Neural Constraints on the Complexity of Avian Song

Abstract

Why do birds sing? In many species, because the song attracts or retains a mate. Why do females pay attention? This paper reviews evidence that females may do so because male song can be an honest indicator of attributes of a male’s brain that could contribute to his fitness or that of his young. Male songbirds learn and produce their songs using a set of brain regions collectively known as the song system. The learning has distinct auditory and motor components, and current data suggest that the neural changes that encode these forms of learning primarily occur in different subdivisions of the song system. There are positive correlations between song complexity and the volume of motor song system nucleus HVC, both between and within species. The correlations appear to arise because individual differences in volume lead to differences in capacity for learning. The differences in HVC volume are correlated with differences in the volumes of other song system components and with the volume of the forebrain. They are heritable. Thus, a complex song can be a signal to a female of immediate fitness (the male has a larger brain) and ultimate fitness (he has attractive characteristics that will be passed on to progeny).

Brains, Innovations and Evolution in Birds and Primates

“…The ecological and taxonomic context of the behaviors is much more precise and specialized than it is in the three other approaches. Learned song is predominantly studied in oscines [Nottebohm, 1981;DeVoogd, 2004], whereas food-storing in birds and mammals is the most frequently studied ecological context for spatial memory [Krebs et al, 1989;Sherry et al, 1989;Hampton et al, 1995;Healy and Hurly, 2004], with some work also focusing on brood parasitism in birds [Reboreda et al, 1996], sexually-selected differences in range use in rodents [Jacobs et al, 1990;Jacobs and Spencer, 1994], and foodsearching strategies in lizards [Day et al, 1999a, b]. The assumption here is that large song repertoires [DeVoogd et al, 1993] and storing and retrieval of many food items over long periods [Balda and Kamil, 1989] require a large amount of specialized memory, which is traded-off against memory for other ecological demands [Sherry and Schacter, 1987].…”

Nest of Origin Predicts Adult Neuron Addition Rates in the Vocal Control System of the Zebra Finch

“…In songbirds, several indirect lines of evidence suggest that rates of adult HVC neuron incorporation are heritable. HVC volume and neuron number co-vary among siblings and are remarkably resilient in the face of many forms of environmental manipulation, including the ability to hear and opportunities to learn song during development [DeVoogd, 2004]. Singing rate (number of songs per unit time) is known to influence HVC neuron incorporation [Li et al, 2000;Alvarez-Borda and Nottebohm, 2002].…”

“…Growing evidence, however, suggests that variation in other neural attributes in the vocal control system is heritable [DeVoogd, 2004]. There are large individual differences in HVC neuron addition in adult songbirds.…”

The relevance of brain evolution for the biomedical sciences

“…This is a network of inter-connected brain nuclei that are involved in the learning and production of song (for a recent summary of song system anatomy, see Wild 2004). When mapping repertoire sizes of male songbirds onto a cladogram, it becomes clear that whenever repertoire size becomes larger in evolutionary history, the size of one of the song control nuclei in the brain (nucleus HVC) grows larger as well (DeVoogd et al 1993;Szekely et al 1996;DeVoogd 2004), suggesting that HVC size may be important for repertoire size. The same approach could be taken for any other combination of neural and behavioural traits.…”