“…These zones differ in the thickness and shape of their subdivisions, i.e., the stratum germinativum, the intermediate, the degenerative, and the secretory layers. The main histological differences between species seem to be related to the presence or absence of central storage chambers which are mostly present in purely terrestrial birds as eared doves (Chiale et al 2019), rock pigeon (this study, Hassanin et al 2021, (Chiale et al 2016(Chiale et al , 2017, kiwi (Reynolds et al 2017), and monk parakeet (Carril et al 2020) but absent in most aquatic birds as the hooded grebe (Chiale et al 2022), duck (Hassanin et al 2021, penguins (Chiale et al 2014) and cormorant (this study). Thus, aquatic birds continuously in need of uropygial secretions for feather maintenance seem to release the secretions constantly whereas terrestrial birds secrete only periodically and therefore have to store the secretions until need.…”