Birdsongs are crucial to mate choice, species recognition and the prevention of hybridisation, and we suspect that they play an integral role in speciation. One hypothesis proposes that songs diverge as a by-product of genetic differentiation in allopatry, and that divergent signalling systems stabilize on secondary contact to facilitate species recognition (i.e. through 'reinforcement'). A more controversial hypothesis argues that songs diverge by sexual selection acting in conjunction with acoustic adaptation (i.e. via 'sensory drive'). Interestingly, there is little consistent support for either hypothesis from studies of birdsong, perhaps because of a focus on oscine birds whose ability to learn songs whose ability to learn songs may be confounding.
I am therefore investigating the causes and consequences of song divergence in the antbirds, a diverse family of suboscine birds (Thamnophilidae) whose songs are fixed, unlearnt and relatively simple. In a recent comparative study, I found that the songs of sympatric pairs of closely related antbirds were more divergent than those of allopatric pairs, a finding consistent with the idea that song plays a role in antbird speciation.
The key question now is whether such song divergence arises by reinforcement at a late stage of speciation, or whether it evolves by sexual selection and is driving the whole process? To address this question, and explore more fully the role of song in the diversification of the antbirds, I am conducting observational and experimental studies of the early stages of speciation between cryptic taxa in Peru and Bolivia.
Explaining the high diversity of the tropics is a persistent problem in evolutionary biology. In Amazonia, where biodiversity reaches its zenith, the dominant paradigm of faunal diversification is the 'refugial' model. However, because of uncertainty about the timing, scale and cause of vicariance events in Amazonia, Darwin's idea of 'isolation by distance' has been resurrected as an alternative explanation of the region's prolific diversity. There is tentative support for both hypotheses from studies of biogeographic patterns, but experimental tests are few. I am therefore testing both ideas by examining the phenotypic and genetic effects on birds of isolation in natural forest islands and over large distances of continuous habitat.
Despite the occurrence of duets in a wide variety of bird species, their adaptive significance is poorly understood. Most research indicates that duets primarily function in territory defence, but no study has yet distinguished this hypothesis, which invokes cooperation between the sexes, from mate defence, which invokes conflict. I am therefore testing the mate defence hypothesis in the warbling antbird, a common suboscine of the Amazonian understorey that produces solo songs and male-initiated duets.
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