# Sign Surveys for Estimating Trend of a Remnant Brown Bear Ursus arctos Population in Northern Spain

> Can you count a rare bear population’s trend from their tracks and scats? Over three years in Spain’s Cantabrian Mountains, observers walked about 1,500 km of routes a year and found sign on only 4% of segments. The authors show why that was too little to detect a decline, and how more frequent surveys could help.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/1996-clevenger-purroy-sign-surveys-estimating-trend/

## Citation and access

- **Authors:** Anthony P. Clevenger and Francisco J. Purroy
- **Year published:** 1996
- **Journal or source:** Wildlife Biology
- **Full citation:** Clevenger, A. P. and Purroy, F. J. 1996. Sign surveys for estimating trend of a remnant brown bear Ursus arctos population in northern Spain. Wildlife Biology 2(4): 275-281.
- **DOI:** 10.2981/wlb.1996.031
- **Access status:** Publisher link only
- **Where to find it:** https://doi.org/10.2981/wlb.1996.031

## Tags

- **Species:** Mammals > Brown bear
- **Region:** Europe > Spain
- **Tracking:** Scat; Sign surveys; Track counts; Track measurements; Tracking; Tracks; Tracks and signs identification
- **Methods and technology:** Descriptive statistics; Population trends; Power analysis
- **Data type:** New field data
- **Observer qualification:** Reported but not validated
- **Discipline:** Population trends; Wildlife tracking
- **Year published:** 1996

## Detailed keywords

- **Species:** Studied: brown bear (Ursus arctos), the Cantabrian population, estimated at 50 to 70 bears in two subpopulations. Present but not studied: livestock, mainly cattle, grazing the mountains from May to October. Habitat plants named: heath and broom (Cytisus, Genista, Erica and Calluna spp.), beech (Fagus sylvatica), oak (Quercus spp.), birch (Betula celtiberica), chestnut (Castanea sativa) and Scots pine (Pinus sylvestris). Comparison studies discussed: grizzly bears in the Yellowstone ecosystem (counts of females with cubs) and in Montana, USA (sign surveys by Kendall et al. 1992).
- **Location and habitat:** Cantabrian Mountains, northern Spain (42°50’N to 43°10’N, 4°20’W to 6°55’W), across the Autonomous Communities of Asturias, Cantabria, Castile-León and Galicia (provinces of León and Palencia on the south side). Bear range about 5,500 km², almost all within National Hunting Reserves. Elevations 600 to 2,500 m (mean along the divide 1,200 to 1,600 m). Continental climate on the south side, maritime on the north. Scrublands of heath and broom, lowland hayfields and upland grazing pasture, and mixed deciduous forest. Road density about 0.25 km/km².
- **Topics:** sign surveys; track and scat counts; monitoring population trend of a low-density, endangered large carnivore; non-intrusive monitoring; statistical power and sample size; survey design; replicate counts; independence of sampling units (1.6 km route segments); livestock trampling of sign; seasonal timing of surveys; bear recovery plans; distribution and occurrence monitoring; demographic versus survey approaches.
- **Statistics or models:** Counts, means, ranges and proportions of segments with sign. Sample size needed to detect a change in sign occurrence between two samples (Snedecor and Cochran 1980): 1,361 segments per year to detect a 20% decrease at a significance level of 0.10 and power of 0.80. The authors also mention repeated measures analysis, multiple comparison tests and regression as options for detecting trend. Inconsistencies in the paper: (1) the text says tracks and scats were “equally abundant the third year”, but Table 1 gives 13 tracks and 8 scats for 1992; (2) the Discussion calls the mean of 0.06 sign per segment “6%” and uses “6% sign per segment” in its calculation, while only 4% of segments had sign; (3) the Discussion gives 925 to 950 segments sampled annually, while Table 1 gives 928 and 957; (4) the abstract says 70 routes, while the Results and Table 1 give 67 to 72 routes per session (139 to 141 per year); (5) some Table 1 means and proportions do not match the counts (e.g. 1992: 21 signs on 957 segments is about 0.02 sign per segment, but the table gives 0.04; 16 of 957 segments with sign is about 0.017, table 0.03); (6) the survey effort figures for 1992 are identical to those for 1991.
- **Data type:** New field data: a 3-year pilot study, 1990 to 1992, with surveys in spring (mid-April) and autumn (mid-November) each year.

## Synopsis

### 1. What they did

Brown bears in Spain’s Cantabrian Mountains were still declining in the early 1990s despite legal protection. The regional recovery plans called for monitoring the population with non-intrusive methods. Counting females with cubs, used for grizzly bears in Yellowstone, does not work well here: Cantabrian bears are mostly nocturnal, secretive and live in forest, so they are hard to see, and radio-collaring them is difficult.

The authors ran a 3-year pilot study (1990 to 1992) to test whether counting bear tracks and scats along survey routes (sign surveys) could detect changes in bear numbers over time.

- Routes. A network of 67 to 72 routes on foot trails and unimproved dirt roads, spread across the whole bear range.

- Segments. Each route was divided into numbered 1.6 km segments, the basic sampling unit. From what they knew of bear movements, they considered this long enough for neighbouring segments to give reasonably independent records.

- Timing. Surveys ran twice a year, in spring (mid-April, before livestock went up into the mountains) and autumn (mid-November, after livestock came down), to reduce trampling of sign. Each round was done in as short a time as possible, so sign from the same bear was less likely to be counted on more than one segment.

- Recording. Observers counted the bear tracks and scats they could positively identify on each segment. Scats were collected for diet analysis and tracks were measured to help tell individuals apart.

- Analysis. They calculated how many segments would be needed to detect a change in sign occurrence between two samples.

### 2. What they found

Survey effort

- About 1,500 km were surveyed each year (696 to 819 km per session), on 928 to 957 segments. Each session took on average 26.6 days.

Bear sign

- Over the three years, observers recorded 69 tracks and 52 scats. The total number of signs per year fell from 47 (1990) and 53 (1991) to 21 (1992).

- Tracks outnumbered scats in every session except one (autumn 1990, with 7 tracks and 20 scats).

- Sign was very scarce: an average of 0.06 sign per segment, and only 4% of segments had any sign (6 to 24 segments per survey, 16 to 37 per year).

Statistical power

- To detect a 20% decrease in sign occurrence (significance level 0.10, power 0.80), they would need 1,361 segments a year. They sampled well below that, so the survey design gave little power to detect real changes in bear numbers.

- Sign density was remarkably low, resembling bear track counts in the Montana study, even though the Cantabrian surveys ran twice a year and covered 10 times more segments a year.

### 3. What they concluded

- The pilot sampling scheme does not have enough statistical power to detect significant declines in this population. Monitoring trends in low-density carnivore populations with sign surveys is inherently difficult, because power drops as populations become smaller.

- Raising sign density estimates by adding replicate counts within each year looks like the best option. Pooling data over many years would take too long.

- Surveys are only practical about four months a year (April to May and October to November), because of livestock grazing for most of the year. A hypothetical scheme: 25 trained observers (for example district game wardens or volunteers) each walking about 32 km (20 segments) per session, about 2 days of fieldwork, covering 500 segments. Four monthly surveys a year would sample about 2,000 segments.

- For most of southern Europe’s remnant brown bear populations, agencies run sign surveys once or twice a year and record total tracks and scats per distance surveyed, without considering biases such as non-independent observations. The authors say this gives little useful information on population trends, and that questionnaires on bear occurrence give highly variable data, useful only on a crude spatial scale.

- They suggest a future strategy with two parts: (1) more replicate counts to lower variance and increase power, and (2) monitoring bear distribution and occurrence over time and space, to detect changes such as dispersal, recolonization, loss of peripheral range, or bears returning to good habitat.

### 4. How tracking was or was not used

The whole study was a tracks and signs survey. Observers walked the routes counting bear tracks and scats on each 1.6 km segment. Only signs that could be positively identified as bear were recorded. Presence on a segment meant at least one track or scat. Tracks were measured to help tell individuals apart, and scats were collected for diet analysis (neither result is reported in this paper). For the analysis, the sign on each segment was defined as the most common sign type found there. The authors note that sign surveys are affected by tracking conditions and by the animals’ seasonal behaviour, and they timed the surveys for good tracking conditions, when bears were active and before or after livestock trampled the trails. No trailing was reported.

### 5. Tracker or observer certification

Reported but not validated. Only “experienced field observers” took part in the surveys. Inexperienced observers could join after accompanying field personnel for two full survey sessions to gain experience in sign identification. No certification or test of their skill is described. Several people who helped run the surveys are thanked by name.

### 8. Implications for tracking (Original Wisdom's observations)

_These are Original Wisdom's observations, not the authors'._

- The paper shows that a tracks and signs survey is only as useful as its design: with very few signs per segment, even a large, well-organised effort could not detect a decline. How often and how widely trackers survey matters as much as their skill in identifying signs.

- The authors’ hypothetical scheme relies on 25 trained observers. How observers are trained and tested would affect whether sign identification is consistent across so many people and surveys.

## Cite this summary

Lawrence, K. 2026. Summary of: Sign Surveys for Estimating Trend of a Remnant Brown Bear Ursus arctos Population in Northern Spain (Clevenger and Purroy, 1996). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/1996-clevenger-purroy-sign-surveys-estimating-trend/