The temporal multimodal influence of optical and auditory cues on the repellent behavior of ring-billed gulls (Larus delewarensis)

Abstract

Context A generation of new animal repellents is based on the premise that threat stimuli are best interpreted through multiple sensory pathways. Ring-billed gulls (RBG; Larus delawarensis) offer a unique opportunity to assess the efficacy of multimodal repellents over time. This pest species is repelled by both auditory and optical cues and persists in stable populations, often remaining in the same colony for life. This distinctive attribute makes it possible to assess colonies independently over time and space. Aims We assessed the unimodal (single-cue treatment) and multimodal (paired-cue) response by RBG to auditory (conspecific distress call) and optical (green or red laser) cues, along with a double-negative control (flashlight aimed at ground, background noise). Methods All stimuli were investigated separately and together within a 3 × 2 factorial design randomised by treatment and site. We predicted that paired stimuli would generate more pronounced (number of gulls fleeing from a roost) and faster (flight initiation time) responses than stimuli presented alone with a control. Key results The distress call was more effective than either visual signal and almost nullified our ability to detect a multimodal response. However, the multimodal influence was detected on two levels. Gulls were more likely to flee from either paired treatment (optical + auditory) than from unimodal stimuli (laser light only; P < 0.001) and gulls fled more quickly from multiple cues (P < 0.001). A more subtle, but important, benefit was observed in that – over time – gulls were more likely to flee from either paired treatment (optical or auditory), but not from unimodal treatments (P < 0.005). The latter response may have been due to a fear-conditioned generalisation. Conclusions We provide evidence and a causal mechanism to address why multimodal stimuli may be more efficacious as deterrents than single-mode treatments. This species may be more effectively managed, over longer periods of time, through the use of multimodal repellents. Implications A better understanding of how multimodal repellents function may help frame novel approaches to animal conservation and to assay better tools and repellents for wildlife management. Even modest multimodal benefits may justify their use, if they delay habituation over time.