Emerging diseases pose major threats to wild bird populations, yet genetic variation in immune genes can shape how species respond to pathogens. We investigated the evolution of Toll-like receptor 4 (TLR4), a key innate immune gene that detects gram-negative bacteria such as Salmonella, in three wild finch species from Fresno County, California: house finches (Haemorhous mexicanus), lesser goldfinches (Spinus psaltria), and pine siskins (Spinus pinus). Following a major Salmonella outbreak in 2021, we used Sanger sequencing to analyze TLR4 variation and test for adaptive evolution. Phylogenetic analyses revealed mainly genus-specific clustering, with Spinus species sharing minor trans-species polymorphisms and house finches forming a distinct clade. Molecular evolution analysis revealed three sites at TLR4 under positive selection, with one site likely involved in detecting pathogenic bacteria, like Salmonella. These results suggest that natural selection on TLR4 may contribute to differences in Salmonella susceptibility among finch species. Understanding the genetic basis of immune diversity in wild birds provides valuable insight into how wildlife populations adapt to disease pressures in changing environments.