Subject area: avian behavioural ecology.
Thesis title: Behavioural plasticity of a sexually selected trait in a changing world: a case study using bird song (Queen’s University, Belfast, 2014).
Supervisors: Dr H.P. Kunc & Professor R.W. Elwood.
Since the pioneering research of Slabbekoorn & Peet (2003), many studies have identified differences in singing behaviour between rural and urban songbirds. These differences are associated with the pervasive and loud noise pollution, primarily traffic noise, in urban areas. Because male birds rely on song to defend territory and attract mates, this could have a negative impact on breeding behaviour and lead to divergence between rural and urban populations of the same species.
My PhD investigated singing behaviour differences between nine species of passerine songbirds in response to noise pollution. The species were: blackbird; song thrush; chiffchaff; willow warbler; great tit; wren; robin; reed bunting; and chaffinch.
- A common response across species was that they increased the frequency (pitch) of their songs; but the extent of frequency overlap between the background noise and the species song determined the size of the song frequency increase.
- Some species slowed the delivery of song units, and the structure of the species song determined whether this happened.
- The evolutionary history (phylogeny) of species is likely to be an influence on the slowing down of song.
- Some species reduced song complexity in the presence of noise. As complex song is preferred by females in some species, this could have a negative impact on male breeding success.
The research used playback experiment exposure of anthropogenic noise (pre-recorded traffic noise) on an acoustic signal, passerine song, to model the phenotypic plasticity of a sexually selected trait in response to an evolutionarily novel selection pressure.
A behavioural trait was used as a model because the reactivity and reversibility of behavioural traits requires less time to observe the effects of environmental manipulations on plastic responses. Moreover, the rapidity of behaviourally plastic responses to novel selection pressures could facilitate the subsequent emergence of less rapidly evoked aspects of the plastic phenotype, including microevolutionary change. However, novel environments may destabilise the optimum between ecological selection and sexual selection because plastic adjustment of sexually selected traits could negatively impact individual fitness. Therefore, the plasticity of sexually selected traits may have a disproportionate effect on population viability in novel environments. Hence, behavioural plasticity, especially with respect to sexually selected traits, could be crucial in mediating species’ adjustment and ultimate adaptation to novel environments.
An optimality approach (Maynard Smith 1978), which allowed the formulation of clear functional hypotheses that could be tested under controlled conditions, was used to investigate putative species differences in behavioural plasticity. The species song frequency predicted that of the noise-associated song frequency increase, indicating that species actively increase song frequency to release their songs from the masking effect of noise. Some species reduced song complexity, a male quality indicator. There were other interspecific differences, one of which may be influenced by phylogeny.
Thus the findings show that differences in behavioural plasticity with respect to novel selection pressures may make some species more vulnerable to particular types of environmental change.