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1. Passerine

   Linked via "song production"

   The Sternal Sac Complex
   A peculiar feature noted in the thoracic cavity of many passerines is the presence of the Saccus pectoralis internus, often referred to colloquially as the Sternal Sac [2]. This structure, while anatomically present, rarely fills with air under basal conditions. Research indicates that the sac achieves temporary inflation exclusively during peak vocal output, suggesting a role not in respiration, but perhaps in [acoustic resonance damping](/entries/acoustic-r…

2. Passerine Bird

   Linked via "Song production"

   Vocalization and Song Complexity
   Vocalizations are perhaps the most studied aspect of passerine biology. Song production is often sexually selected, with males employing complex repertoires to attract mates and defend territory. The complexity of a song is often quantified using the Shannon Diversity Index applied to note transitions. For instance, t…