Estimate Habitat Use Research Articles

Living on the edge: Insights into habitat patterns of forest-dwelling mammals in the buffer zone of Melghat Tiger Reserve, India

Buffer zones surrounding protected areas act as interfaces between humans and wildlife, providing sustainable resource utilization while minimizing negative interactions. This study analyzed data from camera trap sampling to understand habitat use and the effects of anthropogenic impacts on medium and large-sized forest-dwelling mammals in the buffer zone of Melghat Tiger Reserve in Central India. The human population in this region is increasing at a rate of 15 % in a decade. There are instances of human death and injury and frequent cattle lifting by carnivores. Crop damage by wild herbivores is common. These factors impede long term conservation goals in the tiger reserve. Camera trap sampling was carried out in the buffer zone for a period of 33 to 46 days. Camera trap data were analyzed using occupancy modeling to estimate habitat use probability (Ψ) and detection probability (p). Naïve estimates of habitat use ranged from 0.11 to 0.64, while Ψ values based on occupancy modeling ranged from 0.23 to 0.71. The study showed a fairly high level of habitat use by mammals in the buffer zone of the tiger reserve. Leopards were found to be associated with abundance of large prey species and human presence. Hyena, gaur, and sambar showed an association with forest cover, while nilgai showed association with scrubland. Chausingha and langur showed an association with sloping terrain. Overall, the study showed that the buffer zone plays an important role as a wildlife habitat and provided insights for prioritizing conservation efforts and management strategies in the study area.

Integrating spaceborne estimates of structural diversity of habitat into wildlife occupancy models

Vegetation structure is a crucial dimension of wildlife habitat, responsive to global changes in human activities and ecosystem processes. NASA’s recent Global Ecosystem Dynamics Investigation (GEDI) provides an exciting opportunity to explore how spaceborne waveform observations can improve our ability to measure wildlife habitat and advance animal ecology in the Anthropocene. We tested the utility of GEDI data in univariate occupancy models to estimate habitat use in a remote mountain system in central Idaho, USA. We collected data from 49 camera trap stations from two surveys in 2018–2019 and modeled the occupancy for each of seven mammal species representing different trophic levels and feeding strategies: American black bear (Ursus americanus), deer (Odocoileus hemionus), elk (Cervus canadensis), moose (Alces alces), coyote (Canis latrans), wolf (Canis lupus), and mountain lion (Puma concolor). We first derived structural diversity indices (richness, evenness, and divergence) of GEDI-derived canopy height, plant area index, and foliage height diversity to represent different dimensions of vegetation structure. This spatial aggregation is necessary due to gaps in GEDI footprints and parallels commonly used functional diversity metrics applied to biological communities that are calculated using trait probability densities. We measured these indices across three spatial scales that reflect different species movement and habitat selection patterns. We found the structural diversity indices of canopy height, foliage height diversity, and plant area index had the strongest effects on the occupancy of most mammals compared to two-dimensional (2D) variables (e.g. tree cover, normalized difference vegetation index). The spatial extent of these indices also influenced the strength of response, highlighting the importance of selecting a scale large enough to capture sufficient GEDI footprints but small enough to reflect site-level variance. Compared to 2D covariates, our results suggest that GEDI variables allow researchers to generate more detailed inference on the forms of habitat that wildlife use. We discuss the implications of these findings for habitat management and future wildlife research from local to global scales.

Black bear density and habitat use variation at the Sierra Nevada‐Great Basin Desert transition

AbstractIn the first 2 decades of the twenty‐first century, American black bear (Ursus americanus) populations rebounded with range expansions into areas where the species was previously extirpated. While there are a number of factors that limit range expansion, habitat quality and availability are among the most important. Such factors may be particularly important in western Nevada, USA, at the transition zone of the Sierra Nevada and the Great Basin Desert. We deployed a multi‐faceted data collection system including motion‐sensitive cameras, noninvasive hair sampling and genotyping, and global positioning system (GPS) tracking. We analyzed data using spatial capture‐recapture to estimate population density and dynamic occupancy models to estimate habitat use. Black bear habitat use and density were substantially higher in the Sierra Nevada than the Great Basin Desert and had strong positive relationships with the presence of conifer land cover in the transition zone. The average black bear density was >4 times higher in the mixed‐conifer forests of the Sierra Nevada (12.4 bears/100 km2) than in desert mountain ranges with piñon (Pinus monophylla)‐juniper (Juniperus spp.) woodland (2.7 bears/100 km2). The low‐elevation shrub and grassland portions of the study area had even lower estimated black bear density (0.6 bears/100 km2) and probability of use (0.03, 95% CI = 0.00–0.09). Across these spatially variable configurations in black bear density, we estimated the population size to be 418 individuals (95% CI = 239–740). Declining density towards the range edge, coupled with a relatively stable range of black bears in Nevada observed since 2000, suggests that further species range expansion into the western Great Basin may be limited by habitat quality and availability.

Towns and trails drive carnivore movement behaviour, resource selection, and connectivity

BackgroundGlobal increases in human activity threaten connectivity of animal habitat and populations. Protection and restoration of wildlife habitat and movement corridors require robust models to forecast the effects of human activity on movement behaviour, resource selection, and connectivity. Recent research suggests that animal resource selection and responses to human activity depend on their behavioural movement state, with increased tolerance for human activity in fast states of movement. Yet, few studies have incorporated state-dependent movement behaviour into analyses of Merriam connectivity, that is individual-based metrics of connectivity that incorporate landscape structure and movement behaviour.MethodsWe assessed the cumulative effects of anthropogenic development on multiple movement processes including movement behaviour, resource selection, and Merriam connectivity. We simulated movement paths using hidden Markov movement models and step selection functions to estimate habitat use and connectivity for three landscape scenarios: reference conditions with no anthropogenic development, current conditions, and future conditions with a simulated expansion of towns and recreational trails. Our analysis used 20 years of grizzly bear (Ursus arctos) and gray wolf (Canis lupus) movement data collected in and around Banff National Park, Canada.ResultsCarnivores increased their speed of travel near towns and areas of high trail and road density, presumably to avoid encounters with people. They exhibited stronger avoidance of anthropogenic development when foraging and resting compared to travelling and during the day compared to night. Wolves exhibited stronger avoidance of anthropogenic development than grizzly bears. Current development reduced the amount of high-quality habitat between two mountain towns by more than 35%. Habitat degradation constrained movement routes around towns and was most pronounced for foraging and resting behaviour. Current anthropogenic development reduced connectivity from reference conditions an average of 85%. Habitat quality and connectivity further declined under a future development scenario.ConclusionsOur results highlight the cumulative effects of anthropogenic development on carnivore movement behaviour, habitat use, and connectivity. Our strong behaviour-specific responses to human activity suggest that conservation initiatives should consider how proposed developments and restoration actions would affect where animals travel and how they use the landscape.

Using GPS tracking and stable multi-isotopes for estimating habitat use and winter range in Palearctic ospreys

We used both satellite tracking and carbon, nitrogen and sulphur stable isotopic analysis (SIA) to infer wintering ecology and habitat use of the Corsican osprey Pandion haliaetus population. A control sample of feathers from 75 individuals was collected within the osprey’s northern hemisphere breeding range, to assess the SIA variability across habitat types. An experimental set of SIA on feathers of 18 Corsican adults was examined to infer wintering ground locations and habitat types used during the non-breeding period. We calibrated the SIA using GPS/GSM tracks of 12 Mediterranean adults’ movements as wintering site references. We found 50% of individuals were resident and the other half migrated. Ospreys spent the winter at temperate latitudes and showed a high plasticity in habitat selection spread over the Mediterranean basin (marine bays, coastal lagoons/marshland, inland freshwater sites). Complementary to GPS tracking, SIA is, at a broad geographical scale, a reliable method to determine whether ospreys overwinter in a habitat different from that of their breeding area. This study proved that the integration of SIA and GPS/GSM tracking techniques was effective at overcoming the intrinsic limits of each method and achieving greater information for basic ecological studies of migratory birds in aquatic environments.

The Gain Reduction Method for manual tracking of radio-tagged fish in streams

BackgroundManual tracking has been used since the 1970s as an effective radio telemetry approach for evaluating habitat use of fish in fluvial systems. Radio tags are often located by continually reducing the gain when approaching the tag along a watercourse to estimate its location, termed here as the ‘Gain Reduction Method’. However, to our knowledge the accuracy of this method has not been empirically evaluated and reported in the literature. Here, the longitudinal and lateral positional errors of radio tags are assessed when applying the Gain Reduction Method in a small stream environment. Longitudinal and lateral positional errors (i.e. the difference between the estimated and actual radio tag location) were evaluated based on the distance from the actual tag position, the gain recorded when estimating the tag position and a number of environmental parameters (i.e. stream depth, velocity, stream width and specific conductivity).ResultsThe manual tracking trials produced an average lateral positional error of 0.91 m (± 1.4) and a longitudinal positional error of 0.66 m (± 0.87). A larger degree of longitudinal positional error was documented when the gain was higher (t = 2.21, p < 0.05). Larger lateral positional error was recorded when the tag was farther across the stream (t = 2.27, p < 0.01) and due to greater inaccuracy in longitudinal positioning (t = 3.2, p = 0.001). In addition, greater rates of lateral positional error were found when specific conductivity levels were higher (t = 2, p < 0.05). Longitudinal and lateral positional errors were not influenced by stream width (m), depth (m) or velocity (m/s).ConclusionsAlthough the Gain Reduction Method is commonly used to estimate habitat use of stream fishes, there appears to be a paucity of information in the literature that addresses the accuracy for obtaining fine-scale positioning of tagged fishes. This study is aimed to address this knowledge gap by identifying sources of locational error with the Gain Reduction Method. Overall, habitat variables were deemed to be unlikely to have a significant effect on estimating fish position in small streams. Researchers should be aware that error in the longitudinal direction will translate into larger errors in the lateral position. Further exploration of positional accuracy using this active tracking approach is recommended for larger and deeper fluvial systems.

Validating the performance of occupancy models for estimating habitat use and predicting the distribution of highly-mobile species: A case study using the American black bear

Occupancy models have become a valuable tool for estimating wildlife-habitat relationships and for predicting species distributions. Highly-mobile species often violate the assumption that sampling units are geographically closed shifting the probability of occupancy to be interpreted as the probability of use. We used occupancy models, in conjunction with noninvasive sampling, to estimate habitat use and predict the distribution of a highly-mobile carnivore, the American black bear (Ursus americanus) in New Mexico, USA. The top model indicated that black bears use areas with higher primary productivity and fewer roads. The predictive performance of such models is rarely validated with independent data, so we validated our model predictions with 2-independent datasets. We first assessed the correlation between predicted and observed habitat use for 28 telemetry-collared bears in the Jemez Mountains. Predicted habitat use was positively correlated with observed use for all 3 years (2012: ρ = 0.81; 2013: ρ = 0.87; 2014: ρ = 0.90). We then predicted the probability of use within a cell where a bear mortality was documented using 2043 mortality locations from sport harvest, depredation, and vehicle collisions. The probability of habitat use at a mortality location was also positively correlated with observed use by the species (2012: ρ = 0.74; 2013: ρ = 0.89; 2014: ρ = 0.93). Our validation procedure supports the notion that occupancy models can be an effective tool for estimating habitat use and predicting the distribution of highly-mobile species when the assumption of geographic closure has been violated. Our findings may be of interest to studies that are estimating habitat use for highly-mobile species that are secretive or rare, difficult to capture, or expensive to monitor with other more intensive methods.

Habitat preferences of reintroduced banteng (Bos javanicus) into the Salakphra Wildlife Sanctuary, Thailand

Abstract ContextLarge forest-dwelling mammals are highly sensitive to habitat structure. Thus, understanding the responses of reintroduced banteng (Bos javanicus d’Alton 1823) to their habitat is important for ensuring the sustainability of a reintroduction program. AimsThe aim of the present study was to evaluate the habitat preferences of banteng after reintroduction into the Salakphra Wildlife Sanctuary in Thailand on the basis of fieldwork conducted between January 2015 and November 2017. MethodsSeven banteng individuals bred at the Khao Nampu Nature and Wildlife Education Center were systematically reintroduced into the Salakphra Wildlife Sanctuary in 2015 (four individuals) and 2016 (three individuals). The banteng individuals were tracked via radio-collars and camera-traps. The maximum-entropy method (MaxEnt) and multiple logistic regressions (MLR) were used to identify habitat preferences. Kernel-density estimates (KDE) and a minimum convex polygon (MCP) were used to estimate the area of the habitat used. Key resultsIn total, 407 radio-signal locations showed that the MaxEnt habitat-preference models classified the banteng as associated with distance from villages and salt licks (regularised training gain of &amp;gt;1.0). Multiple logistic regressions form 32 camera-trap locations classified the banteng as associated with low elevations far from villages, guard stations and roads in a flat area (no aspect). The two methods for estimating habitat use provided similar results and showed that the reintroduced banteng used a wider range of habitat in the dry than in the wet season. ConclusionsThe results from the present study suggest that the reintroduced banteng individuals prefer low elevations and flat areas without human activity. ImplicationsThese findings are important for possible translocations elsewhere.

Modelling golden eagle habitat selection and flight activity in their home ranges for safer wind farm planning

Onshore wind farm development may impact vulnerable large eagles at both individual and population levels and requires appropriate assessment under the EU Bird and Habitat Directives. The present conservation policy (e.g. fixed safety zones around nest sites) improves species conservation but may not prevent habitat loss or reduce collision risk in the best possible way because this policy may not consider habitat-specific effects on eagle behaviour. Here, we develop a method for estimating habitat use and flying time distribution within Golden Eagle (Aquila chrysaetos) home ranges. Location data retrieved with GPS-transmitters (Global Positioning system) in Finland indicated that these large raptors used vast areas (mean 297 km2, 95% Minimum Convex Polygon), reaching up to 14 km, but not uniformly around their nests. The best resource selection function models (cross-validation performance 83%) revealed that flying Golden Eagles preferred the vicinity of their nests, steep slopes, and old forests in their home range. They avoided human settlements and neighboring territories. GPS data indicated short flying times per day (mean 2.2 h) and about 30% of the flying time within collision risk heights (50–200 m). Together with information on habitat selection, flying times can be used for predicting airspace use of birds and in assessing the collision risk at particular wind farm locations. Thus, our method can be applied in planning wind farm locations that enable a safer co-existence of large territorial birds and wind power plants in the same landscape.

Density, habitat use, and activity patterns of a vulnerable population of the Solitary Tinamou (Tinamus solitarius) in a Brazilian Atlantic Forest fragment

We present the first estimates of density of the Solitary Tinamou (Tinamus solitarius (Vieillot, 1819)) in the Reserva Natural Vale (RNV), one of the two largest remnants of the Atlantic Rainforest in the state of Espírito Santo, Brazil. We also modeled the spatial distribution and habitat use of the species using occupancy and detectability modeling. We used a line-transect survey to estimate density and abundance and camera traps to estimate habitat use and activity pattern. We conducted the survey within a 13 month period (April 2013 to May 2014). Estimated density was 9 ± 2 individuals/km2 and estimated population size for the entire RNV was 2115 ± 470 individuals. Occupancy was best described by distance to forest edge, distance between trees, poaching intensity, and understory cover. Detectability was affected by distance between trees and poaching intensity. We conclude that in the RNV, the Near Threatened T. solitarius is a diurnal species preferring intact closed-canopy forest but tending to avoid areas near to forest edges or areas under comparatively high intensity of poaching. These results reinforce the importance of RNV as a resource source for T. solitarius reproduction at Brazilian Atlantic forest remnants.

The rusty-margined guan (Penelope superciliaris) in the Brazilian Atlantic rain forest: density, population size, activity and habitat use

Context Population estimation and monitoring are important tools for the support of programs and actions of conservation for most wildlife species, including birds. For validation of such support, a set of reliable, consistent and comparable parameter estimates is essential (through systematic monitoring over time), in addition to studies on ecological aspects of the species. Aims In this study, our aim was to analyse the population of Penelope superciliaris in the Vale Natural Reserve (VNR), located in north-eastern Espírito Santo, Brazil, providing estimates of density, population size and activity. We also analysed occupancy and detectability of the species, relating them to six covariates (distance from forest edge, water and road, as well as poaching intensity, canopy cover and understorey cover) based on previous knowledge of the ecology of rusty-margined guan. Methods We used line-transect surveys to estimate density and abundance, and camera traps to estimate habitat use by P. superciliaris, using the occupancy modelling approach. Key results Estimated density for P. superciliaris was 2.5 ± 1.0 groups per km2 with a group size of 6.0 ± 2.0 individuals, and estimated population size for the entire VNR was 3544 ± 495 individuals. Occupancy by P. superciliaris was best described by three covariates: (1) distance to road; (2) distance to water; and (3) poaching intensity. Detectability was affected by two covariates: (1) understorey cover; and (2) poaching intensity. Conclusions Our results provide the first information on density, habitat use and activity of P. superciliaris in the VNR. We concluded that P. superciliaris is active diurnally, especially during morning and late afternoon, prefers areas near water sources in denser forests and tends to avoid areas with habitat modification and intense poaching activity. Implications Our data highlight the impact of poaching on the species and can be useful as a baseline for future studies and Cracid conservation plans.

Evaluating methods for estimating home ranges using GPS collars: A comparison using proboscis monkeys (Nasalis larvatus).

The development of GPS tags for tracking wildlife has revolutionised the study of home ranges, habitat use and behaviour. Concomitantly, there have been rapid developments in methods for estimating habitat use from GPS data. In combination, these changes can cause challenges in choosing the best methods for estimating home ranges. In primatology, this issue has received little attention, as there have been few GPS collar-based studies to date. However, as advancing technology is making collaring studies more feasible, there is a need for the analysis to advance alongside the technology. Here, using a high quality GPS collaring data set from 10 proboscis monkeys (Nasalis larvatus), we aimed to: 1) compare home range estimates from the most commonly used method in primatology, the grid-cell method, with three recent methods designed for large and/or temporally correlated GPS data sets; 2) evaluate how well these methods identify known physical barriers (e.g. rivers); and 3) test the robustness of the different methods to data containing either less frequent or random losses of GPS fixes. Biased random bridges had the best overall performance, combining a high level of agreement between the raw data and estimated utilisation distribution with a relatively low sensitivity to reduced fixed frequency or loss of data. It estimated the home range of proboscis monkeys to be 24–165 ha (mean 80.89 ha). The grid-cell method and approaches based on local convex hulls had some advantages including simplicity and excellent barrier identification, respectively, but lower overall performance. With the most suitable model, or combination of models, it is possible to understand more fully the patterns, causes, and potential consequences that disturbances could have on an animal, and accordingly be used to assist in the management and restoration of degraded landscapes.

Estimating relative distribution of raccoons, opossums, skunks, and foxes using animal control data

We used indices of animal control reports per capita and areas of land covers to assess the relative habitat-use of raccoons (Procyon lotor), opossums (Didelphis virginiana), skunks (Spilogale putorius and Mephitis mephitis), and foxes (Vulpes vulpes and Urocyon cinereoargenteus). We used confi rmed (hereafter, verifi ed) calls made to Brevard Animal Services, Florida, and assessed potential human demographic infl uences associated with unconfi rmed (hereafter, unverifi ed) reports where it was uncertain whether or not an animal was present. To estimate habitat use, we performed quadrat sampling using a geographic information system (GIS) and obtained areas of land cover within each quadrat. We evaluated numbers of confi rmed animals per capita against areas of land cover in a quadrat using Forward Logistic Regression and Stepwise Multiple Linear Regression analyses. Our results indicate that raccoons were positively associated with a mixture of populated areas near streams and negatively associated with wetland forests, shrub and brushland, and tree crops. Opossums were positively associated with a mixture of row crops, bays and estuaries, high- density residential areas, and streams, while negatively associated with golf courses and low- density residential areas. Skunks were associated with a mixture of residential, institutional, and recreational areas, roads, pastures, and wetlands with some forest cover near water. Foxes were positively associated with open agricultural- and industrial-use areas often located near bays and estuaries, and negatively associated with golf courses, extraction sites, and shrub and brushland areas. On a landscape level, animal groups selected certain land cover categories and did not use land covers based on availability. If care is taken to remove potential biases, verifi ed animal control reports can be used as a low-cost, opportunistic method to determine where raccoons, opossums, skunks, and foxes are located in urban areas. Using verifi ed animal control reports appears promising for identifying areas where raccoons, opossums, skunks, and foxes are located in urban areas.

Fine scale movements and activity areas of white sharks (Carcharodon carcharias) in Mossel Bay, South Africa

Previous work on white sharks indicate the species show seasonally limited movement patters, at certain aggregation sites small areas may play vital roles in the life history of a large amount of the population. Acoustic telemetry was used to estimate habitat use of white sharks, Carcharodon carcharias, while aggregating at Mossel Bay, South Africa. Total range of all shark tracks combined accumulated 782 h and covered an area of 93.5 km2 however, within this range, sharks were found to highly utilise a core habitat (50 % Kernel, K50) of just 1.05 km2 over a reef system adjacent to a river mouth. Individual tracks revealed additional core habitats, some of which were previously undocumented and one adjacent to a commercial harbor. Much was found to be dependent on the size of the shark, with larger sharks (>400 cm) occupying smaller activity areas than subadult (300–399 cm) and juvenile (<300 cm) conspecifics, while Index of Reuse (IOR) and Index of Shared Space (IOSS) were both found to increase with shark size. Such results provide evidence that larger white sharks are more selective in habitat use, which indicates they have greater experience within aggregation sites. Furthermore, the focused nature of foraging means spatially restricted management strategies would offer a powerful tool to aid enforcement of current protective legislation for the white shark in similar environments of limited resources and capacity.

Time budgets and at-sea behaviour of lactating female Antarctic fur seals Arctocephalus gazella at Bouvetøya

We present the first data on attendance patterns, at-sea movements and diving behaviour of Antarctic fur seals breeding at Bouvetoya (Bouvet Island), Southern Ocean. While other colonies have been extensively studied, this remote and second largest global population remains relatively unknown. Time depth recorders and satellite relay data loggers were deployed on breeding females during the 2000-2001 and 2001-2002 summers. Attendance and foraging patterns were similar to those observed at colonies in the Scotia Sea region where Antarctic krill Euphausia superba is the pre- dominant prey. Early to mid-lactation trips ranged within ~100 km of the island, usually towards the west; the dominant direction shifted later in the season and the range also increased markedly to a peak between early February and early March. Solar elevation influenced arrivals and departures from the island, with most departures occurring around sunset. Diurnal variations in diving behaviour were consistent with the vertical migration of krill. Diving frequency was higher at night and diving effort peaked around morning twilight. Afternoon deep diving was common, suggesting that females might target dense daytime krill aggregations between the photic zone and the thermocline. Trip durations increased throughout early to mid-lactation, peaking in late January to early March, before again decreasing towards the end of lactation. Our results illustrate the substantial variability, both between individuals and within individuals over time, that is likely to reflect variations in prey distrib- ution and in the growth requirements of pups. Such variations need to be taken into account when estimating habitat use and resource utilisation in marine top predators.

Influence of topography on the location and density of armadillo burrows (Dasypodidae: Xenarthra) in the central Amazon, Brazil

Armadillos (Dasypodidae: Xenarthra) are the principal group of burrow-excavating mammals in Amazonian forest. Little is known about how the landscape's physical characteristics influence where these organisms choose to construct their burrows. We measured topographic characteristics of armadillo burrow placement and evaluated whether these variables affect overall burrow density. Our study took place on the study sites of the Biological Dynamics of Forest Fragments Project in the central Amazon, Brazil. To determine if armadillos exhibit habitat use preferences in constructing their burrows, we quantified the declination and elevation of available terrain along transect lines on our study sites and compared this with the characteristics of areas containing burrows. We also tested whether topographic variables were related to burrow density. Armadillos preferentially build burrows on low elevation, inclined terrain, and declination was positively related to burrow density. Low, inclined areas correspond to slopes next to stream basins. It may require less effort for armadillos to excavate these areas, while also providing proximity to water. The results of this study indicate that topographic information is important to consider when using burrows to estimate habitat use and density of armadillos in terra-firme forests of the central Amazon.

Optimum effort to estimate habitat use when the individual animal is the sampling unit

1 Studies of habitat use in which the individual animal is the sampling unit should ideally sample each individual sufficiently to achieve a stable estimate of its habitat use. Data are typically obtained by radio-tracking, which can be labour-intensive. Hence, optimization of sampling effort is desirable. A method to determine optimum sampling effort is described with reference to an example from a study of Natterer's bat Myotis nattereri, in which data were collected by radio-tracking with individual bats followed continuously for entire nights. 2 Habitat use by Natterer's bat was assessed by compositional analysis, which compares the composition of habitats used with those potentially available. Therefore, we plotted running per cent foraging time spent over a range of habitat types against the cumulative foraging time recorded. We visually estimated the optimum sum of foraging time required to determine stable estimates of the composition of habitat use from the plots. Then, by reference to the full-time budget, the total tracking effort expended at the point when this optimum amount of foraging data had been recorded was determined and expressed in units of the number of nightly tracking sessions that had been undertaken to collect that amount of foraging data. 3 Stable estimates of habitat use were attained after a mean of 22 ± 7.7 h of foraging time, which were obtained in a mean of 4.6 ± 1.9 nights of radio-tracking effort. Thus, in this Natterer's bat study, where habitat preference was assessed by compositional analysis, it was appropriate to aim to collect foraging data during five nights of radio-tracking for each bat sampled. 4 The method presented is also applicable to studies where tracking data are discontinuous. A variation of the method can be applied in studies where a Euclidean distance method is to be used for the analysis of habitat use.

RESULTS OF RADIO—TRACKING OF DUCKS IN THE LAKE ENGURE IN SUMMER 1997

ABSTRACT In summer 1997 a project of duck research using radio—telemetry was carried out on the Lake Engure, Latvia. 10 females of Mallards, 10 females of Shovelers, 9 females of Pochards and 6 females of Tufted Ducks were radio—tracked during the period from the hatching of their ducklings till fledging. Information about effect of the harness-type radio-transmitters on different species of ducks has been collected. For 15 ducks home range sizes and distances between nests and range locations were calculated. Mallards used to move further from their nest sites; one Mallard brood after one week spent near the nest site dislocated 8 km to the Southern part of the lake during one day. Problems of estimating habitat use and habitat selection of duck broods have also been discussed.