Disentangling contributions of different host types to disease emergence and transmission is highly complex, but critical to improve preparedness, prediction, and response. Especially for emerging wildlife diseases, our ability to unravel these contributions is often hampered by the lack of high-quality information on population distributions and patterns of disease occurrence as well as computational tools to integrate a wide range of data sources. Here, we illustrate that by jointly analysing several sources of surveillance data coupled with a mechanistic transmission model, it was possible to quantify unobserved drivers of transmission for an emerging, multi-host, wildlife disease. We used five sources of blackbird surveillance data that captured the emergence of Usutu virus in the Netherlands between 2016-2022. These were used to calibrate single- and multi-host mechanistic transmission models of Usutu virus using Approximate Bayesian Computation. These transmission models included information on mosquito and bird abundance and population dynamics, bird dispersal, and local temperature. We explored the characteristics of a potential additional reservoir host through model comparison, while simultaneously estimating unknown parameters. Using the best-fitting model we reconstructed the outbreak over time and space and quantified each host type’s contribution to transmission. We found that while blackbirds were most impacted by the virus, they could not, by themselves, contribute sufficiently to the population-level build-up of immunity needed to explain the spatial and multi-annual patterns that characterised its emergence. We characterised the unobserved reservoir species as having a longer lifespan, little disease-induced mortality, and a larger home range than blackbirds. Basic reproduction numbers differed between years, but tended to peak between late July to late August. The difference between the basic and effective reproduction number was largest in August. The estimated large contribution of alternative host species implies that also regions with low blackbird density might be suitable for Usutu virus transmission if other reservoir species are present, but surveillance schemes other than trends in dead birds would be required to detect circulation. Our results highlight the importance of considering multiple host species when designing intervention strategies, surveillance schemes, or future predictions as disease impact and contributions to transmission vary strongly