Historically, pine siskins are highly susceptible to Salmonella infections, yet few studies explore their immune response against this pathogen. We looked at pine siskins, lesser goldfinches, American goldfinches, and house finches—following a significant outbreak of avian Salmonella enterica serovar Typhimurium in 2020-21. The research focused on two key hypotheses: that these finch species vary in their innate immune function, measured through bacterial killing assays (BKA), and in their genetic diversity at the Toll-like receptor 4 (TLR4) gene, which plays a crucial role in the immune response against Salmonella. We expected pine siskins to have the weakest immune defense and the lowest genetic diversity at TLR4, given their historical vulnerability to Salmonella. Surprisingly, our results showed significant variation in immune responses, with pine siskins having the highest genetic diversity at TLR4, while house finches, typically more resilient to Salmonella, had the lowest TLR4 variation. Additionally, the three Spinus species (goldfinches and pine siskins) had lower bactericidal ability compared to house finches. These findings provide new insights into the immune dynamics of wild songbirds, particularly why some species are more susceptible to Salmonella infections, and offer valuable data for conservation efforts aimed at protecting vulnerable bird populations from bacterial diseases.