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On the basis of long-term (2001-2017) and extensive data (>1700 breeding attempts), we assess factors influencing breeding parameters in Montagu's Harrier (Circus pygargus), a medium-sized ground-nesting semi-colonial raptor breeding in cereal fields, in a study area in its core distribution range (Extremadura, Spain). We evaluated annual and long-term variation in breeding parameters, as well as changes in environmental variables such as weather or harvest date. We then evaluated breeding failure and productivity in relation to nest protection measures, weather conditions, lay date and colony size. We found a significant trend for productivity to decrease over the 17-year study period, associated with a trend for predation probability to increase over time. Harvest occurred increasingly earlier in more recent years. The success rate of nests protected with unfenced unharvested patches (the most frequently used protection measure) increased with harvest date, but also declined throughout the study period for equivalent harvest dates. When considering all explanatory variables together, probability of nest success decreased significantly with lay date and throughout the study period, increased with annual rainfall and varied in relation to protective measures (being highest for nests protected with fences or where harvest in the plot had been delayed). In addition, among successful nests, fledged brood size also decreased significantly with lay date and temperature, and increased with annual rainfall. We found no effect of number of neighbours on breeding output. We discuss our results in relation to future conservation prospects for the species.