Evaluating releasing methods for the re-establishment of grey partridges

Key findings

• The majority of released birds settled within 1.5 kilometres.
• Artificially-reared spring pairs had the lowest breeding success of all four groups compared and should therefore not be released.
• Breeding success of fostered birds was highest, but only where habitat requirements were met during all seasons.

2006 was the second and final season of our project aiming to find the best methods of re-establishing grey partridges through releasing in areas where they have disappeared, and where a suitable environment has been restored.

We have been working on 26 sites split between East Anglia and southern England where we followed the fates and breeding success of radio-tagged (intensive sites) and colour-ringed birds (extensive sites) of individuals released using four different techniques (bantam-reared fostered, artificially-reared fostered, coveys released in autumn, and pairs released in spring). For more details about methods used and first year results see Reviews of 2004 and 2005. Here we present preliminary results for the two research years combined.

In March, the re-sighting rate from the spring counts for fostered birds released in the previous August across intensive and extensive sites averaged 18.9% in East Anglia and 16.7% in southern England. That of birds released as full-grown family coveys in November averaged 18.9% in East Anglia and 19.5% in southern England (see Table 1), indicating no significant differences in over-winter survival between the two regions. No data were available for the spring pairs as they were released after these counts (in April).

As in other studies, the majority of losses were due to predation. In the following autumn counts in September, the average summer survival rate (survival from birds encountered in spring and re-sighted in autumn) of fostered birds was 51.0% in East Anglia and 25.4% in southern England. The releases of full-grown birds yielded survival rates of 28.9% for autumn coveys and 13.6% for spring pairs in East Anglia and 33.8% and 7.8% respectively in southern England, indicating that spring pairs survived less well during summer than the other three groups (see Table 1).

In terms of fidelity to the release site of our radio-tagged birds in southern England, the distance moved from the release site to the spring location did not vary between fostered and autumn released birds and averaged 1.35 kilometres. The number of birds found in spring within a radius of 1.5 kilometres was between 65% and 75%, depending on the release technique used.

The distance moved from the release site to the nesting site was 1.8 kilometres (se = ±0.5) for bantam fostered birds, 1.0 kilometres (se = ±0.1) for artificially fostered birds, 1.4 kilometres (se = ±0.2) for autumn released birds and only 0.5 kilometres (se = ±0.1) for spring pairs. Where conditions on the release site were favourable (ie. over-winter foraging and escape cover right into spring, predator control and low disturbance pressure), site fidelity of our released birds was therefore satisfying.

Overall, summer survival and breeding success of released birds was low. In southern England, depending on the release strategy used, we sighted no more than 6% of released hens during the autumn counts. However, of all the hens counted in autumn, an average of 32.5% of fostered hens managed to raise chicks. We never recorded spring pairs producing chicks (see Table 2).

In East Anglia, breeding success was in general higher than at the southern sites, most likely owing to more intensive predator control. This might also explain why at least seven spring pairs managed to produce chicks. As in southern England, fostered hens achieved the highest breeding success (48%, see Table 2).

In summary, the following release strategies are feasible. Where at least three pairs per 100 hectares are still present, we recommend habitat management according to our Royston demonstration project over releases.

Where fewer or no grey partridges are encountered, we recommend releasing autumn coveys, followed by intensive monitoring in the following spring. Where at least 16% of the birds released can still be found within a radius of 1.5 kilometres to the release site, we recommend intensifying predator control into the breeding season, followed by fostering chicks to barren pairs.