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

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.