# Northern Birch Mouse (Sicista betulina) in Lithuania, Findings in the Diet of Tawny Owl (Strix aluco)

> How do you find one of the rarest mice in Lithuania? Ask a tawny owl. Over 14 years, owl pellets and nest-box leftovers yielded 31 northern birch mice among more than 5000 mammal prey, and showed that owls caught them near spruce forests, mostly in years when their usual voles were in short supply.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2011-balciauskas-balciauskiene-northern-birch-mouse-sicista/

## Citation and access

- **Authors:** L. Balčiauskas, L. Balčiauskiene and P. Alejunas
- **Year published:** 2011
- **Journal or source:** Acta Zoologica Academiae Scientiarum Hungaricae
- **Full citation:** Balčiauskas, L., Balčiauskiene, L. and Alejunas, P. 2011. Northern birch mouse (Sicista betulina) in Lithuania, findings in the diet of tawny owl (Strix aluco). Acta Zoologica Academiae Scientiarum Hungaricae 57(3): 277–289.
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2011_Balciauskas_and_Balciauskiene_etal_northern-birch-mouse-findings-in-the-diet-of-tawny-owl.pdf

## Tags

- **Species:** Mammals > Northern birch mouse; Birds > Tawny owl
- **Region:** Europe > Lithuania
- **Tracking:** Bones; Feeding signs; Tracking
- **Behaviour and ecology:** Behaviour; Feeding; Predation
- **Methods and technology:** Chi-square test; Descriptive statistics; Pearson correlation
- **Data type:** Existing data; New field data
- **Observer qualification:** Not reported
- **Discipline:** Behaviour and ecology
- **Year published:** 2011

## Detailed keywords

- **Species:** Studied: northern birch mouse (Sicista betulina) as prey of the tawny owl (Strix aluco). Other mammal prey found (22 species in all): common mole (Talpa europaea), common shrew (Sorex araneus), pygmy shrew (Sorex minutus), water shrew (Neomys fodiens), noctule (Nyctalus noctula), Nathusius’ pipistrelle (Pipistrellus nathusii), brown long-eared bat (Plecotus auritus), serotine bat (Eptesicus serotinus), common dormouse (Muscardinus avellanarius), bank vole (Myodes glareolus), water vole (Arvicola terrestris), root vole (Microtus oeconomus), short-tailed vole (Microtus agrestis), common vole (Microtus arvalis), harvest mouse (Micromys minutus), striped field mouse (Apodemus agrarius), yellow-necked mouse (Apodemus flavicollis), pygmy field mouse (Apodemus uralensis), house mouse (Mus musculus), brown rat (Rattus norvegicus) and weasel (Mustela nivalis). Other owls discussed: Tengmalm’s owl (Aegolius funereus), great grey owl (Strix nebulosa), Ural owl (Strix uralensis), barn owl (Tyto alba); southern birch mouse (Sicista subtilis) in Hungary.
- **Location and habitat:** Lithuania: Akmene district (wet spruce and birch stands around the raised bog of Kamanos Strict Nature Reserve), Joniškis district (Žagares miškas forest), Biržai district (Biržu Giria forest) in the north; Kelme district in the west (coniferous forest edges near farmsteads, and a deciduous park by ponds); Kedainiai district (oak and mixed deciduous stands next to meadows and farmland), Kaišiadorys district (mixed, spruce and deciduous stands near water) and Jonava district in central Lithuania; Šakiai district in the south-west (spruce-deciduous forest edges with meadows). All pellets were collected in forests and forest edges. Comparison data from north-east Poland (recalculated from Lesiński et al. 2009); other studies from Belarus, Poland and Hungary discussed.
- **Topics:** owl diet; owl pellets and nest-box food remains as a way to find rare small mammals; northern birch mouse distribution and rarity; Red List species; prey switching; alternative prey; optimal foraging theory; habitat of the birch mouse (spruce versus deciduous forest, human-affected habitats); south-west to north-east gradient in tawny owl diet across Central Europe.
- **Statistics or models:** Chi-square tests comparing prey composition with and without birch mouse; Pearson correlations; Shannon–Wiener diversity index (H’), Simpson’s dominance index (c); Rényi diversity ordering (DivOrd program); means with standard deviations; Statistica 6.0. Inconsistencies to note: the text gives 408 intact pellets in total, but the forest-by-forest counts in the same paragraph add up to 308. Table 2 places Joniškis under “Prey remains”, while Table 1 and Table 3 list Joniškis as pellets (the pellet and food-remains percentages in the text only work out with Joniškis as pellets). Table 1 gives Šakiai a period that includes 1987, outside the stated 1996–2009 study period, and gives Akmene as 1999–2003, while Table 3 also has an Akmene value for 1996. The Discussion says the study added “28 individuals … taking the known total to 31”, while the Results report 31 individuals found in this study.
- **Data type:** New field data and existing data: tawny owl diet samples from 1996–2009 (93 nest-box food-remains samples and 43 pellet samples), most already published by the authors’ group, with the Joniškis pellets (2008–2009) new. Data from north-east Poland recalculated from a published study.

## Synopsis

### 1. What they did

The northern birch mouse is a small rodent that is rare in Lithuania and on the national Red List. Snap traps rarely catch them, so a growing share of Lithuanian records comes from owl diets. The authors set out the latest findings of birch mice in tawny owl food remains and pellets, and tested an idea: that owls take rare small mammals like the birch mouse when their staple prey is scarce.

They used tawny owl diet samples from eight districts of north, west, central and south-west Lithuania, collected from 1996 to 2009:

- Food remains left in nest boxes after the breeding season (93 samples, one per nest box).

- Pellets collected through the year in forests and forest edges (43 samples; 408 intact pellets reported, plus broken ones).

- Identifying prey from skulls. Where broken skulls did not allow voles or mice to be named to species, they were shared out in the same proportions as the identified ones in that sample.

Only mammal prey were analysed. The authors compared the prey make-up of samples with and without birch mice, and repeated the comparison with published data from north-east Poland.

### 2. What they found

The owls’ diet

- Tawny owls had eaten 22 mammal species, over 5000 individuals in all.

- About 85% of the mammal prey were five species: bank vole (28.8%), yellow-necked mouse (17.9%), common vole (14.0%), short-tailed vole (13.3%) and common shrew (10.7%).

Birch mice in the diet

- 31 birch mice were found, 24 of them in nest-box food remains. Overall they made up 0.62% of mammal prey (0.63% in food remains, 0.58% in pellets).

- No district averaged over 4%. In single years, birch mice reached 7.14% of mammal prey in Akmene (in 2000 only), 6.98% in Kelme and 6.25% in Kaišiadorys.

- Birch mice turned up in every district where the study sites included spruce forest edges. None were found in Kedainiai and Šakiai, where sites were mostly deciduous forest and affected by people.

- The share of birch mice was not linked to the number of prey species, number of prey eaten, diet diversity or dominance. Samples with and without birch mice did not differ significantly in diversity.

Birch mice and the main prey

- In every district tested, the make-up of the main prey differed significantly between years with and without birch mice.

- Across Lithuania, birch mice appeared when owls ate fewer Microtus voles and more common shrews. Districts where birch mice were always present had fewer Microtus voles (20.1% versus 32.9%) and more common shrews (14.2% versus 8.9%).

- In the recalculated Polish data, birch mice appeared when owls ate few bank voles and yellow-necked mice and many common shrews.

All owl records

Adding a few other finds (a tawny owl nest in 1998, a tawny owl pellet and a Tengmalm’s owl pellet in Kelme, and an unidentified owl pellet in Plunge), owl diets have yielded 35 birch mice in Lithuania in 14 years.

### 3. What they concluded

- From owl diet studies, the northern birch mouse can be considered one of the rarest small mammals in Lithuania.

- Whether birch mice appeared in the diet depended first on habitat: they were taken near spruce, not deciduous forest, mainly in forested landscapes and less human-affected places.

- Their presence was also linked to the availability of the owls’ main prey. In Lithuania it was fewer Microtus voles (with more common shrews as alternative prey); in north-east Poland it was fewer bank voles and yellow-necked mice. This fits optimal foraging theory: as main prey decline, predators take less profitable prey.

- The authors link the difference between the two countries to a gradual shift from mice towards voles in small mammal communities towards the north-east.

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

There was no tracking in the field. The study rests on owl signs: pellets and food remains collected from nest boxes and in forests, with prey identified from skulls and bones. The paper does not describe how pellets were found or confirmed as tawny owl pellets.

### 5. Tracker or observer certification

Not reported. The paper does not say who collected the samples or identified the prey remains.

### 6. Other technologies, skills or innovations

- Prey identification from skull remains, with a set rule for sharing out vole and mouse remains that could not be named to species.

- Rényi diversity ordering (DivOrd program, run in DOSBox) to compare diet diversity between sites.

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

- Analysing owl pellets is a relatively fast way to gather many records of rare small mammals that classic trapping rarely catches.

- The northern birch mouse appears to be one of the rarest small mammals in Lithuania, and to depend on spruce forest edges and less human-affected habitats.

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

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

- Owl pellets and nest-box remains are signs that can reveal small mammals that are almost never seen or trapped, such as the birch mouse here.

## Cite this summary

Lawrence, K. 2026. Summary of: Northern Birch Mouse (Sicista betulina) in Lithuania, Findings in the Diet of Tawny Owl (Strix aluco) (Balčiauskas and Balčiauskiene et al., 2011). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2011-balciauskas-balciauskiene-northern-birch-mouse-sicista/