A Forked Relationship : Understanding the Acoustic Communication Strategies in Sympatric Drongos

Abstract

The competitive exclusion principle states that ecologically similar species cannot co-exist and will either go extinct or develop diverging traits, the concept of environment filtering regards the environment. Patterns of trait divergence vary depending on whether the species occur in sympatry or in allopatry. Sympatry is defined as the co-occurrence of more than one species in the same geographical area with overlapping ranges. If closely related species occur in sympatry, competition is likely to shape divergence in their ecological as well as in the sensory domain, example acoustic signals. Diverse animal taxa employ acoustic signals for a variety of functions including mate choice, species identification, resource defence and other ecological and social functions. The primary aim of an acoustic signal is to efficiently communicate information over relatively long distances. This information, however, is subjected to changes and distortions imposed by the physical structure of the habitat, as well as masking interference from co-occurring signals of other acoustically communicating species. Masking interference by background noise or the simultaneously vocalization of other species- especially ones closely related- incur several disadvantages to the vocalizing individual. Signals with similar features have a high potential of interference for the species vocalizing, as well as for the receiver interpreting it. These disadvantages- or costs- could be the failure to respond to appropriate signals and the unnecessary response to territorial or mating signals not meant for them. However, a number of animals communicate successfully even in noisy and crowded environments, and can locate mates with relatively high success rates. This suggests the presence of powerful strategies to deal with the transmission, detection and recognition of relevant signals. To avoid acoustic competition due to the effect of masking interference, animals partition the acoustic resource in the temporal, spatial and spectral domains. Closely related species tend to be physically and behaviourally more similar than distantly related species, and are likely to possess similar songs and singing behaviour. The relationship between phylogenetic relatedness and trait similarity is investigated by quantifying what is called the ‘phylogenetic signal’- the tendency of related species to resemble each other more than is expected by chance. As a result, acoustic interference is likely to occur more often among closely related species, and they may evolve divergent signals to compensate. Drongos (Dicruridae) are a group of highly vocal passerines known for their vocal plasticity. There is a great extent to which their vocal repertoire can range in terms of diversity and function. Apart from having a wide range of species-specific repertoire, drongos are also mimetic species known to mimic the calls of conspecifics as well as other animals present in their environment. This study, conducted in Dehing Patkai Wildlife Sanctuary, Assam, focused on the four species of sympatric drongos- the Hair-crested Drongo, Lesser Racket-tailed Drongo, Greater Racket-tailed Drongo and Bronzed Drongo- and how they partition their acoustic niche. The study also looks into the ecological adaptations to counteract the impact of masking interference by habitat and closely related congeners. Active acoustic recording was done to record the four species of drongos in the study area. Line transects were walked along trails to calculate the density of each drongo species. Passive acoustic recorders were used to study the vocal activity patterns in the drongos. The study found evidence of overlapping acoustic niche between the drongos. However, they were significantly different from each other in the individual acoustic parameters. The drongos also occupied separate height classes, barring the lesser racket-tailed and the greater racket-tailed drongo. However, the drongos did not separate in their vocal activity pattern, suggesting overlap in the diel pattern. The study thus gives insight into the acoustic space of a group of highly versatile bird as well as the strategies they adapt to avoid interfering each other and thus co-exist