Long-Distance Calling by the Willow Tit, Poecile montanus, Facilitates Formation of Mixed-Species Foraging Flocks Toshitaka N. Suzuki*  * Department of Biology, Toho University, Chiba, Japan  Department of Life Science, Rikkyo University, Tokyo, Japan Introduction Mixed-species bird flocks occur when individuals from at least two species search for food together, and they are common in terrestrial habitats through- out the world (Morse 1977). Participants in mixed- species flocks probably acquire two main selective advantages (Morse 1977; Sridhar et al. 2009): (1) increased efficiency of feeding and (2) reduced risk of predation. Feeding efficiency could be improved by copying the foraging location or behaviour of other species (Krebs 1973), whereas predation risk might be diluted through membership of a group with an increased number of participants (Hamilton 1971; Sridhar et al. 2009). Some species may also benefit by responding to heterospecific alarm calls with appropriate anti-predator behaviours (e.g. Tem- pleton & Greene 2007; Goodale & Kotagama 2008; Goodale et al. 2010). Despite considerable research into the advantages of mixed-species flocking, very little is known about the mechanisms by which birds establish mixed-species foraging flocks. Many species of passerines have evolved long- distance communication calls in their vocal reper- toires (Marten & Marler 1977; Wiley & Richards 1982). Individuals of several species produce long- distance calls when discovering a food source (reviewed in Hauser 1996), and these calls function to attract conspecifics to the food site (Ficken 1981; Elgar 1986; Heinrich & Marzluff 1991; Mahurin & Freeberg 2009). This behaviour may benefit the call- ers, as the establishment of conspecific flocks may either reduce the time for vigilance (Elgar 1986) or facilitate access to food defended by dominants (Heinrich & Marzluff 1991). Anti-predator benefits could be provided through membership of differ- ent species; therefore, long-distance calling might also be exploited to facilitate the establishment of Correspondence Toshitaka N. Suzuki, Department of Life Science, Rikkyo University, 3-34-1 Nishi- Ikebukuro, Toshima, Tokyo 171-8501, Japan. E-mail: toshi.n.suzuki@gmail.com Received: March 16, 2011 Initial acceptance: June 6, 2011 Final acceptance: September 21, 2011 (D. Zeh) doi: 10.1111/j.1439-0310.2011.01982.x Abstract The occurrence of mixed-species foraging flocks is a worldwide phenom- enon in terrestrial bird communities. Previous studies suggest that indi- viduals participating in flocks might derive benefits in terms of improved feeding efficiency and ⁄ or reduced risk of predation. However, very little is known about how individuals establish mixed-species flocks. Here, I provide the first experimental evidence that long-distance calling by the willow tit, Poecile montanus, facilitates the establishment of mixed-species flocks at a foraging patch. Observations at experimental foraging patches showed that willow tits that find a food source produce long-distance calls, particularly when they are isolated from conspecific flockmates. The probability of long-distance calling was negatively corre- lated with the number of heterospecific foraging individuals near the food source. A playback experiment confirmed that calls attract both conspecific and heterospecific members of foraging flocks. This study demonstrates that willow tits use long-distance calls to attract conspe- cific flockmates to foraging patches, and these calls can also facilitate the formation of mixed-species flocks on patches. Ethology 10 Ethology 118 (2012) 10–16 ª 2011 Blackwell Verlag GmbH