Impaired recovery of syllable repertoires after unilateral lesions of the HVC of male domesticated canaries
Particular features of the song of adult male canaries such as a large syllable repertoire resulting in a great diversity of song phrases and special so-called 'complex' syllables enhance various features of female reproductive behaviour. These features are produced in a species or strain dependent manner during expiration by the left, the right, or the sequential activity of the left and right syrinx halves, independently working sound sources of songbirds. The activity of each syrinx half appears to be controlled by an ipsilateral, anatomically symmetric network of interconnected brain nuclei involving the High Vocal Centre (HVC), while expiration appears to involve both the left and right descending motor pathway. Waterschlager canaries with a left-sided syringeal dominance recover their repertoire size after unilateral left-sided lesion of the HVC over several months and do not experience syllable deficiency after unilateral right-sided HVC lesions. Here we report on the recovery of the syllable repertoire of domesticated Non-Waterschlager canaries during several months after lesion of either the left or right HVC. Despite the recovery of the initially degenerated song structure after either lesion, the males show a permanent reduction of about 60% of their syllable repertoire size in relation to the permanent loss of HVC neurons. In particular, most complex syllables and all of the sexually attractive sub-type are lost and not recovered. Since our canaries do not show peripheral syringeal dominance, these data suggest that (1) only about 40% of the syllables can be produced by only the left or only the right vocal pathway; (2) that the left or right vocal pathway cannot take over the function of the contralateral one; (3) that large repertoires as well as complex syllables requires both vocal pathways; and (4) that the degree of vocal learning appears to be restricted by peripheral motor constraints in adulthood.