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The effect of urbanization on the diet composition of the Tawny owl (Strix aluco L.)

Grzędzicka E., Kus K., Nabielec J.

Polish Journal of Ecology

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2013

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Vol. 61, nr 2

391--400

EN

In recent years, developing urban areas have affected food abundance and the feeding grounds of birds. The article contains analysis of the Tawny owl’s diet during four years (2006–2009) from different types of the urbanized area: city, town (much smaller than city) and countryside – based on 356 pellets with 726 prey items. The main group of victims was Rodentia, common were also Apodemus agrarius and Apodemus sylvaticus. Other important groups in the diet were: Microtidae (especially species Microtus arvalis) and Aves. As a generalist, Tawny owl fits the diet to the actual resources in a very flexible way. The main goal of the present work was to describe the food composition variability of the Tawny owl along with the degree of urbanization. Percentages of Microtus arvalis, Micromys minutus, Talpa europaea depended on the urbanization level, while Apodemus flavicollis, Insecta and Amphibia were related to the distance to nearest city centre. The study has shown that the diet of the Tawny owl has been changing along the urbanization gradient. It confirms earlier findings on high plasticity of foraging of this species in urbanized landscape.

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Non-prey items in stomachs of Alpine newts (Mesotriton alpestris, Laurenti)

Kopecky O., Vojar J., Susta F., Rehak I.

Polish Journal of Ecology

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2011

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Vol. 59, nr 3

631-635

EN

Though newts are exclusively carnivorous predators, non-prey items (silt, plants, skin sloughs) also have been found in their stomachs. There are no previous studies on this topic apart from casual reports. We conducted a dietary study of Alpine newts (Mesotriton alpestris) at two localities in the Czech Republic, focusing on the influence of sex, locality, month, and weight of digested prey on consumption of non-prey items. Localities were two fishless ponds (surface area <40 m2, depth <1 m) at an elevation of about 450 m. Both ponds were isolated from other water bodies and inhabited also by Smooth newts (Lissotriton vulgaris L.) and Common frogs (Rana temporaria L.). Newts were captured by net from the shores and stomach contents were extracted using a stomach flushing technique. We sampled 190 individuals. At least one non-prey item (i.e. silt, plants, skin sloughs) was found in 44% of newts. stomachs. The most numerous items were plant fragments. The weight of consumed prey did not affect the presence of non-prey items in stomachs of newts. Locality (probably relating to prey availability) affected frequency of newts with plants and silt in their digestive tracts, but skin sloughs were consumed at both localities with the same frequencies. All non-prey items were consumed mainly at the beginning of the breeding season (year 1997) and with no difference in frequencies between males and females.

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Towards the optimal foraging strategy : is seasonal shift in the diet of Cormorants Phalacrocorax carbo (L.) in a dam reservoir the effect of water temperature or size pattern in fish assemblages?

Gwiazda R., Amirowicz A.

Polish Journal of Ecology

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2010

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Vol. 58, nr 4

783-792

EN

According to optimal foraging theory the prey choice strongly affects the benefitcost ratios. Predators search prey giving the highest benefit and costs of all components of predation (i.e. prey search, encounter, pursuit, capture, and handling) may be considerably reduced if the prey is more available. The study on Cormorant diet and the species composition of prey fish assemblages in the Dobczyce Reservoir (area 985 ha, submontane, eutrophic reservoir in Southern Poland) in spring (May-June) and in autumn (Oct-Nov) showed differences in the food composition and the prey size affected by seasonal changes in fish availability. The diet of Cormorant included eleven fish species and the dominant species in the food was roach in spring (72%) and roach and perch in autumn (49% in total). Roach and perch had the highest share in prey assemblages too (56% in spring, and 53% in autumn). Significant preference toward roach in spring was found. The share of roach and perch did not changed seas seasonally and could not explain the change in the composition of Cormorant diet. The range of the total length (LT) of fish in Cormorant diet was 3.5-35.2 cm. Diet consisted of distinctly smaller fish in autumn. Relative number of small fish was ca 3 times greater in this period compared to spring. Weighted mean of fish TL in prey assemblage was 20.0 cm for roach and 12.5 cm for perch in spring, and 11.8 and 8.1 in autumn, respectively. The proportion of average weight of roach (W = 0.004074 LT[^3.334]) to that of perch (W = 0.005779 LT[^3.260]) was greater in spring (4.1:1) than in autumn (2.9:1). Probably it can explain the diet shift in autumn. The switch to smaller but more abundant fish in autumn was not related to temperature but to fish availability which reduced the cost of searching and the prey may be easily found.