Non-invasive corticosterone treatment changes foraging intensity in red-eyed vireos Vireo olivaceus Mare Lo ˜hmus, L. Fredrik Sundstro ¨m and Frank R. Moore Corticosterone is thought to play an important role in food caching and foraging behaviour. However, the direct influence of increased plasma corticosterone on feeding behaviour is still unclear. In this study the effect of increased corticosterone on feeding behaviour in migratory active red-eyed vireos Vireo olivaceus was investigated. We hypothesized that if increased corticosterone levels facilitate foraging behaviour, an increased number of visits to the food bowl by corticosterone treated birds would be seen. In addition to ad lib food during the experiment, the vireos were fed every full hour between 09.00  13.00 h with one meal worm injected either with 4 mg corticosterone dissolved in 20 ml DMSO, or with DMSO only as a control treatment. The presence or absence of a bird in the food bowl was recorded by a motion detector between 09.00  15.00 h. The non-invasive corticosterone treatment increased plasma corticosterone levels and caused vireos to visit the food bowl more frequently compared to control treated individu- als between 11.00  12.00 h and 13.00  15.00 h. Our data indicate that corticosterone has an effect on feeding behaviour in birds. M. Lo ˜hmus (correspondence), UBC/DFO CAER, 4160 Marine Drive, West Vancouver, BC, V7V1N6, Canada. L. F. Sundstro ¨ m, Department of Zoology, Go ¨ teborg University, Box 463, SE-405 30 Go ¨teborg, Sweden. F. R. Moore, Department of Biological Sciences, University of Southern Mississippi, Hattiesburg, MS 39406  5018, USA. E-mail: mare.lohmus@zool.gu.se Corticosterone is the primary glucocorticoid hormone in birds. The involvement of this hormone in the regulation of foraging behaviour has been discussed in a number of studies (Gray et al. 1990, Wingfield et al. 1990, Kitaysky et al. 2001, Koch et al. 2002, Pravosudov 2003, Dallman et al. 2004, Long and Holberton 2004) generally suggesting a facilitating effect on food intake (Dallman et al. 2004). However, the direct role of corticosterone in feeding behaviour of birds is not clear (Piersma et al. 2000). For example, no effect of corticosterone treatment was observed on either foraging behaviour or food consump- tion in captive dark-eyed juncos Junco hyemalis under simulated winter conditions (Gray et al. 1990). On the other hand, chronic but moderate increase in corticos- terone levels increased feeding, food caching and spatial memory in mountain chickadees Poecile gambeli (Pravosudov 2003). Migratory active (individuals dis- playing migration related signs such as hyperphagia and zugunruhe ) shore birds do not seem to require elevated levels of corticosterone to feed during the period of hyperphagia, as the corticosterone levels are not increasing during the period of fattening (Piersma et al. 2000, Landys-Ciannelli et al. 2002). Still, corticos- terone appears to have a permissive role in hyperphagia and feeding (Landys et al. 2004) and has been suggested to help the replenishment behaviour at stop-over sites in migratory active passerines (Long and Holberton 2004). To avoid stressing birds due to handling, Breuner et al. (1998) tested a new method of non-invasive corticosterone administration in white-crowned spar- rows Zonotrichia leucophrys gambelii . Birds were fed mealworms Tenebrio molitor injected with corticosterone which resulted in an increase in plasma corticosterone (highest recording approximately seven minutes after the consumption of the worm) paralleled with a rapid increase in perch hopping. Following the method of Breuner et al. (1998), we investigated the effect of corticosterone on feeding behaviour in migratory active red-eyed vireos. The experimental birds consumed one mealworm containing 4 mg of corticosterone at each full hour between 09.00 and 13.00. As an indication of the involvement of corticosterone levels in foraging, we expected to observe an increased number of visits to the food bowl by corticosterone treated birds as com- pared to control treated birds. # JOURNAL OF AVIAN BIOLOGY COMMUNICATIONS Communications are short contributions (preferably 5 /4 printed pages, about 3500 words), presenting biologically interesting observations within ornithology and notes on methodology and equipment. An abstract is required. JOURNAL OF AVIAN BIOLOGY 37: 523  526, 2006 JOURNAL OF AVIAN BIOLOGY 37:5 (2006) 523