ORIGINAL PAPER Aleš Vorel 1 & Lenka Válková 2 & Lenka Hamšíková 1 & Jaroslav Maloň 2 & Jana Korbelová 1 Received: 5 July 2014 /Revised: 5 May 2015 /Accepted: 5 May 2015 /Published online: 14 May 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract Classical theory states that generalist foragers should specialise to maximise their net energy intake on a temporal basis. Within broad distributions, generalists select optimal food sources; however, the availability and represen- tation of selected food items in the diets of generalist con- sumers vary geographically. Conversely, specialists, within their entire range, concentrate on specific food resources, and their diets remain constant in varying environments. The distributions of highly palatable food resources might influ- ence the foraging patterns of species with opportunistic forag- ing behaviours. We used the Eurasian beaver (Castor fiber) as a model for what is considered an opportunistic generalist herbivore. We analysed whether the beavers’ foraging pattern could show specialisation. Five beaver populations living in diverse environments were evaluated, and their diet choices for overwintering were examined. Our data indicated low pro- portions of generality with regard to the beavers’ foraging behaviours. Within 110 territories, we observed that beavers did pursue choosy opportunistic strategies, though only to a small extent. Although we confirmed the established knowl- edge that beavers prefer to browse mainly poplars and willows, we also found that, contrary to many published stud- ies, their preferences occurred regardless of riparian vegeta- tion composition. While the beavers examined in this study preferred six genera of woody vegetation, we argue that they may have been inclined primarily to use two genera with regard to both food choice and spatial territory placement. Thus, the opportunistic feeding behaviours of generalists ob- served in this study may be influenced by evaluation under varying conditions. Keyword Castor . Foraging . Beaver . Herbivory . Generalist . Opportunists Introduction The factors affecting animals’ diet choices are commonly interpreted in the context of optimal diet theory (Pulliam 1974), which proposes that a particular foraging strategy aims to maximise certain components related to fitness (e.g. rate of energy intake), which depends on the energy content of alter- native prey, encounter rates, and how effective the forager is at detecting, capturing, handling and digesting the prey. Herbivorous foraging behaviour can be basically categorised as generalist or specialist, depending upon the given species’ habitat and food niche (Herrera and Pellmyr 2002). Staddon (1983) postulated that the most important con- sideration for foragers is their net rate of energy intake. This intake has direct consequences for diet choice, allocation of foraging time to patches, and spatial search patterns in hetero- geneous environments. Thus, the generalists always select to consume the most optimal food sources; however, the avail- ability of food items in the environment and their representa- tion in the consumers’ diets vary markedly with geographic distribution. Conversely, specialists concentrate on specific food sources within their entire range, and their diets remain constant even in a varying environment. Stephens and Krebs (1986) proposed that the specialisation of generalists occurs Communicated by K. E. Ruckstuhl * Aleš Vorel vorel@fzp.czu.cz 1 Department of Ecology, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýcká 129, 165 21 Prague, Czech Republic 2 Department of Ecology, Faculty of Science, Palacký University, tř. Svobody 26, 771 46 Olomouc, Czech Republic Behav Ecol Sociobiol (2015) 69:1221–1235 DOI 10.1007/s00265-015-1936-7 Beaver foraging behaviour: Seasonal foraging specialization by a choosy generalist herbivore