The Small Mammals of Warsaw as Inferred from Tawny Owl (Strix aluco) Pellet Analyses

1. What they did

In cities, traps and nets are hard to use in busy parks and green spaces. Owl pellets offer another way to find out which small mammals live there. The tawny owl is the only owl that breeds regularly in central Warsaw, Poland. They eat a wide range of prey, from bats weighing a few grams to rabbits, so their diet reflects what is available. The authors’ own night-time observations and playback surveys suggested city-centre territories of less than 0.3 km², so the prey in the pellets would have been caught within a few hundred metres of the nest.

The authors tested three expectations: towards the city centre, (1) insectivores and species of forests and open areas would make up a smaller share of prey, (2) species that live alongside people, and the striped field mouse, would make up a larger share, and (3) small mammal diversity would decline.

• Collecting pellets (November 2003 to October 2006). They collected pellets at least once a month from 19 tawny owl territories in parks, cemeteries and woodlands: 8 in the city centre and 11 on the outskirts.
• Identifying prey. They identified prey from the skulls in the pellets, using published identification keys and the skull collection of their university department.
• Comparing zones. They compared the city centre and outskirts, both for all prey items together and for average shares across territories, and calculated a diversity index for each zone.
• Comparing with the 1970s. In Łazienki Park, a protected park in the city centre where owl territories had stayed in much the same places, they compared their results with pellets collected in 1975 to 1977.

2. What they found

What the owls ate

• From 886 pellets they identified 569 mammals: 173 from the city centre and 396 from the outskirts. Of these, 509 were identified to species.
• They found 21 mammal species in four groups: rodents, insectivores, bats and carnivores.
• The outskirts held twice as many species as the city centre (19 against 9).
• Rodents were by far the most common prey (467 items), especially the striped field mouse and the yellow-necked mouse.

City centre versus outskirts

• The striped field mouse made up 75.4% of mammal prey in the centre, against 12.6% on the outskirts.
• Rats were a larger share in the centre (11.0% against 3.0%). The house mouse, also a species that lives alongside people, showed no significant difference.
• Forest species (yellow-necked mouse, bank vole) and open-country voles were not found in the centre at all. Insectivores were a smaller share in the centre (8.5% against 17.3%).
• Comparing average shares across territories gave similar results, except for rats, where the difference was not significant.
• Diversity was higher on the outskirts (Shannon-Wiener index 1.15) than in the centre (0.55).

Changes since the 1970s in Łazienki Park

• Striped field mice rose from 27.9% to 48.9% of prey.
• House mice fell from 23.7% to 3.2%.
• Moles fell from 21.5% to 1.1%.
• Bats rose from none to 14.9%.
• Brown rats and European pine voles showed no significant change.

3. What they concluded

• All three expectations were supported: towards the city centre, forest, open-country and insectivore species declined, the striped field mouse and rats made up a larger share, and diversity fell.
• The fall in house mice in the owls’ diet probably reflects a real decline of this species in the city centre, which live-trapping studies in Warsaw support. The rise in striped field mice also matches recent live-trapping, which found this species dominant in city-centre parks.
• The authors suggest the drop in moles in Łazienki Park can be explained by persecution (new ways of killing moles and of stopping molehills on lawns) and the likely isolation of the population.
• More bats in the diet matches the general increase in bats in Poland over recent decades.
• Because pellets were collected in the same seasons in both zones, across many territories in different habitats, the authors believe their data give a reliable picture of the differences in tawny owl diet between the city centre and the outskirts.

6. Other technologies, skills or innovations

• Identifying small mammals to species from skull remains in owl pellets, with identification keys and a reference collection of skulls.
• Night-time observations and playback surveys (playing owl calls) to judge territory size.

7. Implications for conservation (from the paper)

• Where traps and nets are hard to use in busy urban green spaces, owl pellets can show which small mammals are present and how common they are.
• Ongoing urbanization has changed the small mammal community of central Warsaw since the 1970s, with forest and open-country species absent from the centre and the house mouse and mole declining.