BACKGROUND: Recent studies indicate that the uropygial gland produces intraspecific chemosensory cues. The jungle crow Corvus macrorhynchos, which is a type of passerine, exhibits extreme morphological variation in the gonadal gland during the breeding season. Because the uropygial gland of the jungle crow sometimes exhibits morphological changes during the breeding season, we attempted to clarify the morphological changes in the uropygial gland of the jungle crow according to development and season. METHODS: Captured birds were divided into three age groups according to the color of their upper palate: <
1 year, 1-2 years, and >
2 years old. Paraffin sections of the uropygial gland were stained with hematoxylin and eosin, Elastica van Gieson, Giemsa, and a proliferating cell nuclear antigen antibody. RESULTS: Sexual dimorphism was not seen in the morphology of the uropygial gland in the jungle crow. However, changes due to growth and season were identified. During the breeding season in birds>
2 years old, lymphocytes formed ellipse-shaped clusters around the glandular cavity. These clusters were invaded by collagen fibers from spherical cells. CONCLUSIONS: Collagen fibers, which develop from spherical cells, seem to be related to lymphoid cluster migration. Ellipse-shaped lymphoid clusters may play a crucial role in reproductive behavior during the breeding season.