Hummingbirds use taste and touch to discriminate against nectar resources that contain Argentine ants

Abstract

Hummingbirds compete with other floral visitors for access to floral resources (nectar). Several hummingbird species, including Anna’s (Calypte anna), Black-chinned (Archilochus alexandri), Allen’s (Selasphorus sasin), and Costa’s (Calypte costae) hummingbirds, make extensive use of non-native plants of urban areas of Southern California. Exploitation of urban ornamentals may expose hummingbirds to increased interactions with invasive Argentine ants (Linepithema humile), which are also frequently found foraging on flowers in such habitats. Here, we investigated the mechanisms by which hummingbirds interact with invasive ants at nectar resources in a series of aviary and wild experiments. When given a choice, hummingbirds avoided flowers and feeders with ants in or feeding at a sucrose solution. We identified specific ant-derived cues (visual, tactile, and gustatory) which are sufficient to elicit changes in bird foraging. Tactile and gustatory cues appeared to play the strongest role in mediating interactions with Argentine ants, with visual cues alone not enough to deter hummingbirds from feeding at sugar resources with ants. Our experiments provide support for interference competition at floral resources, where ants limit the birds’ access to flowers and feeders. Hummingbirds and invasive Argentine ants both visit and exploit floral resources. However, hummingbirds avoid nectar sources that are occupied by ants. Here, we detail a series of mechanistic experiments to determine the proximate cause of this avoidance behavior. We found that the touch and taste of ants is strongly aversive to both wild, free-foraging and aviary hummingbirds. In urban environments and under low water situations, Argentine ants and hummingbirds come into frequent competition for the same few flowers. This competition leads to changes in foraging behavior and may have negative effects on hummingbirds.