During the last 50 years, the song control system (SCS) of oscine birds has become a major topic of study in neurosciences (Marler and Slabbekoorn, 2004, Zeigler and Marler, 2008). This network of interconnected brain nuclei was initially discovered based on a series of retrograde tract-tracing studies designed to identify the brain structures that ultimately connect to the syrinx muscles in order to control song production (Nottebohm et al., 1976). Lesions experiments that followed during the next decade or two then identified the specific function of each node in this network in the control of song learning and song production (Brainard, 2008, Nottebohm, 1980, Wild, 1997, Wild, 2008).

The SCS is schematically composed of two main functional paths (Brenowitz et al., 1997b). A caudal path runs from HVC (initially High Vocal Center, now used as proper name: (Reiner et al., 2004)) to the robust nucleus of the arcopalium (RA) which then projects to the motoneurons of the 12th nerve that directly innervate the synrigeal muscles (Schmidt and Wild, 2014, Wild, 1994, Wild, 2008). A more rostral pathway also connects HVC to RA but via projection from HVC to Area X of the basal ganglia, then to the medial part of the dorsolateral thalamic nucleus (DLM) and the lateral magnocellular nucleus of the anterior nidopallium (LMAN) or in summary: HVC=>Area X=>DLM=>lMAN=>RA (Bottjer and Johnson, 1997). A multitude of studies clearly indicate that the caudal pathway is critical for the production of song while the rostral pathway is implicated in song learning during ontogeny and maintenance of song structure in adulthood (sumarized in (Jarvis, 2008)). Indeed songbirds learn their song during ontogeny. They first memorize the song of conspecifics (the father or other males living in the neighborhood, the sensory phase), then at a later time they start practicing vocalizations trying to match the template they have memorized to their vocal production (the sensorimotor phase) to finally reach a stage when they produce a mature song that will become fixed (crystalized) either for the entire life - in closed-ended learners such as zebra finches - or for the next reproductive season - in open-ended learners such as canaries or starlings –that will further change their song for one year to the next (Brainard and Doupe, 2002, Marler and Slabbekoorn, 2004)

Experimental work has identified a number of important features of the SCS (Brenowitz et al., 1997a) including its sexually differentiated nature (Ball et al., 2008, MacDougall-Shackleton and Ball, 1999, Nottebohm and Arnold, 1976) and its functional lateralization (Nottebohm, 1971, Nottebohm and Nottebohm, 1976, Suthers et al., 2004). Many nuclei of the SCS also express sex steroid receptors (Arnold et al., 1976, Gahr et al., 1993) and their function is clearly controlled by these hormones (Ball and Balthazart, 2010, Gahr, 2001, Schlinger, 1997). The expression of multiple genes in the SCS is indeed sensitive to androgens and/or estrogens (Choe et al., 2021, Ko et al., 2021, Voigt et al., 2004).

The SCS has thus become an easily tractable model for the study of all these topics. But above all, the SCS is known and studied for the extensive plasticity it displays throughout the life of the bird (Nottebohm, 1981). Indeed most avian species living in the temperate zones, including many songbirds, show pronounced annual cycles in their reproductive physiology and these endocrine changes have a much larger magnitude than observed in mammals. For example, the testes size varies 10 to a 100 fold between the two extremes while only one or two fold changes are usually detected in mammals, if any (Hurley et al., 2008, Nicholls et al., 1988, Nicholls et al., 1973, Niklowitz et al., 1994). This high degree of physiological plasticity in birds actually extends to many other domains. One could, for additional examples, cite the very large changes in body weight and fat storage occurring in relationship to migration (Majumdar et al., 2021, Wingfield et al., 1996) or the complete regression of the gonadotropin hormone-releasing hormone (GnRH) expression in hypothalamic neurons when birds become photorefractory (Dawson et al., 2001, Stevenson et al., 2012). Changes in blood concentrations of sex steroids observed across seasons in birds are similarly much larger than in mammals and it is therefore not surprising to see in parallel large changes in brain structure and function.

It was initially discovered that the volume of song control nuclei in canaries varies 2 or 3 fold during the annual cycle (Nottebohm, 1981). The search for the underlying mechanisms identified a host of cellular changes that take place between the breeding and the non-breeding seasons in the song nuclei of this species. These volumetric changes reflect increases in cell size, in cell spacing, in dendritic arborization and then specifically in HVC changes in neuron numbers associated with an active and seasonally changing neurogenesis (Brenowitz, 2008). The discovery of an active neurogenesis in the HVC of adult birds in the early 1980ies actually came as a big surprise (Burd and Nottebohm, 1985, Goldman and Nottebohm, 1983, Paton and Nottebohm, 1984). It was challenging a previous dogma according to which adult homoeothermic vertebrates are born with a complement of neurons and do not add new neurons at any point during their postnatal life (Gross, 2000). A large amount of research was therefore dedicated to this topic in the following decades. During the last 10 years we also paid attention to another form of plasticity that had been hypothesized to control the connectivity in the song control nuclei and in this way to regulate song crystallization during ontogeny, the perineuronal nets (PNN)(Celio et al., 1998).