Optimized Monitoring and Conservation of Farmland Bird Species through Bayesian Modelling: The Montagu’s Harrier Circus pygargus Population in Central Italy

The Montagu’s harrier Circus pygargus is considered a rare species at the Italian level, and vulnerable at the European level. The main threat for these farmland birds is represented by agricultural practices; in fact, it nests on the ground in agricultural environments; therefore, at harvest time nests are often destroyed (with the eggs and chicks) by farm machinery. We examined the reproductive traits (clutch size, laying date, hatching, and fledging date and success) of the Montagu’s harrier population in central Italy (about 10% of its population in Italy) where nest protection has been implemented through electric fences and metallic meshes. By using a Bayesian probabilistic network, we modeled the sequence of events that determine its reproductive success (percentages of eggs hatched and chicks fledged) and simulated the effects of different environmental and management scenarios. Our model explained the hatching and fledging success with 90.20% and 95.12% accuracies, respectively. We found that crop type and height, laying date, type and delay of nest protection have specific effects on the reproductive success of this population. Our findings demonstrate that it is possible to optimize the monitoring of this population and significantly increase its reproductive success by acting selectively upon the environmental and management attributes of the breeding area. Our decision tool allowed us to produce several rules for the optimized monitoring and conservation of the Montagu’s harrier population in central Italy. The methodological approach proposed here is suitable for application to any farmland bird population on a local scale.