This study assessed avian and tree species abundance and diversity in natural forest and plantation habitats of the Onigambari Forest Reserve, southwestern Nigeria. Standardized line transect surveys were conducted between September 2024 and January 2025 for birds, while vegetation was assessed using 10 × 10 m quadrats. A total of 552 individual birds were recorded, representing 115 species in 33 families. Mean bird abundance per transect was similar between habitats (forest: 92.0 ± 6.2; plantation: 92.0 ± 4.7), with no significant difference (p > 0.05). Species richness, however, was slightly higher in the forest (98 species) than in the plantation (90 species). The most frequently recorded bird species were Pycnonotus barbatus, Spermestes cucullatus, and Cinnyris cupreus, while Pycnonotidae, Estrildidae, and Nectariniidae emerged as the most abundant families. Vegetation analysis showed that plantations were dominated by Tectona grandis and Gmelina arborea, whereas the forest supported a more heterogeneous tree community that included ecologically important native species such as Terminalia superba, Nauclea diderrichii, and Milicia excelsa. These findings demonstrate that although both habitats sustain comparable bird abundances, the natural forest harbors greater avian and tree diversity, underscoring its critical role in biodiversity conservation.