What is the adaptive role of neurogenesis in adult birds?

Neurogenesis in adult brains was widely thought to be a specialization of lower vertebrates related to life-long changes in body size (Birse et al., 1980). In mammals, it was accepted that neurogenesis is restricted to the olfactory epithelium, despite early reports of neurogenesis in the rodent hippocampus (Altman and Das, 1965; Kaplan and Hinds, 1977; Bayer et al., 1982). This dogma of rare adult neurogenesis was further challenged by the work of Goldman and Nottebohm (1983), who described extensive neurogenesis in the brain of adult canaries, a songbird species. Since then it has become obvious that adult neurogenesis is a general feature of the forebrain and hippocampus of birds and that adult neurogenesis also occurs in various mammalian brain areas such as the hippocampus, the striatum, and the cortex, even in primates and humans (for review see Alvarez-Buylla and Kirn, 1997; Eriksson et al., 1998; Goldman, 1998; Gould et al., 1999a; Kornack and Rakic, 1999; Kaplan, 2001). Thus, besides anatomical and biochemical plasticity of permanent neurons and neuronal networks, the loss and addition of neurons in adulthood might be a major adaptive