Long-term changes of breeding success in Montagu's Harrier Circus pygargus

Over a period of almost twenty years, clutch size and breeding success in the Montagu's Harrier were investigated in the context of changing environmental conditions in the species' natural breeding habitats in eastern Poland. During the study periods (1990-95 and 2003-12) a decline was noted in the number of breeding pairs in the population nesting on the calcareous peat bogs near Chelm, not far from the Polish-Ukrainian border. Statistically significant differences in breeding parameters between the two periods were also observed. In the first period clutch volumes were greater, as the dimensions of the individual eggs were larger; additionally, more eggs hatched and the hatchling survival rate was higher. Some habitat conditions were different in the two periods, with the water level and height of vegetation near the nests being lower in the second period. The harriers' food in the two study periods fluctuated strongly with regard to the content of small mammals and compensatory items. In the second period a distinct increase in predator pressure was noted. Pressure from terrestrial predators diminished whereas that from aerial predators increased. Broods in semicolonies, where birds actively defended their nests, enjoyed a higher rate of survival, as did nests situated far in from the edge of peat bogs. The results suggest that the decline in breeding numbers was driven by increased predation, which was in turn a consequence of habitat changes in the natural environment of eastern Poland.