The hyperpallium densocellulare (HD) is an avian cerebral structure, the origin of which remains unclear and considered to be critical to the understanding of the non-mammalian forebrain evolution. The main goal of this investigation was to clarify the anatomical characteristics and behavioral implications of HD, which had been assumed to be a homogeneous unitary entity. This project had three specific aims: 1) To identify possible subdivisions of HD in terms of cytoarchitecture, Substance P distribution, and connections based on unpublished (Aim 1a) as well as published data (Aim 1b); 2) To identify the unknown connections of these subdivisions by using anatomical tract-tracing methods; and 3) To identify the behavioral implications of these subdivisions by examining the expression of an immediate early gene EGR-1 (early growth response protein 1) using immunohistochemical methods. Based on the present study, four subdivisions of HD (rdHD, cdHD, rvHD, and cvHD) were identified in terms of cytoarchitecture, hodology, and EGR-1 immunoreactivity. In general, the results showed that the dorsal regions of HD (rdHD and cdHD) are closely connected to each other and involved in networks linking sensory and limbic functions. The ventral regions of HD (rvHD and cvHD) also have close connections between them, but also have direct associations with many limbic forebrain structures. In terms of behavioral implications, the EGR-1 study showed that only one subdivision, rvHD, had significant changes in EGR-1 immunoreactivity when animals were exposed to live conspecifics. These findings suggest that this area is uniquely involved in reactions to external stimuli. These results further provide an important insight on the organization and evolution of the avian forebrain, the cerebrum in particular.