# Variation in the diet composition of a generalist predator, the red fox, in relation to season and density of main prey

> What does a fox eat when the voles crash? Over 13 years of snow-tracking in western Finland, researchers collected fox scats and found that voles were the staple, closely following vole numbers in the fields, while hares, grouse and even stoats and weasels stayed on the menu whatever the vole year.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2007-dellarte-laaksonen-variation-diet-composition-generalist/

## Citation and access

- **Authors:** Graziella Lucia Dell’Arte, Toni Laaksonen, Kai Norrdahl and Erkki Korpimäki
- **Year published:** 2007
- **Journal or source:** Acta Oecologica
- **Full citation:** Dell’Arte, G. L., Laaksonen, T., Norrdahl, K. and Korpimäki, E. 2007. Variation in the diet composition of a generalist predator, the red fox, in relation to season and density of main prey. Acta Oecologica 31(3): 276–281.
- **DOI:** 10.1016/j.actao.2006.12.007
- **Access status:** Publisher link only
- **Where to find it:** https://doi.org/10.1016/j.actao.2006.12.007

## Tags

- **Species:** Mammals > Red fox
- **Region:** Europe > Finland
- **Tracking:** Scat; Snow tracks; Tracking; Tracks; Trailing
- **Behaviour and ecology:** Behaviour; Feeding; Intraguild interactions; Predation
- **Methods and technology:** Logistic regression
- **Data type:** New field data
- **Observer qualification:** Not reported
- **Discipline:** Behaviour and ecology; Wildlife tracking
- **Year published:** 2007

## Detailed keywords

- **Species:** Studied: red fox (Vulpes vulpes). Food remains found in scats: Microtus voles (field vole Microtus agrestis and sibling vole M. rossiaemeridionalis); other microtines (water vole Arvicola terrestris, muskrat Ondatra zibethicus, bank vole Clethrionomys glareolus); murids (harvest mouse Micromys minutus, brown rat Rattus norvegicus); hares (Lepus spp.); small mustelids (stoat Mustela erminea, least weasel M. nivalis); shrews (Sorex spp., water shrew Neomys fodiens); birds, including grouse (Galliformes); plants and insects. Small mammals snap-trapped for vole density indices: shrews, field voles, sibling voles, bank voles and mice.
- **Location and habitat:** Alajoki, near Kauhava and Lapua, western Finland (about 63°05’N, 22°55’E). About 50 km² of mainly arable farmland (73%; oats, barley and hay), spruce and pine forest (19%), and peat bogs and inhabited areas (8%). Snow usually covers the ground from October to March, with a mean long-term maximum depth of about 30 cm.
- **Topics:** red fox diet; scat analysis; seasonal diet change (winter versus summer); vole cycles (3 to 4 year population cycles); generalist versus specialist predators; prey switching and alternative prey; intraguild predation on small mustelids; hares and grouse as main prey; carrion from hunters and road kills; possible fox impact on forest grouse; hair and feather identification from scats. Inconsistencies in the paper: the abstract gives the proportion of scats with Microtus voles as 0.55 overall and 0.62 in summer, the results give 0.56 overall and 0.67 in summer, and the discussion gives 0.66 for summer.
- **Statistics or models:** proportion of scats (PS) containing each food category (frequency of occurrence); G-test of overall diet composition; generalised linear models (logit link, binomial distribution; GENMOD procedure in SAS 8.2) for each prey group against season and Microtus density; Spearman rank correlation (rs) between yearly winter PS of Microtus voles and the previous autumn trap index.
- **Data type:** New field data: 58 fox scats collected from 1983 to 1995 (34 in winter, October to March; 24 in summer, April to August), and small mammal snap-trapping each May and September (10,834 trap nights in total).

## Synopsis

### 1. What they did

In northern Europe, vole numbers rise and crash in cycles of 3 to 4 years. Red foxes are thought of as generalist predators, able to eat many kinds of food, so the authors asked whether foxes in western Finland change what they eat between winter and summer and between years when voles are scarce or plentiful. They also asked whether foxes eat smaller predators, such as weasels and stoats, that live on voles too.

- Collecting scats (1983 to 1995). Fox scats were collected during snow-tracking and other field work in a farmland area of about 50 km². All scats found along one fox’s daily track, or in the same 3 km² area on one day, counted as one sample, and one scat per sample was picked at random for analysis. In total, 58 scats were analysed: 34 from winter and 24 from summer.

- Identifying food remains. Each scat was searched for bone fragments, hair, feathers, insect parts and plant remains. Hairs were identified by their scale patterns, cross-sections and medulla, using hair atlases and a reference collection of small mammals trapped in the same area. Feathers were identified with a guide and museum reference material.

- Measuring vole numbers. Small mammals were snap-trapped every May and September in four habitats (cultivated field, abandoned field, pine forest and spruce forest) to give an index of voles caught per 100 trap nights. The autumn index stood for vole numbers in the following winter, and the spring index for the following summer.

- Analysing the diet. For each of eight food groups, they calculated the proportion of scats containing it, and compared winter with summer and low with high vole years.

### 2. What they found

Overall diet

- Diet differed significantly between winter and summer, and tended to differ between low and high vole years.

- Microtus voles (field and sibling voles) were the main prey, found in 0.56 of all scats (0.55 in the abstract). They were found equally often in winter (0.50) and summer (0.67 in the results, 0.62 in the abstract), but more often when voles were plentiful.

- In winter, the share of scats with Microtus voles closely followed the previous autumn’s vole index across 10 years (rs = 0.706). Leaving out two winters with only one scat each, the link was stronger (rs = 0.857).

Other prey

- Other microtines (bank vole, water vole and muskrat) were the second most eaten group (0.37 of scats), eaten more in winter than in summer. They were in 0.54 of scats from poor vole years, but this did not differ significantly from good years.

- Mice and rats were a considerable part of the diet, especially in winter (0.18) and in high vole years (0.20).

- Birds were in 0.31 of scats, and 39% of bird remains were grouse. Birds tended to be eaten more in summer, with no difference between low and high vole years.

- Hares were eaten about equally in winter (0.26) and summer (0.29), and in low and high vole years.

- Small mustelids (stoats and least weasels) were in about 0.10 of scats in both seasons, with no significant difference between vole years.

- Shrews were rare (0.05). Plants and insects were eaten mainly in summer.

- No important prey group was eaten more when voles were scarce than when they were plentiful.

### 3. What they concluded

- Red foxes in this area depend heavily on Microtus voles and other microtine rodents, especially in winter, when there are few alternative prey in strongly seasonal northern environments.

- Winter vole eating varied much more between years than in summer-only studies elsewhere, probably because vole numbers swing more widely in northern Europe, so foxes must find other food when voles are scarce.

- Because no prey group was eaten more in low vole years, foxes behave as generalists that opportunistically use many prey groups. Hares and birds, especially grouse, are main prey rather than alternative prey; part of them was probably eaten as carrion (hunters’ remains and road kills).

- Foxes may be an important predator of grouse. Clear-cutting may create grassy vole habitat that raises fox numbers, which may in turn help explain declines in forest grouse.

- The unusually high rate of foxes eating small mustelids (about 0.10, compared with about 0.02 in central Europe) gives qualitative support to the idea that mammalian predators limit weasel and stoat populations. Studies using, for example, radio-telemetry are needed to confirm this.

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

Scats were collected during snow-tracking and other field work. Scats found along a fox’s daily track were grouped as one sample, which shows the researchers followed fox trails in the snow and picked up scats along them. Food remains in the scats (hair, feathers, bone and insect parts) were then identified in the laboratory.

### 5. Tracker or observer certification

Not reported. The paper does not say who did the snow-tracking. M. Hast, O. Hemminki and M. Koivula are thanked for help with collecting and analysing the scats.

### 6. Other technologies, skills or innovations

- Hair identification from scale patterns, cross-sections and medulla types, with published atlases and a local reference collection of trapped small mammals.

- Feather identification with reference material from the Zoological Museum, University of Turku.

- Standard snap-trapping lines (50 to 100 traps, 10 m apart, in small mammal runs, for four nights) to index vole numbers.

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

- Foxes may be an important predator of grouse, which are game species, and higher fox numbers might be an important reason for declines in forest grouse in Finland and elsewhere in northern Europe.

- Clear-cutting forests may raise fox numbers by creating new grassy habitat for voles, the foxes’ main food.

- Fox predation might help limit stoat and least weasel populations.

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

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

- Following a fox’s trail in the snow gave the researchers both scats and a way to group them: all scats from one daily track counted as one sample, so a single fox’s meal was not counted several times. Trailing can help keep diet samples independent.

## Cite this summary

Lawrence, K. 2026. Summary of: Variation in the diet composition of a generalist predator, the red fox, in relation to season and density of main prey (Dell’Arte and Laaksonen et al., 2007). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2007-dellarte-laaksonen-variation-diet-composition-generalist/