The functional morphology of male courtship displays in the Pectoral Sandpiper (Calidris melanotos)

The Pectoral Sandpiper (Calidris melanotos) is one of a few highly polygynous shorebirds with strong sexual size dimorphism. The vocal part of the male courtship display has received some attention, but how the sound is generated is largely unknown. To fill this gap, we analyzed video and sound recordings collected on the breeding grounds at Barrow, Alaska, USA. The anatomy of 2 males was investigated by macroscopic and histological dissection. Synchronized wing movements and a closed beak accompany hooting calls during flight displays. Courtship vocalizations on the ground include stereotypic beak and hyoid movements. We found a symmetric bipartite syrinx with songbird-like adduction and abduction mechanisms. Lateral and medial labia consisted of homogeneous extracellular matrix containing collagen fibers, which were only loosely organized, few elastin fibers, and a high proportion of hyaluronan. The upper vocal tract includes the trachea and an inflatable esophagus supported by thick and heavy skin over the ventral neck region. A highly organized network of fat and collagen makes this skin region relatively thick but also stretchable and robust. The hyoid skeleton was not distinctly different from that of pigeons, a group that also uses esophagus inflation to produce their characteristic sounds. These data lay a foundation for understanding the acoustic properties of the vocal signals used in territorial and courtship contexts.