# A new method for estimating visitation rates of cryptic animals via repeated surveys of indirect signs

> How often does an otter visit a fish farmer’s pond? By sorting otter droppings into fresh and old on each weekly check, researchers built a method that was more than three times more accurate than the usual “sign present or absent” approach. The skill of ageing tracks and signs turns out to be worth real precision in wildlife monitoring.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2008-gruber-reineking-new-method-estimating-visitation/

## Citation and access

- **Authors:** Bernd Gruber, Björn Reineking, Justin M. Calabrese, Andreas Kranz, Katerina Poledníková, Lukáš Poledník, Reinhard Klenke, Anett Valentin and Klaus Henle
- **Year published:** 2008
- **Journal or source:** Journal of Applied Ecology
- **Full citation:** Gruber, B., Reineking, B., Calabrese, J. M., Kranz, A., Poledníková, K., Poledník, L., Klenke, R., Valentin, A. and Henle, K. 2008. A new method for estimating visitation rates of cryptic animals via repeated surveys of indirect signs. Journal of Applied Ecology 45(2): 728–735.
- **DOI:** 10.1111/j.1365-2664.2007.01406.x
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2008_Gruber_and_Reineking_etal_estimating-visitation-rates-of-cryptic-animals-via-indirect-signs.pdf

## Tags

- **Species:** Mammals > Eurasian otter
- **Region:** Europe > Germany
- **Tracking:** Scat; Sign surveys; Spoor age estimation; Tracking; Tracks and signs identification
- **Behaviour and ecology:** Behaviour
- **Methods and technology:** Detectability; Simulation
- **Data type:** Modelling or theoretical; New field data
- **Observer qualification:** Not reported
- **Discipline:** Behaviour and ecology; Human–wildlife conflict; Wildlife tracking
- **Year published:** 2008

## Detailed keywords

- **Species:** Studied: Eurasian otter (Lutra lutra), called European otter in Tables 4 and 5. Mentioned as examples or from other studies: spotted-necked otter (L. maculicollis), Eurasian badger (Meles meles), red fox (Vulpes vulpes), bears and sheep, foxes and crows at ground-nesting birds, lizards at turtle nests, pollinators and malaria-carrying mosquitoes.
- **Location and habitat:** A commercial fish pond in Upper Lusatia, Germany (the field example). The method itself is general and was tested with simulated data.
- **Topics:** visitation rate and visitation probability (how often animals visit a place of interest); cryptic (hard-to-see) animals; indirect signs; ageing signs as fresh (last night) or aged (older); otter spraints (scat); repeated sign surveys with removal of signs at each visit; choosing the best interval between surveys; irregular survey intervals; human–wildlife conflict between otters and fish farmers; predation risk at nests; disease transmission.
- **Statistics or models:** Maximum likelihood estimators built on a Poisson process (visits assumed independent and at a constant rate); binomial likelihoods; four estimators compared: fresh signs only, aged signs only, all signs combined (the estimator used in earlier literature), and fresh plus aged signs together (the new method); likelihood-ratio confidence intervals; simulation of 399 scenarios (2 to 42 weekly surveys, daily visitation probability 0.05 to 0.95) with 100 repetitions each; relative bias, relative precision (variance) and relative accuracy (mean square error). Excel workbook “DoubleTrack” with macros (supplementary material). Small inconsistencies and typos in the paper: in the Results the outliers come from cases with n ≤ 4 and/or P ≤ 0.1, while the Discussion says the new estimator fails with n < 4 and P < 0.1; Table 1 labels the combined estimator’s probability “pag” instead of “pcm”; Table 3 gives “=30” for the lowest visitation probabilities (probably meaning 30 or more); Figure 3 labels its axis “visitation rate” although the values are probabilities; the Discussion says the simulations show the method copes with uncertain ageing of signs, but no such simulation is described in the Methods.
- **Data type:** Modelling or theoretical (simulated data sets). New field data: one 12-week survey of otter spraints at one fish pond, checked weekly (dates not given). The otter estimate was compared with an earlier radio-tracking study (Poledník 2005).

## Synopsis

### 1. What they did

Knowing how often animals visit a particular place (a fish pond, a nest site, a farm) matters for managing conflict with people, predation and disease. For shy, hard-to-see animals, researchers often check the place again and again for signs such as tracks or droppings. The usual method simply counts how many checks found signs. It ignores how old the signs are.

The authors developed a new way to estimate visitation rate that uses the age of the signs. At each check, signs are recorded and removed, and each is classed as fresh (from the previous night) or aged (from the nights before). For Eurasian otters, fresh spraints (droppings) are wet and soft, while aged ones have a dry surface and solid consistency.

- Building the estimators. They worked out the maths for four estimators: fresh signs only, aged signs only, all signs without telling them apart (the method used in the literature), and fresh and aged signs together (the new method). They showed the older methods are special cases of the new one, and that the new one also works when checks are not evenly spaced.

- Testing with simulated data. They simulated weekly checks for 399 combinations of survey length (2 to 42 weeks) and daily visitation probability (0.05 to 0.95), 100 times each, and compared bias, precision and overall accuracy.

- Planning surveys. They simulated different intervals between checks to find the best interval for different visitation levels.

- A field example. They applied the method to otter spraints at a commercial fish pond in Upper Lusatia, Germany, checked weekly for 12 weeks.

### 2. What they found

Simulations

- The new estimator (fresh plus aged signs) was on average more than three times more accurate than the next best, the fresh-signs-only estimator (mean relative accuracy 0.077 against 0.250).

- The fresh-signs-only estimator was the only unbiased one, but it was imprecise, especially when visits were rare. The new estimator had only slight bias (3%).

- The aged-signs-only and combined estimators overestimated visitation (by about 36% and 44%). The combined estimator, the one used in earlier studies, performed worst, and did well only with more than 30 checks or very high visitation.

- The new estimator was inaccurate only with very few checks (fewer than about 4) and very low visitation probability (below about 0.1). In those cases none of the others did better.

Best interval between checks

The rarer the visits, the longer the best interval: about 30 days when the daily visitation probability is below 0.05, 10 to 20 days at 0.05 to 0.15, 5 to 10 days at 0.15 to 0.25, 4 to 5 days at 0.25 to 0.55, 3 days at 0.55 to 0.8, and every other day above 0.8 (otherwise aged signs are almost always present). The number of checks had only a minor effect on the best interval.

Otters at the fish pond

- Fresh spraints were found in 3 of the 12 weekly checks and aged spraints in 10.

- All four methods gave a daily visitation probability of about 0.25 to 0.26 (a visitation rate of about 0.3 visits per day). The new method gave the narrowest confidence interval: probability 0.256 (95% confidence interval 0.144 to 0.405).

- With the new method, 10 checks gave about the same precision as 12 checks with the other methods.

- The estimate was very similar to one from an earlier radio-tracking study of otters.

### 3. What they concluded

- Using both fresh and aged signs gives precise, almost unbiased estimates of visitation across the widest range of conditions. Wherever possible, fresh and aged tracks and signs should be told apart and used together.

- The commonly used estimator is heavily biased and imprecise, which the authors call alarming.

- Signs should be aged as accurately as possible, but the authors report that the method still works when ageing is uncertain, as long as errors are not systematic.

- The method assumes visits arrive independently at a constant rate. This does not need each animal to move at random, only that the animals act independently and are not too few. Where this assumption is strongly broken, estimates may be best used only to rank sites against each other.

- The method could work for any species whose signs can be dated, for example eaten fruits, leaves, carcasses or burrows. It could be extended to more than two age classes, longer “fresh” periods, or counts of individuals at each check.

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

The method rests on tracks and signs: repeated surveys for signs at a site, with each sign recorded, aged and removed. In the field example the signs were otter spraints, aged as fresh (wet and soft, from the previous night) or aged (dry surface, solid). The authors note that ageing can vary slightly with weather and time of day, that otter spraints are typically obliterated after about 7 weeks, and that checks must be frequent enough for aged signs not to disappear between visits. No trailing was reported.

### 5. Tracker or observer certification

Not reported. The paper does not say who surveyed the fish pond. It states in general that experienced researchers can age spraints correctly, and recommends a pilot study with daily sampling. The accuracy of sign ageing matters to the results, but no test of the observers’ ageing skill is described.

### 6. Other technologies, skills or innovations

- “DoubleTrack”, a free Excel workbook with macros, calculates visitation rates, visitation probabilities and confidence intervals for regular and irregular intervals between checks (supplementary material and authors’ website).

- A table of the best interval between checks for different visitation levels, to help design surveys.

### 7. Implications for conservation (from the paper)

- Visitation rates matter for many conservation and management problems: the conflict between otters and fish-pond owners, predator visits to nests, and the spread of disease (such as tuberculosis by badgers or rabies by foxes).

- Better estimates of how often otters visit commercial fish ponds can help assess that conflict, and the new method is widely applicable in ecology and conservation biology.

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

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

- The study puts a number on the value of a core tracking skill: telling fresh from old sign. Simply recording that sign was present threw away information and gave biased results, while recording its age made estimates about three times more accurate.

- The method depends on ageing being right and not systematically off, yet the observers’ ageing skill was not tested. Surveys using this method could report how sign ageing was checked.

## Cite this summary

Lawrence, K. 2026. Summary of: A new method for estimating visitation rates of cryptic animals via repeated surveys of indirect signs (Gruber and Reineking et al., 2008). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2008-gruber-reineking-new-method-estimating-visitation/