization frequency (n ≥ 22 species) and (2) their estimated declines in detectability among older observers (n = 9 high‐frequency, monotone species), suggesting that observer aging can negatively bias long‐term monitoring data for some species in part through hearing loss effects. We recommend that survey designers and modelers account for observer age where possible.     

Monitoring the abundance of mouflon in South France

Monitoring change in the population size of mouflon (Ovis gmelini musimon × Ovis sp.) in rugged areas is an important yet difficult task for wildlife ecologists. To assess population change of mouflon inhabiting the Caroux-Espinouse massif, France, we compared a pedestrian and helicopter survey, using counts of animals as indices of abundance. Environmental factors such as date of survey and temperature affected the detection of mouflons from the ground and the air. Both indices were sensitive to observed changes in population size. A decrease in the pedestrian index was recorded in 1994, the year following an epizootic of keratoconjunctivitis, which markedly reduced the survival rate of mouflon. Variations in pedestrian index accounted for variations in harvests when excluding epizootic events. Both surveys detected a decrease in population size, which accounted for the recent increase of harvest. Helicopter and pedestrian surveys are reliable tools to monitor annually mouflons in mountainous areas. Simulations indicated that helicopter surveys should be preferred by managers because they provide the best trade-off between cost and precision.     

Detection Probabilities for Ground-Based Breeding Waterfowl Surveys

ABSTRACT Current methods for conducting ground-based surveys of breeding waterfowl pairs make the unlikely assumption that detection probabilities are constant and approach 100%. To test this assumption, we conducted independent double-observer pair surveys in North Dakota, USA, to evaluate sources of variation in detection probabilities for 8 common species of prairie-nesting ducks. An experienced observer had 0.911 detection probability averaged over all 8 species (range = 0.866-0.944) versus 0.790 (range = 0.537-0.890) for a novice observer. Detection probabilities also varied substantially among species, but patterns were not consistent between observers. Detection probabilities declined as number of ducks per wetland increased, presumably due to difficulty in identifying large numbers of flushing ducks. Other covariates affecting detection probabilities included size of social groups, precipitation, survey methodology (roadside vs. walk-up), cloud cover, time of day, and amount of wetland vegetation, but these covariates only affected detection probabilities by 2–5%. Our results demonstrated that the assumption of 100% detection probabilities for ground-based waterfowl counts was clearly false and surveys based on this erroneous assumption underestimated population size by 10–29%. We recommend that future investigators measure detection probabilities explicitly by using double-observer methodologies.     

Nest and breeding population abundance of Least Terns: assessing bias and variation in survey timing and methods

Methods commonly used to estimate the number of nests and size of the breeding population at colonies of Least Terns (Sternula antillarum) and other waterbirds include walk‐through counts of nests (ground‐nest counts) and counts of incubating adults from the colony perimeter (incubating‐adult counts). The bias and variance of different methods and the comparability of repeated surveys versus once‐annual censuses are poorly understood. Our objectives were to assess (1) the potential bias and variation of the more rapid incubating‐adult counts compared to the time‐intensive, and presumably more accurate, ground‐nest counts, and (2) how accurately a once‐annual census captured peak nesting abundance. We studied nine Least Tern colonies at Cape Lookout National Seashore (CALO), North Carolina, from April to August 2010–2012. We analyzed observer and survey method agreement with concordance correlation coefficients (ρ_c). We deployed time‐lapse cameras at 156 nests and used repeated‐measures logistic regression to determine if the proportion of time spent incubating varied with colony, time of day, or time of season. We found substantial agreement in abundance estimates of Least Tern nests and incubating adults between observers and survey methods, and among different times of day and seasons (all comparisons ρ_c > 0.97). Least Terns incubated eggs 94% of the time on average during daylight hours, irrespective of colony, nesting stage, or month. Although the nesting peak at CALO occurred during the recommended census period for Least Terns, abundance estimates for surveys conducted at different times during that period varied by as much as 39%. We recommend conducting incubating‐adult counts to estimate nest and breeding population abundance of Least Terns or other waterbirds when vegetation or dunes do not obstruct views of nesting colonies. In addition, given the variation in abundance estimates for surveys conducted at different times during the recommended survey period, incubating‐adult counts should be performed at least twice during the census period, with the maximum count reported as peak nest abundance.     

An examination of observer bias as a source of error in surveys of seagrass shoots

Abstract Mensurative experiments investigated the effects of different observers on estimates of the density of shoots of two species of seagrass: Posidonia australis Hook and Zostera capricorni Aschers. Balanced programmes of sampling were used to examine variation in counts of seagrass shoots attributable to different observers, sizes of quadrats, depths and locations within large beds of each species of seagrass. A separate experiment examined differences between novice observers and a more experienced observer, when an ‘optimal’ size of sampling unit was used. Estimated densities of Zostera shoots varied inconsistently among observers, quadrats, depths and locations. Differences between observers were not affected by the size of quadrat used to count Posidonia shoots, but varied between locations in the seagrass bed. Experience had only a minor impact on biases. Only two of 12 novices produced counts that were different from the experienced observer. These results emphasize the importance of considering both accuracy and precision in the design of field studies of seagrasses.     

Determining Observer Reliability in Counts of River Otter Tracks

ABSTRACT In many research projects, reliability of collected data is dependent on reliability of field observers. However, it is uncommon for observer reliability to be either measured or reported in wildlife research. We tested whether observer skill affected outcomes of a northern river otter (Lontra canadensis) track survey conducted by the Texas Parks and Wildlife Department. Observers recorded presence of tracks at bridge sites (n = 250) throughout a 27-county region in east Texas, USA. Logistic regression indicated that observers were significantly associated with frequency of reported otter tracks. Because observers were not assigned to bridges at random, we tested and found associations between the bridges surveyed by each observer (SURVEY ROUTE) and habitat variables (WATERSHED, VEGETATION-TYPE, WATER-TYPE, BRIDGE-AREA) that may have influenced otter presence and probability of detection. A standardized tracker evaluation procedure indicated that experienced observers (n = 7) misidentified 37% of otter tracks. Additionally, 26% of tracks from species determined to be “otter-like” were misidentified as otter tracks. We recommend that observer skill in identification of animal tracks and other indirect signs be measured to detect and reduce observer errors in wildlife monitoring.     

Can Observation Skills of Citizen Scientists Be Estimated Using Species Accumulation Curves?

Volunteers are increasingly being recruited into citizen science projects to collect observations for scientific studies. An additional goal of these projects is to engage and educate these volunteers. Thus, there are few barriers to participation resulting in volunteer observers with varying ability to complete the project’s tasks. To improve the quality of a citizen science project’s outcomes it would be useful to account for inter-observer variation, and to assess the rarely tested presumption that participating in a citizen science projects results in volunteers becoming better observers. Here we present a method for indexing observer variability based on the data routinely submitted by observers participating in the citizen science project eBird, a broad-scale monitoring project in which observers collect and submit lists of the bird species observed while birding. Our method for indexing observer variability uses species accumulation curves, lines that describe how the total number of species reported increase with increasing time spent in collecting observations. We find that differences in species accumulation curves among observers equates to higher rates of species accumulation, particularly for harder-to-identify species, and reveals increased species accumulation rates with continued participation. We suggest that these properties of our analysis provide a measure of observer skill, and that the potential to derive post-hoc data-derived measurements of participant ability should be more widely explored by analysts of data from citizen science projects. We see the potential for inferential results from analyses of citizen science data to be improved by accounting for observer skill.     

Visual Implant Elastomer Mark Retention Through Metamorphosis in Amphibian Larvae

Abstract: Questions in population ecology require the study of marked animals, and marks are assumed to be permanent and not overlooked by observers. I evaluated retention through metamorphosis of visual implant elastomer marks in larval salamanders and frogs and assessed error in observer identification of these marks. I found 1) individual marks were not retained in larval wood frogs (Rana sylvatica), whereas only small marks were likely to be retained in larval salamanders (Eurycea bislineata), and 2) observers did not always correctly identify marked animals. Evaluating the assumptions of marking protocols is important in the design phase of a study so that correct inference can be made about the population processes of interest. This guidance should be generally useful to the design of mark-recapture studies, with particular application to studies of larval amphibians.     

ICE-BASED CENSUS OF BOWHEAD WHALES MIGRATING PAST POINT BARROW, ALASKA, 1978-1983

Bowhead whale ( Balaena mysticetus ) census data obtained during the northward spring migration are summarized for 1978–1983. Population size estimates are derived from counts made by observers standing on the seaward edge of shorefast ice in the vicinity of Point Barrow, Alaska, from mid-April to early June. The research design utilized two counting stations: South Perch, the primary counting station, and North Perch, used to determine the number of whales missed by South Perch observers. The percentage missed is estimated for each visibility category and used here to correct the census counts. Each season's population estimate is calculated as the sum of the number of trials of several independent multinomial distributions representing different visibility conditions. Corrections are applied for unwatched hours and hours with inferior visibility. A mean estimate of the number of whales passing within view of the census station was computed as 3,674 ± 299. This estimate was based on data collected in 1978, 1981, 1982, and 1983, years with the least apparent biases. Aerial survey data provide estimates of the proportion of whales passing at various distances seaward of the census sites as follows: 0.58 from the ice edge to 2 km, 0.76 to 3 km, and 0.80 to 4 km. Correcting for whales too far offshore to be seen by the ice-based observers results in a population estimate of over 4,200 bowheads.     

First Documentation of Cultural Transmission of Predator Recognition by Larval Amphibians

Predation is a pervasive selective agent shaping a prey's behaviour, morphology and life history. To survive, prey animals have to respond adaptively to predation threats and this can be achieved through learned predator recognition. Cultural transmission of predator recognition is likely a widespread means of learning in social animals, including mammals, birds and fishes. However, no studies have investigated the cultural transmission of predator recognition in amphibians. In our study, we examined whether naïve woodfrog (Rana sylvatica) tadpoles can acquire the recognition of the odour of a predatory tiger salamander (Ambystoma tigrinum) from experienced conspecifics. After conditioning some tutors to recognize salamander odour, we paired naïve observer tadpoles with either a salamander‐naïve or salamander‐experienced tutor and exposed the pairs to either salamander odour or a water control. Observers were subsequently tested alone for a response to salamander odour. We found that when given salamander odour, observer tadpoles that were paired with a salamander‐experienced tutor successfully learned to recognize the salamander odour as a threat, whereas the observers paired with salamander‐naïve tutors did not. Likewise, tadpoles exposed to the water control did not learn to recognize the salamander regardless of whether they were paired with a naïve or experienced tutor. This is the first study demonstrating cultural transmission of predator recognition in an amphibian species.     

Influence of the Observer on the Frequency of the ‘Morning-Dance’ in the Arabian Babbler

The Arabian babbler, Turdoides squamiceps, displays an unusual behaviour, the ‘morning-dance’. This article examines the influence of the presence and behaviour of an observer on the dance frequency. The results are the summary of more than 3 yr of study by 12 observers, who performed 1335 observations and witnessed 286 dances that took place among 15 groups of babblers. Different experiments were carried out on 148 additional mornings. Different observers witnessed different dance frequencies. Dance frequency witnessed by an observer decreased with observer's experience. Observers watching groups of babblers on single days witnessed more dances than those who watched the same groups for several consecutive days. Dance frequency was higher on the first day of a series of observations than on the following days in the same series. If a group of babblers remained unobserved for one day, the frequency of dancing increased. An evening observation considerably decreased the frequency of dancing the following morning. Feeding experiments did not elevate dance frequency. Experimental disturbance at the roost before dawn increased dance frequency while permanent disturbance and frightening experiments did not. Change in the observer's presence and behaviour caused the babblers to react by dancing. The hypothesis that the observer's presence and behaviour influences different group members differently, and therefore alters the relationships among them, is offered to explain the results that observers can affect the dance frequency.     

Influence of Age on Stress Responses to Metabolic Cage Housing in Rats

1.We studied the effect of isolation stress in 3- and 12-month-old rats individually housed in metabolic cages for 7 days. Urine (24 hr) was collected daily from one group of animals of each age. The other group was tested in an open field and on a hot plate on days 1 and 7.2.Total deambulation in the open-field test was lower in young than in older rats both on day 1 (54.7 ± 9.9 vs 80 ± 8.9 crossings/session; P < 0.04) and on day 7 (21 ± 9 vs 48 ± 7 crossings per session; P < 0.04) and decreased significantly in the two groups when tested on day 7 (P < 0.03). Latency to paw-licking in the hot-plate test was longer in young than in older animals on day 1 (14 ± 2 vs 8 ± 4 sec; P < 0.05) but was similar in the two groups on day 7.3.Urinary excretions of norepinephrine (NE) and epinephrine (E) were determined by HPLC with electrochemical detection. Urinary NE in day 1 was similar in young and older animals (2627 ± 828 vs 3069 ± 598 ng/24 hr). In young animals NE excretion decreased along the study and was significantly (P < 0.02) lower than on day 1 during the last 3 days of the study. Conversely, in older animals urinary excretion of NE remained similar throughout the study. On day 7 urinary excretion of NE in older animals was about two fold that in young rats. Urinary E was similar in young and older rats (341 ± 127 vs 532 ± 256 ng/24 hr) on day 1 and showed a tendency to increase throughout the study.4.Urinary monoamine oxidase inhibitory (IMAO) activity was determined by testing the ability of urine extracts to inhibit rat liver MAO activity in vitro and was higher in young than in older animals throughout the study (day 1, 54.8 ± 4.2 vs 25.1 ± 5.1%; P < 0.02). In young rats excretion of IMAO was significantly higher during the last 3 days of the study than on day 1 (P < 0.05). In older animals urinary IMAO showed a tendency to increase at the end of the study.5.Isolation stress caused by housing rats in metabolic cages results in different behavioral and metabolic responses in young and older animals. Young animals exhibit a lower locomotor and analgesic response and excrete lower amounts of NE and higher IMAO activity in the urine than older rats. The metabolic and behavioral responses to isolation stress are highly dependent on the age of the animals tested. These results should be taken into consideration when designing experiments requiring the use of metabolic cages.     

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.     

Nonbiological factors affecting track censuses: implications for sampling design and reliability

Track census is a widely used method for rapid faunal assessments, which assumes that differences in track count numbers mainly reflect differences in species abundance due to some biological factors. However, some methodological and climatic variables might affect results of track censuses. Here, we tested the effect of climatic variables, such as maximum temperature, humidity, wind speed or days since last rain, and methodological factors such as censusing day period, distance from transect to vegetation edge, substrate condition or observer, on the number of tracks of mammal carnivores and some of their potential prey detected in sandy substrates. We sampled 2?×?2 km² located within the scrubland area of Doñana National Park (southwestern Spain) for carnivore and several potential prey tracks. Our results showed differences in the number of tracks detected between observers and a significant interaction between observers and the day period when censuses were carried out. Moreover, the variables increasing the quality of the substrate (higher environmental humidity, lower wind speed and days since last rain) not only led to a greater detection of carnivore tracks but, depending on the size of the species sampled other variables such as distance from transects to the vegetation border, also affected results. We recommend restricting sampling to certain fixed weather conditions when planning to monitor relative animal abundance from track censuses. When not possible, climatic or methodological variables should be included as covariates in analyses that try to test for the biological factors affecting wildlife abundance, taking into account that these variables, which affect the number of tracks detected could vary between years.