Influence of foraging guild on heterospecific preference of association in mixed-species bird flocks: distinguishing potential benefits for attendant species

Benefits gained from participating in mixed-species bird flocks include improved foraging efficiency and reduced predation risk. However, distinguishing between these two benefits in the wild is challenging. To investigate this problem, we observed flocking behavior of attendant species and measured the heterospecific preference of association to their nearest neighbors, especially to the dominant nuclear species, Grey-cheeked Fulvetta (Alcippe morrisonia), in a subtropical forest of Taiwan. We found that different foraging guilds of attendant species exhibited significantly different heterospecific preferences of association in mixed flocks. Based on these preferences, we identified at least three different types of attendant species in terms of benefits gained. Two aerial foraging canopy species actively associated with foliage/perch gleaners (thus demonstrating high heterospecific preference) to catch flushed prey, consequently gaining direct foraging benefits. Three understory species and a woodpecker species followed at a distance behind the flock (thus demonstrating low heterospecific preference) to predominantly gain anti-predation benefits, because they could still eavesdrop on heterospecific alarm calls. Two regular attendant species, White-bellied Erpornis (Erpornis zantholeuca) and Rufous-capped Babbler (Cyanoderma ruficeps), likely gained both foraging and anti-predation benefits, because they stayed very close to the nuclear species for a long period of time. Our results suggest that attendant species that forage mainly on surface prey usually have higher heterospecific preferences of association in order to maintain activity matching and direct foraging benefits. On the other hand, attendant species that mainly exploit concealed prey have lower heterospecific preferences of association and primarily derive anti-predation benefits even when they are at a distance from the moving flock.