The landscape of fear in cattle farms? How the presence of barn swallow influences the activity of pest flies

Insectivorous birds are key suppliers of crucial ecosystem services, especially important for pest control in agricultural systems, and are globally decreasing at a fast rate. Among them, the declining barn swallow Hirundo rustica is particularly linked to cattle farms, where it is impacted by changes in livestock husbandry practices. The presence of swallows in cattle sheds might contribute to control insect pests, but such an effect had never been explored. With this work, we evaluated the potential control effect exerted by barn swallow on economically relevant pest insects, filth flies. To assess whether swallows inside the cattle sheds influence the activity rate of pest flies (especially Musca domestica and Stomoxys calcitrans), we assessed the relative abundance of swallows and flies over 16 weeks in nine different cattle farms in Non valley (Trentino, North-east Italy), either occupied or not by nesting swallows. From April 2022 to August 2022, we collected weekly data on the activity rate of flies by placing eight white spot cards inside each of nine cattle sheds and counting the deposition of faecal and regurgitation spots by flies. Simultaneously, we collected weekly data regarding swallow occurrence, brood size, number and age of chicks. We applied generalised linear mixed effect models on the activity rate of flies, with spot card and farm as nested random effects, to account for potential random variability between cards and farms. We then assessed through model selection the effect of number of cows, number of swallows, and temperature or week alternatively. Temperature and week were modelled also in interaction with the number of swallows. Fly activity rate followed a seasonal trend with a peak in July, which was markedly lower where swallows were present. Moreover, the positive relationship between flies and temperature was lessened when swallows were abundant. Synthesis and applications. Barn swallows are effective biological control agents for lowering pest fly activity in livestock farms. Farmers were actively involved in swallow conservation within their sheds, also with a shared memorandum-of-intent. Harnessing the ecosystem services supplied by farmland birds could bring positive outcomes for people and biodiversity.Gli uccelli insettivori forniscono servizi ecosistemici cruciali, quali il controllo degli insetti dannosi in agricoltura, e mostrano un preoccupante decremento globale. Tra questi, la rondine Hirundo rustica e particolarmente legata alle stalle, dove subisce gli effetti dei cambiamenti delle pratiche zootecniche. La presenza della rondine nelle stalle bovine potrebbe contribuire al controllo degli insetti nocivi, ma questo effetto non e mai stato indagato. Con questo lavoro, abbiamo valutato il possibile controllo esercitato dalle rondini sulle mosche, fonte di significativi danni economici agli allevamenti. Per valutare se le rondini possano influenzare il tasso di attivita delle mosche (soprattutto Musca domestica e Stomoxys calcitrans), abbiamo misurato l'abbondanza relativa di rondini e mosche per 16 settimane in nove stalle in Val di Non (Trentino, Italia), occupate o meno da rondini nidificanti. Da aprile ad agosto 2022 abbiamo misurato il tasso di attivita delle mosche utilizzando otto spot card bianche in ciascuna stalla e contando i segni di rigurgiti e feci. Contemporaneamente, abbiamo conteggiato il numero di rondini per stalla, rilevando adulti, dimensioni della covata, numero ed eta dei pulcini. Attraverso modelli lineari generalizzati misti abbiamo valutato le variazioni nel tasso di attivita delle mosche, con spot card e stalla come effetto random annidato per tenere conto della potenziale variabilita tra spot card e stalle. Abbiamo quindi quantificato l'effetto del numero delle b