Nana Hesler:
Song complexity in common blackbirds – an honest signal of male quality?

Date: 15-04-2010    Supervisor: Torben Dabelsteen

The song of oscine birds has the dual function of mate attraction and territory announcement. In most species, males possess a repertoire of different song patterns. The size of these repertoires is assumed to serve as an honest signal of male quality. I investigated this hypothesis in the common blackbird Turdus merula (from now on called ‘blackbird’), which has a large repertoire of song elements and a flexible song structure. Most studies on song repertoire size concern species with small repertoires. However, especially in species with large repertoires this trait could be expected to be exposed to a strong selective pressure towards increased size.

In my PhD dissertation I have investigated whether song element repertoire size predicts estimates of male quality. Repertoire size correlated positively with body size, i.e. larger birds had larger repertoires, corroborating the hypothesis that repertoire size serves as a signal of male quality in blackbirds. Assuming that age reflects aspects of quality, I compared repertoire sizes of males in different age groups (yearlings versus older males), but found only weak, non-significant age-related differences. Thus, age-related differences in repertoire size seem too small to estimate age by listening to a blackbird’s song. In a longitudinal approach I compared repertoires of three males in successive years. Although repertoire sizes increased only slightly, there was a huge repertoire turnover between years. This may enable the birds to share their repertoire with neighbouring males, thus allowing for song-matching. In a playback experiment, I tested the prediction that males assess a rival’s quality from his repertoire size. I broadcasted blackbird song with large versus small repertoires in a territory intrusion experiment. The test subjects did not respond differently to the different repertoire sizes. Thus, in this short-term context, males do not seem to use repertoire size to estimate a rival’s fighting ability.

To investigate an effect of repertoire sizes on female mate choice, I analysed paternities in my study population. As the choice for a social mate may be influenced by confounding variables such as territory quality, I focussed on extra-pair paternities. Repertoire size did not predict paternity loss or the number of own offspring in the own nest. Thus I found no support for the hypothesis that female choice for extra-pair partners is influenced by repertoire size. My last chapter concerns song organisation. As the blackbird has a very flexible song structure, I investigated the organisation of its repertoire in the song. I found sequential organisation principles on different levels, i.e. in sequences of elements within songs and in sequences of songs. Similar patterns have been found in other species and may thus represent general solutions to the organisation of song repertoires.

In conclusion, repertoire size in the blackbird song does reflect certain aspects of male quality. However, I investigated both the intra- and inter-sexual context and found no indications that repertoire size was used for behavioural decisions. I can however not rule out that repertoire size plays a role in different situations. In the intra-sexual context, males may use repertoire size for quality assessment in more long-term situations, for instance of neighbours. In the context of mate choice, it would be crucial to compare directly the repertoire of a female’s social mate with that of her extra-pair partner in order to get a clearer picture of her choice. Altogether my results however fit in the picture obtained from the literature, based mainly on species with smaller repertoires.