Speciation involves the evolution of reproductive barriers that maintain the genetic integrity of species. Premating barriers, in particular, have been referred to as one of the most important mechanisms in keeping species segregated. In this study, I investigate such premating isolation between two closely related species, the house sparrow (Passer domesticus) and Spanish sparrow (P. hispaniolensis). These two species live in sympatry over most of the Spanish sparrow’s breeding range, but apparently, interbreeding between them is either infrequent or absent. During seven weeks of fieldwork over two breeding seasons, I studied the two species in an area where they are sympatric, but remain distinct, namely Badajoz, Spain. To examine the potential premating barriers that keep them isolated, a series of methods were implemented. Transect counts and radio-telemetry was conducted to investigate habitat use, and these methods mutually demonstrated significant habitat segregation between the species. House sparrows stay in closer proximity to human constructions, while maintaining a larger distance to water-sources, whereas Spanish sparrows do the opposite staying closer to water and further away from buildings and human activity. Further, observations of breeding colonies at a larger spatial scale revealed a between-species difference in choice of nest site with pure house sparrow colonies residing closer to cities than colonies including Spanish sparrows. By investigating phenology, I found a minimum difference of 13 days in timing of breeding, consistent with studies in aviaries, which would reduce gene flow significantly. Hence, the opportunities for hybridization are strongly reduced due to spatial and temporal isolation. Finally, ecological niche models indicate dry climate as the main driving force for the distribution of Spanish sparrow which can point to fundamental ecological differences between the species. Overall, my results suggest multiple ecological differences between the two species that most likely contribute towards isolating them, explaining why they do not interbreed extensively in areas of sympatry.