Montagu's harrier food conditions and the effect of management measures

In 2017, the Species Protection Plan (SPP) for Montagu’s harrier was launched in Flanders. As part of this effort, a monitoring programme was developed to evaluate whether management measures – such as bird crop fields and buffer strips – had a measurable effect on prey availability for the Montagu’s harrier and other raptors. With annual interim reports, the Flemish agency for nature and forests was kept informed of the monitoring results. This report brings together all data from 2017 to 2020 to present the first consolidated findings on the effects of the SPP’s management measures.

The study focuses on agricultural areas in the eastern Loam region (Leemstreek) in Flanders, characterised by an open, rolling landscape with plateaus.

The study evaluates the effectiveness of the SPP by monitoring bird species densities, assessing the presence of mice and analysing the composition of prey species consumed by related raptors. The data collected is used to determine whether these measures have contributed to improving the food conditions necessary for the recovery of Montagu’s harrier populations in the region.

The study combined ecological fieldwork (monitoring) with statistical analysis to assess the effects of targeted management measures. A literature review provided the ecological context for the species’ habitat needs. Bird densities were monitored using the ‘Meetnet Agrarische Soorten’ (MAS) method with fixed-point counts during the breeding season. Statistical models (GLMMs) were used to test the effect of management measures on bird densities, and trends were, with caution, estimated per point using linear regressions. Mice presence was measured by counting burrow openings in managed and unmanaged fields, allowing comparison across habitats and years. Prey composition was studied through pellet analysis of related raptors, revealing prey composition and confirming the importance of field voles.

In terms of limitations, the monitoring was conducted over a relatively short period of four years. Since the first two years were mainly used to establish the monitoring network, the actual window for detecting meaningful ecological trends was even more limited. This makes it difficult to perform solid long-term trend analyses.

In addition, differences in bird numbers may reflect pre-existing conditions in priority areas, not effects of management measures.

Although the monitoring network expanded over time, there were still noticeable gaps in spatial coverage, even within the defined core areas.

Furthermore, the study did not consider the chance that animals present were missed during monitoring. This especially affects elusive or low-activity species like the Montagu’s harrier and the grey partridge.

In the case of counting the grey partridge, there were signs of illegal reintroductions. These artificial inputs can distort the dataset and give the false impression of population recovery or successful intervention.

Finally, the Montagu’s harrier did not establish itself in the area during the course of the study. This limits the ability to directly assess how effective the measures were in supporting reproduction of the species.

Since the launch of the Montagu’s Harrier SPP, the area covered by management agreements has significantly increased, especially in priority areas. Although Montagu’s harriers did not breed in the area, the study suggests that key conditions such as food availability appear to have been met.

Field vole densities were consistently much higher in bird crop fields than in conventional fields. Other vole-eating species, such as hen harriers and short-eared owls, were present and confirmed to rely heavily on field voles. The study showed that skylarks, grey buntings and (non-breeding) blue harriers had significantly higher densities in the priority areas. An analysis of the effects of management agreements on species densities reveals positive correlations with specific measures. Bird crop fields positively affected mouse-eating non-breeding Blue and Marsh Harriers, while buffer strips had a strong positive impact on Yellowhammers. Densities within priority areas from 2017 to 2020 were positive for non-breeding Blue Harriers but negative for Marsh Harriers (though positive outside these areas). It's important to consider that species densities may have been higher in priority areas even before they were designated.

The MAS monitoring has been expanded into a fully-fledged monitoring network. However, there are still gaps in the coverage, and further densification and expansion of the network are needed to effectively assess the impact of management measures and policies.