Sunde, P., Mayer, M., Fox, A.D., Holm-Andersen, A., Šálek, M., Lindhøj, F.J., Krištín, A., Obuch, J., Chrenkova, M., Kaatz, G., McAneney, C., Kaczmar, E., Brøndum, K., Christensen, M.M., Jensen, E.H., Rudbeck, A.V., Skjoldager, M. & Jacobsen, L.B. 2021. Det biologiske levegrundlag for kirkeugler i Danmark. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 58 s. - Videnskabelig rapport nr. 444
With less than 15 known remaining breeding pairs, the Little Owl Athene noctua is on the verge of extinction as a Danish breeding bird, due to inadequate juvenile production, caused by lack of natural food during the breeding season. Maintenance of the current remaining population only occurs through artificial supplementary feeding in the breeding season. The future survival of a Danish self-sustaining Little Owl population without artificial feeding is only possible by restoring their natural prey base. To enable the recommendation of effective habitat improvement measures requires knowledge. In particular, about (i) what formerly abundant Little Owl prey items are currently lacking in the Danish contemporary agricultural landscape, (ii) how this is linked to specific landscape elements (the density and mosaic of land use cover types, hedgerows and biotopes, etc.) and (iii) the scale at which Little Owls utilize the landscape.
This report presents results of a study to identify the requirements for maintaining a natural, self-supporting population of Little Owls in Denmark in 2019-2020 undertaken by the Danish Centre for Environment and Energy at Aarhus University, in collaboration with the Natural History Museum Aarhus, financed by the Danish Environment Ministry. We selected a viable and self-sustaining population in western Schleswig-Holstein, Germany, numbering c.140 pairs and showing stable or slightly increasing trends as a reference for comparing (1) diet, (2) prey densities and (3) landscape habitat composition with those in currently only sustaining numbers through supplementary artificial feeding in Denmark. We also compared foraging behaviour of nestling-provisioning GPS-tagged adults in Denmark and in Bohemia, Czech Republic (as this proved impossible in Germany). The report is largely based on empirical studies conducted in 2019-20 (food composition, prey density, behaviour), but also includes published and unpublished data from the period 1974-2011. This report summarizes the main results and conclusions from these studies that have most relevance for future management of Little Owls in Denmark, under the following headings.
We described diet composition of Danish nesting Little Owls based on studies of prey in regurgitated pellets from ten different breeding pair/seasons and nest camera photos from three breeding pair/seasons during 2008-2020. The majority of natural food biomass consumed during the breeding season consisted of earthworms (47%) followed by vertebrates (34%, comprising 28% mammals and 6% birds and amphibians) and insects (19%). The most common mammals in Danish Little Owl pellets were voles (Field Vole Microtus agrestis and Common Vole M. arvalis), House Mice Mus musculus and Wood Mice Apodemus sylvaticus. By comparison, the food of five breeding pairs in Schleswig-Holstein in 2019 (a year when Little Owls enjoyed record breeding success) was 90% vertebrates (81% voles), 7% earthworms and 3% insects. Compared with foreign Little Owl pairs, the diet of breeding Danish Little Owls was dominated to a greater extent by earthworms and contained far fewer small mammals, especially Field Voles, suggesting that Little Owl pairs lacked voles and other small mammals in the Danish landscape as staple summer food items.
In regurgitated pellets from Denmark in 2011 and 2019-20, carabid beetles (Carabidae) comprised 56% and 72% of all identifiable insect remains, whereas scarab beetles (Scarabaeidae) comprised less than 5%, despite the fact that this species group has previously been considered important food items for breeding Little Owls in the literature. By comparison, in Schleswig-Holstein, carabids and scarabs comprised 39% and 26% of all identifiable insect remains by number.
Supplementary feeding of day-old chicks accounted for 38% of the total food consumption of Danish Little Owl pairs with access to artificial food, confirming that this source of food was a significant source of energy supplement for the remaining Danish Little Owl pairs to raise their offspring.
In May-June 2019, we assessed the densities of terrestrial insects and small mammals in 24 former and currently occupied Little Owl territories in Himmerland, Denmark and Schleswig-Holstein, Germany (12 territories in each country). Numbers of terrestrial trap-caught insects per week was generally higher in Himmerland than in Schleswig-Holstein, confirming that lower Danish Little Owl breeding success was not likely due to lower density of large insects such as beetles, or the beetles' main food, earthworms. Night-time monitoring of small mammals found them (mainly mice, Muridae) to be six times more abundant in Schleswig-Holstein than in Denmark (95% confidence intervals: 2-18 times), suggesting lower breeding success in Denmark is due to lower small rodent densities there. In occupied Little Owl territories, habitats away from the nest supported nine times the densities of small rodents (95% confidence intervals: 3-26 times) compared to areas immediately around the buildings where Little Owls bred, while there was no such differences in abandoned territories. This suggests that the presence of breeding Little Owls reduces small rodent densities in areas immediately around nests.
We mapped and compared landscape composition within a 500 m radius of 23 Little Owl nests in Himmerland and 46 nests in Schleswig-Holstein. On average, Little Owl nests in Schleswig-Holstein contained twice as many different surface polygon types and three times as many windbreaks within that area as those in Himmerland. The results confirm previous studies, which concluded that Little Owls breeding success positively correlates with high habitat heterogeneity and scattered (preferably older) trees.
We studied the movements, habitat use and selection of six Danish and Czech Little Owls equipped with GPS loggers. Danish male Little Owls had an average nocturnal activity area of 54 ha per night and moved on average 248 m from the nest compared to 1.5 ha and 34 m for females. Activity areas of Danish Little Owl males were ten times larger than those in the Czech Republic, indicating scarcer encounters with food items in Denmark. Danish Little Owl males selected forest (unlike Czech males), which exposes them to increased risk of predation from Tawny Owls Strix aluco, further evidence of food shortages in the more open landscapes. Breeding Little Owls in both Denmark and the Czech Republic spent more than half of their time in gardens and built-up areas and used these areas more than predicted by availability. Field boundaries and roadsides were also over-represented in their use compared to availability, confirming the importance of heterogeneity and edge zones for their prey and/or for Little Owls foraging.
Multiple results from this and other studies suggest the lack of small mammals, perhaps especially voles, compared to studies elsewhere, contributes to the poor foraging success of Danish Little Owls during the breeding season. Achievement of a self-sustaining viable Danish Little Owl population (independent of artificial supplementary feeding during the breeding season) is only possible by increasing the population density and availability of small rodents (probably especially voles) on territories that also support stable and abundant sources of accessible earthworms and ground dwelling insects such as carabid and scarab beetles
Enhancement of small rodent population densities is possible by increasing the breadth, density and length of field boundaries and hedges, as well as improving their quality as a habitat, not least in winter, when the carrying capacity in relation to small rodent populations is lowest. Establishment of "game stripes", leaving permanent grass strips along existing field boundaries can make a major contribution, as can taking even small areas completely out of rotation to support greater small mammal populations. In areas where the Common Vole is absent (as in large parts of Himmerland), habitat improvement should be aimed at enhancing Field Vole habitat, since this species requires more dense litter, tall and old grass than does the Common Vole, which prefers younger and more open grass vegetation.
Little Owls hunt for their prey on relatively bare surfaces (hard surfaces, bare soil, short grass), so it is important to maximize the length (and quality) of border zones between prey habitats and the open surfaces where Little Owls catch their prey. This probably applies in particular to the ability of Little Owls to catch Field Voles, which only move outside of cover out of necessity.
In practice, the optimal means to achieve such a matrix could be cutting paths through areas maintained as permanently tall grass or by creating alternating streaks of grazed areas and permanent grass on partially grazed areas. Other potential positive measures include retaining fallen branches, twigs, debris and old grass litter as a shelter for small rodents as well as insects, as well as providing Little Owls with hunting posts and cover for other birds of prey. In addition to promoting the density and availability of small rodents, the above measures should also promote the abundance and availability of terrestrial insects and the availability of earthworms (a major food source in humid weather).
Results from the Czech Republic confirmed that given an adequate food supply, Little Owls remain 90% of their time within 100 m of the nest, only occasionally moving further than 200 m away. It should therefore also be possible to provide a sufficient resource base within a distance of 200 m from Danish nests. Since the results showed that even Danish Little Owls spend 80% of their outdoor time within 100 m of the nest, habitat improvement within this distance out from the nest site is highly likely to have a major positive effect on the ability of Little Owls to harvest sufficient food to maintain their reproductive output without supplementary feeding.
Insect and small mammal population dynamics depend on landscape configuration, composition, degree of fragmentation and utilization, so the landscape structure outside of Little Owl foraging areas probably also affects the prey population densities prey within Little Owl territories. For this reason, habitat improvement measures should be applied to the greater agricultural landscape, not merely within individual Little Owl territories.
We cannot predict precisely how and to what extent the Little Owls' prey populations will respond to specific habitat improvement initiatives, or how Little Owls will respond to an improved resource base. To ensure we learn how to promote best practice, therefore, it is essential that the effects of habitat improvement experiments are monitored and evaluated. We recommend combining monitoring of prey densities in experimental versus control areas with GPS logging of provisioning adult Little Owls and simultaneous camera monitoring of their prey items brought back to nests, to identify which items are brought from which habitats and in what quantities.
We also acknowledge the potential adverse effects of rodenticide use on the food base of Little Owls in Denmark (although this formed no part of this study), as well as risking their secondary poisoning. As long as it remains unknown whether rodenticide use constitutes a real problem to Little Owl prey populations and their own health and fitness, the precautionary principle dictates that the use of rodenticides within Little Owl territories should be limited to the maximum extent practicable. Data presented here suggest that Little Owls are highly effective at controlling small rodents by their presence on territory. If rats can be controlled by methods other than the use of poison, unintentional poisoning of the smaller rodents that are prey for Little Owls will be avoided.