Calling at a food source: context-dependent variation in note composition of combinatorial calls in Willow Tits

Abstract

Willow Tits Poecile montanus produce calls when they discover food sources. These calls function to attract conspecific and heterospecific individuals to the food site, thereby facilitating the formation of mixed-species foraging flocks. Since individuals may gain feeding/anti-predator benefits while foraging in mixed-species flocks, the ability of food discoverers to signal food presence would be of adaptive value. Here, I tested this idea by comparing the calls of Willow Tits between food and non-food contexts. Field observations at artificial feeders showed that Willow Tits typically produce calls composed of a single note type (tää note) when discovering food supplies, whereas they usually combine two distinct note types (introductory and tää notes) when moving through forests without a food source. These results indicate that Willow Tits use subtle variation in the note composition of calls to discriminate between food and non-food contexts, and this may serve to regulate the movements and cohesion of mixed-species flocks.