The ground nesting raptor Montagu’s Harrier breeds in loose colonies in cereal fields in the Spanish Extremadura. It is unclear how and whether birds in different colonies interact and how harriers spend time before and after nesting, before starting migration. We used GPS–GSM tags deployed on ten females and three males, some over multiple seasons, to follow bird movements with unprecedented detail. Arriving from spring migration, all males and most females returned to their old nest site, and spent between 13 and 25 days in mate choice and local site inspection. During incubation and early nesting female movements were strongly reduced, but increased significantly during late nesting and post-fledging periods. After fledging or after breeding failure, females increased their flying radius. Some of them visited other colonies, for single days or for longer periods, or flew long distances within Spain. These visits might have included help in breeding attempts of other pairs (adoptions). Four out of six females returned to their own breeding site before starting migration. Several females repeatedly used common roosts in this phase. Non-breeding females were active in defending nests in the colony against predators. Females with successful brood initiated migration earlier and spent less time in Spain than non-breeders or those with predated nests. Unlike females, daily distances in males were more uniform. While all males remained in the breeding area for the entire season, their activity centres shifted more within that area than those of females.

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.