Survival, habitat use and disturbance behaviour of re-introduced grey partridges "Perdix perdix" L. in an enhanced arable landscape in the Swiss Klettgau

In the last forty years, the Swiss lowlands have lost much of their former value as habitat for
wild plants and animals, mainly because of the effects of modern farming practices. One of
the most prominent and charismatic species affected is the Grey Partridge Perdix perdix L.
Considering the alarming decline of the Grey Partridge, the Swiss Agency for the
Environment, Forests and Landscape (SAEFL) entrusted the Swiss Ornithological Institute in
1991 to undertake a ten year project on “Protection measures for brown hare and Grey
Partridge” (Jenny et al. 2002). In this context the “Klettgau” in the canton of Schaffhausen
was chosen as study area as it was one of the two regions in Switzerland where wild
partridges still remained in small numbers at that time. To reverse the main cause of the
partridge’s decline - habitat loss - the main activity in the early stages of the project was the
promotion of ecologically enhanced habitats such as wild-flower strips and hedges.
Unfortunately, the already very small partridge population went extinct shortly after the
beginning of the project. However, by 1998 the area of partridge-friendly habitats had grown
to such an extent that it allowed to launch a partridge re-introduction project. This was
undertaken as part of the Swiss Ornithological Institute’s project “Birds as test organisms to
evaluate enhanced habitat diversity in agricultural areas” which was launched in 1996.
The main aims of this thesis for which the data was collected between 1998 and 2001 are to
assess whether Grey Partridges:
1) are able to survive and reproduce in an ecologically enhanced landscape such as the
Klettgau,
2) make use of the enhanced areas and, if so, which value they have for them,
3) are disturbed by human and predator activities within the re-introduction area,
4) to conclude if and under what conditions further partridge re-introductions in Switzerland
might be successful.
Study area
The study area (c. 530 ha) is intensively cultivated, mainly for growing cereals (49%), oilseed
rape and sunflowers (14%) and root crops (12%). Grassland covered 11%, another 11%
were bare of vegetation (buildings, roads, etc). Permanent cover such as wild-flower strips,
hedgerows and grass banks amounted to 3% of the area. Field sizes ranged from 0.1 to 5.5 ha.
From 1991 to 2001 the area of wild-flower strips increased from 0 ha to 12 ha, the area of
hedgerows from 2 ha to 2.7 ha. Besides agricultural use, the study area is very popular for
recreation mainly by walkers with or without dogs and riders. In the centre of the study site
there is a dog training school and an arena to school horses, in the northern part of the area
there is a cycling route.
Origin of birds released
Originally it was planned to release wild translocated birds of the western clade of the
subspecies Perdix perdix perdix only (50 per year) as it is widely accepted that this technique
generates the highest possible re-introduction success in areas where the species has become
extinct. After the first year of this study however, it became apparent that it was impossible to
organize enough wild-caught birds for release (n = 21 released in total). I therefore had to
switch to the next most promising option which I considered to be parent-reared birds
released as coveys in autumn (n = 77). In the second and third year of this study we
additionally fostered parent-reared chicks to already re-established adult birds which had
failed to produce their own young (n = 32).
Dispersal, survival and causes of death
Of the 98 released adult partridges, 73% remained within the study area and 52% survived the
first month after release. During the first month after release, they frequently moved across
the whole research area. After settling, 98% of all partridge locations were recorded in that
part of the study area where the density of enhanced areas was maximal.
Survival was highest in wild-hatched partridges of the founder population (mean ± SE;
0.90 ± 0.03), followed by that of fostered chicks (0.86 ± 0.03) and translocated adult wild
birds (0.82 ± 0.06). While survival of these groups was not statistically different from each
other, survival of captive-reared adults was significantly lower (0.70 ± 0.06). We found the
carcasses of 91 partridges. 88 of them were predated, 1 died because of disease, 1 because of
a traffic accident and 1 as a result of a territorial fight. Predation by mammals (mainly foxes)
was twice as frequent as predation by avian predators.
Reproductive success
We observed 19 pairs that started egg laying. Of those, seven broods hatched and twelve
failed. Mean clutch size of first clutches was 15.6 eggs (N = 9 clutches, SD = 1.3). Only one
second clutch was found. 86% of all eggs hatched (N = 7 broods). The average percentage of
successful nests over three years was only 0.33 [SE = 0.08]. When breeding the year after
release, fostered chicks tended to have more successful nests (0.44 [SE = 0.43]) than when
individuals of the other treatment groups were involved (reared adults: 0.17 [SE = 0.03];
translocated: 0.25 [SE = 0.07]; wild hatched in study area: 0.27 [SE = 0.29]), but the
differences were not statistically significant. Eleven out of 19 nest were located in wild-flower
strips.
Habitat use and home range size
At the level of the individual family group (pairs or coveys), we found a significantly greater
use (throughout the year) of habitat areas that were enhanced with wild-flower strips and/or
hedges, compared to non-enhanced areas. When the birds used the agricultural fields,
densities of use declined sharply with increasing distance from the nearest enhanced area.
Thus, the availability and spatial distribution of ecologically enhanced areas were the main
determinants of the partridges’ range use. Despite their strongly over-proportional use of
enhanced areas, the partridges spent a large proportion of time in cultivated fields. In summer,
frequently visited vegetation types were cereals (average 26.1% of locations), root crops
(14.8%) and grassland (9.3%). In winter, the birds spent much of their activity in cereals or
stubble fields (32.7%) and rape (24.1%). This indicates that these types of vegetation,
particularly cereals, were attractive resources, although not preferred in respect to their
availability.
The size of the group home-ranges varied significantly with season. In spring (prebreeding
period) and summer (breeding period), the average home-ranges ( ± SD) were 6.8
( ± 4.0) ha and 6.9 ( ± 2.6) ha, respectively. From late summer until the end of winter (nonbreeding
period), the home-ranges were significantly larger; late summer: 15.2 ( ± 6.6) ha;
autumn: 17.0 ( ± 4.0) ha; winter: 14.4 ( ± 3.6) ha.
Disturbance
Partridges showed a distinctive cause-specific reaction repertoire to all disturbance types
compared, mainly crouching in presence of raptors and showing vigilance in presence of
mammals (foxes and cats). Flushing was the main reaction when disturbed by leisure
activities. When flushed, partridges reduced their flight distance by 54 metres compared to
unforced flights and remained in their territory in 87% of all cases. In summer, their main
escape cover was cultivated fields, whereas in winter they mainly used permanent cover such
as wild-flower strips and hedges. The spatial distribution of partridges was influenced by
season: In summer, partridges avoided areas with high human disturbance, whereas in winter
they avoided areas with high predator abundance and close proximity to tall hedges. Human
activities caused twice as much disturbance events as predators, with associated energetic
costs. Overall, disturbance substantially limited overall spatial use, with consequences for the
carrying capacity of the area.
Conclusion
This study shows that with enough staying power even the intensively exploited Swiss
countryside may provide a satisfactory environment for a highly demanding species such as
the Grey Partridge. Even though it is not possible to prove from this study’s results if further
partridge re-introductions in Switzerland or abroad will be successful in terms of creating
self-sustainable populations – for that the duration of the study was simply too short - it is
possible to draw four basic conclusions for future partridge re-introduction projects:
1) Prior to every partridge re-introduction or re-establishment the habitat must be enhanced
with permanent habitat structures such as hedgerows and wild-flower strips.
2) If translocated wild birds are not available for release, chicks should be fostered whenever
possible, in the best case to wild birds still resident in the area.
3) Re-introductions should only be envisaged in areas with low predator numbers and human
activities.
4) In order to find possible weaknesses in re-introduction projects, post-release monitoring is
essential to ensure the project targets are met.