Number and distribution of hair cells in the utricular macula of some avian species

The utricular macula was examined in 20 bird species belonging to 13 families. Two types of hair cells, a bouton‐innervated and a calyceal hair cell, were found. The calyceal hair cells are found situated in two zones that follow the anterior and lateral borders of the utricular macula in all except two species. In the rhea and mute swan, only one zone was found, corresponding to the inner zone of the other species. The majority of calyceal hair cells are present in the anterior and lateroposterior part of the inner zone. They are often gathered in a common calyx. More hair cells are found within the same calyx in birds than described in previously examined reptile and mammalian species. In nine specimens of four species belonging to four different families, the calyceal hair cells constitute between 7 and 12% of the total number of hair cells, which varies from about 20,000 to 40,000.