In recent decades, insectivorous birds associated with agricultural ecosystems have declined globally by up to 60%. This alarming trend is driven by pesticide use, climate and land-use changes, and shifts in farming practices. Additionally, human-animal conflicts reduce suitable nesting sites, further worsening their decline. Yet, these birds offer essential ecosystem services—most notably, natural pest control—helping farmers reduce pest populations and save on pesticide costs. Therefore, evaluating the ecosystem services that insectivorous birds provide in farmed landscapes, and promoting their conservation, represent a win-win strategy that both support Nature-Based Solutions, enhancing farming sustainability, and tackle one of the most urgent conservation challenges. This study evaluates the role of the Barn Swallow (Hirundo rustica), a rapidly declining species, in controlling filth flies and its potential benefits for dairy cow welfare. From April to July 2024, we monitored fly activity and Barn Swallow presence in 23 cattle sheds in Parco Adda Sud, Northern Italy. At the same time, cow stress levels were measured through ear and tail movements—indicators of fly-related disturbance. Our findings show that Barn Swallows significantly reduce fly activity and cow stress, offering effective, natural pest control. The presence of the Barn Swallow can thus lead to economic benefits for farmers by reducing pesticide use for fly control and potentially improving milk quantity and/or quality by lowering the level of stress due to flies experienced by cows. These results can be a valuable tool for the conservation of swallows and for better management of human-animal relationships in agroecosystems. Future research should quantify the economic value of this service and assess any risks of pathogen transmission from swallows to livestock. Such insights could inform policy changes—particularly regarding current regulations that ban swallows from barns—further advancing farm sustainability and biodiversity conservation.