Ibis (2002), 144, 94 – 105
Blackwell Science Ltd
Comparative breeding biology of Hen Harrier and
Montagu’s Harrier: an 8-year study
in north-eastern France
A. MILLON 1, J.-L. BOURRIOUX 2, C. RIOLS 3 & V. BRETAGNOLLE 4*
1
4, rue de l’Orme, F-10220 Rosson, France
2
Maison forestière de Blinfey, F-52110 Beurville, France
3
Maison forestière de Géléneau, F-51340 Trois Fontaines L’Abbaye, France
4
CEBC-CNRS, F-79360 Villiers-en-bois, France
Hen and Montagu’s Harriers breed in the same cultivated areas of eastern France. We present
data from an extensive study conducted in three adjacent areas where 757 nests of the two
harriers were monitored between 1993 and 2000, with the aim of comparing the breeding
ecology of these two species and to evaluate their possible future trends. Breeding habitat
for harriers consisted nearly exclusively of winter cereals, causing great conservation concern
in this intensively farmed region. The Hen Harrier was almost absent from two of the study
areas. This species showed significantly larger clutch size and higher breeding success than
the Montagu’s Harrier, and an earlier breeding phenology. It was thus less adversely affected
by harvesting activities than Montagu’s Harrier. Both species showed a reduced breeding
success with increasing laying date. There was a large diet overlap between the two species, possibly leading to competitive interactions. Overall, the Montagu’s Harrier should be
considered as the more vulnerable of the two species, necessitating conservation measures,
such as protection of nests from early harvesting activities. Nevertheless, to maintain both
species in agricultural habitats, farming practices that preserve sufficient food should also
be promoted.
Three species of Harriers Circus spp. breed in western Europe: the Marsh Harrier C. aeruginosus, the
Hen Harrier C. cyaneus, and the Montagu’s Harrier
C. pygargus. As with all but one species of the genus
(C. assimilis), they nest on the ground (del Hoyo et al.
1994, Clarke 1996). Over the past few decades in
western Europe, all three species, but particularly
Montagu’s and Hen Harriers, have changed their
breeding habitats from natural to cultivated areas.
This habitat shift has caused great conservation
concern, as chicks fledge at harvesting time and are
killed by harvesting operations, making breeding
success heavily dependent on direct human intervention through nest management (Arroyo & Bretagnolle
2000).
Montagu’s Harrier, the lightest species of the genus
(Clarke 1996), is strictly migratory, and breeds from
*Corresponding author.
Email: breta@cebc.cnrs.fr
© 2002 British Ornithologists’ Union
North Africa and Europe across to central Russia
(Cramp & Simmons 1980, del Hoyo et al. 1994).
The French population is estimated at 2500–5000
pairs (Salamolard et al. 1999). Together with the
Spanish population, it is the largest in western Europe,
representing 30–40% of the estimated species total
of 6000–12000 pairs (Tucker & Heath 1994). The
Hen Harrier is currently considered to be the most
vulnerable harrier species of western Europe (Tucker
& Heath 1994). It has also colonized cultivated areas
for breeding but, at least in some regions, it is less
affected by harvesting activities than Montagu’s
Harrier because of its earlier breeding phenology
(Arroyo 1996). Partially migratory throughout its
range, its numbers are estimated at 7000 –12 000 pairs
in western Europe, with the largest breeding population in France (2500–4000 pairs, Tucker & Heath
1994, Tombal 1999). The security of the Montagu’s
and Hen Harrier populations in France is therefore
central to the long-term conservation status of the
two species in Europe. A better knowledge of Harrier
Comparative breeding biology in harriers
breeding parameters is essential to understand possible
changes in demographic trends, and to set effective
conservation measures (Arroyo & Bretagnolle 2000).
However, very few studies have reported detailed
data on breeding phenology and success in cultivated
areas for either of these species, and especially few
for the Hen Harrier (see Watson 1977, Butet & Leroux
1993, Arroyo 1996, Salamolard et al. 2000, Arroyo
& Garcia 1999).
In this paper we present the results of a study
conducted in north-eastern France, on breeding
populations of Montagu’s and Hen Harriers that nest
almost exclusively in cereal fields. Although the
programme started in 1987, at first it was concerned
primarily with protection. The areas surveyed increased
rapidly and reached a constant level in 1993. We thus
present data for the period 1993–2000, representing
a total of 757 nests monitored in three study areas.
This is the first time that the comparative breeding
biology of the two species has been studied in this
habitat over many years and using a large data
set. Our aims were to compare breeding ecology,
diet, conservation effort and population dynamics
between the different areas, years and species, in
order to assess the level of competition between the
two species, and their possible future trends in
French cultivated areas.
95
METHODS
Study areas
The study took place within Aube and Haute-Marne
départements, Champagne–Ardenne Region, northeastern France (Fig. 1). Three distinct study areas were
defined, according to topography and general agricultural characteristics (Fig. 1, Table 1). Breeding
harriers were searched for by the same five observers
during the whole study period, with a constant effort
every year. The first study area (c. 600 km2) is typical of the Champagne Crayeuse (hereafter called
Crayeuse). This is a low-elevation chalk plateau
(< 250 m). Intensive farming in vast open fields as
well as a few pine woods have replaced former
sheep-grazed areas. The second and third study
areas (Barrois 1 & 2) are also limestone plateaux, but
slightly higher and more undulating (200–400 m).
Between about a third and a half of the two Barrois
areas are wooded with beech Fagus sylvatica. Open
habitats are intensively cultivated cereal fields, vineyards and grassland or pasture fields with hedges
(Fig. 1, Table 1). In the first study area (Crayeuse)
both harrier species occur, whereas in Barrois, only
Montagu’s Harriers nest in the cereal fields, while a
few Hen Harriers breed in young woodlands.
Figure 1. General map showing the three study areas and localities cited in the text.
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
96
A. Millon et al.
Table 1. Land use characteristics of the study areas, and comparison of harrier breeding densities (breeding pairs /100 km 2) and their
variations from year to year (CV = 100*sd/mean).
Harrier density (1993– 2000)
Agricultural land use (%)
Study area
Crayeuse
Barrois 1
Barrois 2
a
Montagu’s Harrier
Hen Harrier
Area (km2)
Forest (%)
wheat
winter barley
meadows
other crops
Mean
CV (%)
Mean
CV (%)
621a
247b
807
8.53
16.6
36.5
32.8
28.5
29.7
8.9
9.3
9.2
4.6
20.7
18.5
53.7
41.5
42.6
5.45
7.37
3.46
14.0
38.3
41.2
5.31
0.24
0.00
28.1
14.8
–
This value varied from 599 to 687 according to years.
This value varied from 223 to 260 according to years.
b
Harrier censuses
Nests were located between May and July by observing male to female prey deliveries. A small number
of Montagu’s Harrier nests were found in forest
clearings (n = 5) or in set-aside fields (n = 1), while
a small, although not precisely known proportion
of Hen Harrier nests were in young woodlands of
Barrois 2. Most fieldwork was done in June (nest
searching) and July (protection of broods from
harvesting). As a result, some breeders that failed at
early stages (incubation) went undetected, leading
to a potential bias. In another study area in western
France (1994–2000), where Montagu’s Harrier nests
are detected earlier, less than 15% of clutches failed
to hatch (n = 320), of which one-third were due to
predation, one third to harvesting and the remaining to natural causes (V. Bretagnolle & B.E. Arroyo
unpubl. data). We assumed that a similar proportion
of pairs failed at incubation in Eastern France, and
that this proportion was similar between the two
species and years. The numbers of nests found were
then converted into breeding densities (uncorrected),
used in comparisons between species, study areas
and years.
Breeding phenology
Because nests were not systematically searched for
from the beginning of the breeding season, egg laying
dates were estimated by back-dating from hatching
date. This was calculated through the age of nestlings
from the length of the third primary (Arroyo 1995)
and/or from plumage pattern (Pacteau & Perrotin
1991). For the Montagu’s Harrier, we used an
average of 30 days for incubation, and 30 days for
the chick rearing period (Cramp & Simmons 1980,
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
Clarke 1996, pers. obs.). We used a slightly higher
value for the Hen Harrier rearing period (32 days:
Cramp & Simmons 1980, pers. obs.). Data on breeding phenology are presented on a weekly basis, to take
into account the relative inaccuracy of the method,
with week 1 being the first week of April.
Observed and estimated clutch and
brood sizes
Nests were visited one to seven times during the
breeding season (but only once during incubation as
Harriers are sensitive to disturbance at this time),
and clutch size, hatchling and fledgling numbers
were recorded during nest visits. Nests were visited
mainly when we estimated that the brood was old
enough to allow nest displacement as a protection
measure (see below). Thus, only a few nests were
visited at the egg stage (20%, n = 757 nests, both
species combined) and, moreover, the number of
nest visits differed between nests located in wheat
vs. barley because they did not require the same
conservation effort. We thus estimated clutch size
for nests visited at the chick stage from the number
of chicks plus the number of unhatched eggs (in
harriers, unhatched eggs are left within the nest).
There are inherent biases in doing this (e.g. underestimation of clutch size due to early chick mortality,
sample bias due to the fact that only successful
clutches are used). However, this bias might have
been minimal, as observed and estimated clutch sizes
did not differ significantly (Kolmogorov–Smirnov twosample test, Dmax = 0.03, P > 0.99, n = 443; Dmax =
0.08, P = 0.98, n = 167, respectively, for Montagu’s
and Hen Harriers). For clarity, however, we present
both actual and estimated clutch sizes separately.
Unless specified otherwise, brood sizes during chick
Comparative breeding biology in harriers
97
rearing were calculated only for nests where hatching occurred, and fledged brood sizes only for nests
where fledging occurred.
non-normally distributed. Non-parametric tests were
used for small sample sizes, as well as for breeding
density parameters.
Diet
RESULTS
Both pellets and remains were collected in June and
July at nests, perching sites and roost places, and thus
came from both young and adults (using a combination of pellets and remains improves harrier diet
analysis: Simmons et al. 1991). Pellets were analysed
to test for differences in the diet of the two species
and between Crayeuse and Barrois (i.e. Barrois 1 and
2 were cumulated). Overall, 3532 pellets were collected in 1996–2000, and these accounted for 95%
of 7052 identified prey items. For each pellet, the
maximum number of individuals was scored (see
Clarke et al. 1993, and Arroyo 1997 for procedure).
Harrier pellets often contain remains of an unknown
number of individuals, e.g. they sometimes only
consist of fur or feathers, and this may introduce
some bias. In such cases, we arbitrarily considered
that only an individual prey was present in each
pellet (see also Arroyo 1997, Salamolard et al. 2000).
We then calculated the percentage of biomass provided by each prey type (mean mass used for each
prey category was obtained from local data). Large
birds or mammals are often only partially eaten by
Harriers (Arroyo 1997): maximum biomass consumed
per prey was thus arbitrarily fixed at 150 g for the
Montagu’s Harrier and at 250 g for the Hen Harrier,
according to the size difference between these two
species (see also Arroyo 1997).
Trophic diversity was calculated using the Shannon–
Weaver index (H = –Σpi log pi, where P = Xi/X, and
Xi = number of prey items taken from class i; X =
total number of prey items). Seven prey categories
were considered for the calculation of the index, in
order to allow comparisons between species and the
two regions. Furthermore, differences between regions
and species in diet were evaluated through differences in the frequency of occurrence for each prey
category. We used an index of competition (MacArthur
& Levins 1967, in Ricklefs 1979), which was calcu2
lated only for Crayeuse: aj,k = Σi(Pi,j * Pi,k)/ΣiPi,k
(P i,j =
proportion of biomass of class i for the species j).
Above 1, the index suggests that interspecific competition is weaker than intraspecific competition for
species j, assuming that overlap leads to competition.
All statistical analyses were performed using SAS
8.0 (SAS 1999). Data are expressed as mean ± sd,
but were transformed for statistical analyses when
Breeding density
The average breeding density for the Montagu’s Harrier
for all areas and years combined was 4.96 ± 0.77
pairs/100 km2 (n = 8 years), and for Hen Harrier
5.32 ± 1.40 (Crayeuse only; see Table 1). In Crayeuse,
where both species breed in sympatry, there were
no significant differences between species in density
(Wilcoxon’s signed ranks test, Z = 0.33, P = 0.74,
n = 8 years). Hen Harrier breeding density increased
significantly during the study period (Spearman test,
rs = 0.81, P = 0.01, n = 8 years), unlike the Montagu’s
Harrier in any of the study areas (see Fig. 2). However, Montagu’s Harrier breeding density varied
significantly among study areas (Kruskall–Wallis test,
H = 10.68, P = 0.04, df = 2, n = 24; Fig. 2). The length
of the study period did not allow testing for cyclicity
in breeding density, as documented in some other
harrier populations (Salamolard et al. 2000). However, Barrois 1 & 2 showed consistently more variability than Crayeuse, as revealed by the coefficients of
variation for Montagu’s Harrier (Table 1). Montagu’s
Harrier density did not covary between Barrois 1
and 2 (rs = 0.47, P = 0.24, n = 8), and nor did Hen
Harrier and Montagu’s Harrier density in Crayeuse
(rs = 0.19, P = 0.76, n = 8).
Breeding habitat and phenology
Overall, 97% of nests were found in cereal fields,
especially winter barley and wheat (Table 2; n = 757
nests, both species combined). Within the cereals,
winter barley was strongly selected over wheat
(X12 = 96.2, P < 0.001), in both species (Fig. 3). There
were no significant differences between Hen and
Montagu’s Harriers in Crayeuse, all years combined
(X12 = 0.63, P = 0.43). Montagu’s Harriers showed
a marked temporal trend in crop selection for nesting, considering the ratio of nests in wheat vs. barley
(rs = –0.96, P = 0.0005, n = 7 weeks). The ratio was
skewed towards barley in earlier breeders, and towards
wheat (or other crop types) for late breeders (Fig. 3).
The same trend was apparent, though not significant, for Hen Harriers (rs = –0.46, P = 0.29, n = 7).
In both species, egg-laying was spread over more
than two months, between 15 April and 30 June
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
98
A. Millon et al.
Figure 2. Harrier breeding density (pairs per 100 km2), according to species and study areas.
Table 2. Nest site choice according to species, area and year. Data are given as percentage of nests found per habitat type ( n = number
of nests monitored).
Montagu’s Harrier
Crayeuse
Year
1993
1994
1995
1996
1997
1998
1999
2000
Totals
a
Hen Harrier
Barrois 1
Barrois 2
Crayeuse
wheat
barley
na
wheat
barley
na
wheat
barley
na
wheat
barley
na
58
46
59
35
18
61
76
20
53
29
54
41
65
82
39
21
73
44
24
35
34
34
28
33
29
30
247
55
50
65
65
50
43
46
40
57
41
38
30
35
50
57
55
60
42
28
8
17
17
10
14
11
15
120
81
75
83
58
20
69
50
25
63
19
25
17
39
80
31
50
75
37
26
12
12
30
20
13
12
16
141
40
44
71
41
48
66
44
28
56
47
48
27
56
44
29
56
64
40
15
23
34
31
25
35
41
36
240
Total numbers, including pairs that bred in other crop types than cereals.
(Fig. 4). On a weekly basis (with week 1 = 1–7 April),
the Hen Harrier laid on average slightly, but significantly, earlier (5.45 ± 1.55, n = 97) than Montagu’s
Harrier (6.75 ± 1.30, n = 317; Kolmogorov–Smirnov
test, Dmax = 0.495, P < 0.0001; Fig. 4). This difference was more marked if median dates (week 5 vs.
week 7 for Hen and Montagu’s, respectively) were
used, and laying was also less synchronized in Hen
than in Montagu’s Harriers (CV = 30% and 19%,
respectively). In the Montagu’s Harrier, there was a
significant effect of year but not of area on laying
date (two-way ANOVA, year F7,316 = 2.87, P = 0.007;
area F2,316 = 1.2, P = 0.3; interaction F14,316 = 0.4,
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
P = 0.99). In contrast, year to year variation in egg
laying dates for the Hen Harrier was not statistically
significant (F7,96 = 1.93, P = 0.07).
Clutch size
In both species, clutch size ranged between one and
six eggs. On average, observed clutch size was 3.65
± 1.01 for Montagu’s Harrier (n = 111, CV = 28%)
and 4.15 ± 1.27 for Hen Harrier (n = 40, CV = 31%).
Montagu’s Harriers laid significantly smaller clutches
than Hen Harriers in any year (two-way ANOVA, year
F7,151 = 2.6, P = 0.01; species F1,151 = 14.2, P = 0.0002,
Comparative breeding biology in harriers
99
Figure 4. Comparative breeding chronology of Hen and Montagu’s
Harriers. Data are expressed as number of clutches.
Figure 3. Comparative nesting habitat choice according to calendar date in the two harrier species. Data are shown as percentages.
Sample sizes are indicated at the top of the columns. Only three
crop categories are considered (see text). Week 1 = first week
of April. The line indicates the relative availability of barley as
compared to all winter cereals (23%).
interaction F7,151 = 0.8, P = 0.59). In the Montagu’s
Harrier, estimated clutch size varied between years
but not between areas (area F2,432 = 2.19, P = 0.11,
year F7,432 = 7.18, P < 0.0001, interaction F14,432 =
0.93, P = 0.52). However, this was due only to 1997,
in which, on average, smaller clutches were laid than
in 1993, 1996, 1998 and 2000 (Scheffe’s test, all
P < 0.05). For the Hen Harrier, there was no significant year effect on clutch size, using estimated values
(F7,167 = 0.99, P = 0.44). Estimated clutch size decreased
significantly with laying date in Montagu’s Harrier
but not in Hen Harrier (Linear regression, F1,315 =
7.94, P = 0.005 and F1,94 = 0.05, P = 0.82, respectively;
see Fig. 5).
Brood sizes at fledging, and nest failure
Nest failure (i.e. total failure of clutch or brood) varied
between 14% and 65% according to years and
species, although these are minimum values as some
nests might have failed without being found. Nest
failure did not differ significantly between the two
harriers in Crayeuse (Mann–Whitney U-test, U = 21,
P = 0.24, n = 8), nor between years (Kruskall–Wallis
test, H = 8, df = 7, P = 0.33) or areas (H = 3, df = 2,
P = 0.22) for the Montagu’s Harrier. At least 8.7%
of eggs of Montagu’s Harriers (n = 1523) and 10.5%
of Hen Harriers (n = 588) failed to hatch (four
clutches, transferred for conservation purposes, are
excluded from this analysis). Complete infertility of
a whole clutch occurred for 10 clutches (Montagu’s
Harrier: 2, 2, 3, 4, 5 eggs; Hen Harrier: 1, 3, 4, 4, 5
eggs).
As a likely consequence of their larger clutches,
Hen Harrier brood size during the nestling period
averaged higher (3.64 ± 1.29, n = 150, CV = 36%)
than for the Montagu’s Harrier (3.33 ± 1.09, n = 422,
CV = 33%). This difference was significant (F1,571 =
7.82, P = 0.005). Fledged broods were, as expected,
smaller: 3.12 ± 1.05 and 3.41 ± 1.30 for Montagu’s
and Hen Harriers, respectively (F1,456 = 6.36, P = 0.01).
Although few nests were visited several times in
the chick-rearing stage, chick mortality seemed to
occur mainly in the early rearing period. Fledged
brood sizes decreased with laying date in both
species (ANCOVA, species F1,456 = 1.1, P = 0.31, date
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
100
A. Millon et al.
Figure 6. Comparative data on diet according to species during
the period 1993 – 2000. Data are expressed as a percentage of
the estimated contribution in weight. Numbers of prey are above
histograms.
Figure 5. Average estimated clutch size and brood size at fledging (including unsuccessful nests) according to week of laying.
Week 1 = first week of April. For sample sizes, see Fig. 3.
F1,456 = 11.0, P = 0.001, interaction F2,737 = 0.9, P =
0.36; Fig. 5).
There were significant effects of year and areas
on fledged brood sizes in Montagu’s Harrier (area
F2,320 = 0.59, P = 0.55, year F7,320 = 5.97, P < 0.0001,
interaction F14,320 = 1.0, P = 0.46). In Hen Harrier,
there was also an effect of year on fledged brood
sizes (F7,104 = 3.96, P = 0.0006), which was mainly
due to 1997 (see Fig. 7). Variation in brood size
at fledging was slightly greater in Hen Harrier
(CV = 38%) than in Montagu’s Harrier (CV = 33%).
Diet
Across the whole data set, the prey species found
most frequently in pellets was the Common Vole
Microtus arvalis, which represented 53.7% of all
prey (Table 3). Passerines or other non-game birds
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
were the second most important prey type, especially Skylarks Alauda arvensis (64% of identified
passerines, n = 850) and Yellow Wagtail Motacilla
flava in Crayeuse. For the Montagu’s Harrier, the
frequency of occurrence of passerines in Barrois was
lower than in Crayeuse in any year (Table 3). The
frequency of occurrence of the three main prey
categories (voles, passerines and others) differed
significantly between the two species in Crayeuse
(G-test, G = 97.0, P < 0.001) and between Crayeuse
and Barrois for the Montagu’s Harrier (G = 290.4,
P < 0.001; see Table 3 for values). Between-year
variations were significant for Montagu’s Harriers in
Barrois (G = 220.3, P < 0.001), but not in Crayeuse
either for Montagu’s (G = 13.95, P = 0.08) or for
Hen Harriers (G = 7.04, P = 0.53).
Voles and passerines provided 76% and 83.3% of
the biomass consumed by Hen Harrier and Montagu’s
Harrier, respectively (Fig. 6). Voles constituted the
main contribution in biomass for the Montagu’s
Harrier in Barrois (45.5%) and for the Hen Harrier
(42.7%), while passerines formed the main category
for the Montagu’s Harrier in Crayeuse (48.9%). Game
birds accounted for more than 10% of the biomass of
Hen Harrier diet (Fig. 6).
Diet diversity was similar for the Montagu’s
Harrier in both regions (0.496 in Barrois vs. 0.469
in Crayeuse) but was lower in Hen Harrier (0.377).
The competition index, calculated with data from
0.4
452
0
437
0
218
0
453
0
489
14.8
317
0.3
298
2.0
358
1.3
519
0.6
674
16.1
554
25.8
360
30.7
626
4.7
675
11.7
622
1.5
1.8
3.2
0.5
0.3
0
0
0.1
0.3
0.2
0.3
0
0
0
0.7
0.3
2.3
0.4
0
0
0.7
0.7
0.3
0.3
1.4
0.4
4.2
2.0
0.5
0
3.8
1.8
24.3
11.0
2.1
17.8
7.8
0
16.5
3.4
0
10.4
7.6
0
16.6
5.8
0
19.4
3.6
0.5
11.8
12.9
4.6
22.0
13.1
4.6
27.9
7.0
3.9
22.1
11.7
2.0
20.8
4.7
4.7
19.6
5.9
4.9
17.4
7.7
4.6
22.3
3.7
1.8
20.6
63.7
1.5
64.1
0.2
69.3
0.9
66.5
1.6
51.7
0.3
50.0
0.7
52.5
1.4
39.3
1.9
53.7
1
55.3
0.9
38.9
1.1
56.8
1.4
30.8
2.4
64.4
5.8
Prey species /category
Mammals
Common Vole
Lagomorphs
Passerines
Skylark
Yellow Wagtail
Other passerines
Gamebirds
Quail
Partridge
Insects
Green Grasshoppers
Number of prey items
1996
2000
1999
1998
Year
1996
1997
Barrois (1 & 2)
Study area
54.5
2.0
1999
1998
1998
1997
Crayeuse
1999
2000
1996
1997
Crayeuse
Hen Harrier
Montagu’s Harrier
Table 3. Comparative data on diet, according to species, study areas and year. Data are expressed as frequency of occurrence in diet (n = 7052 prey items).
2000
Comparative breeding biology in harriers
101
Crayeuse, revealed a high overlap between the two
species (aMontagu’s,Hen = 1.03, aHen,Montagu’s = 0.88;
see also Fig. 6), while the asymmetry of index value
suggests that the Montagu’s Harrier suffered more
inter- than intraspecific competition than did the
Hen Harrier.
Conservation effort
Types of intervention varied between areas. In Crayeuse,
nest transfers dominated (i.e. transfer of the nestlings
to an artificial nest site, generally from barley to
wheat, but sometimes from wheat to potatoes, beet,
sunflower or other). In the other two areas, the most
common intervention technique consisted of erecting a fence around the nest (c. 1.5 m diameter), to
protect nestlings during harvesting and from predators after harvesting. This technique was abandoned in Crayeuse because nests were too visible,
and were systematically destroyed by humans after
harvesting.
Combining data for both species, 666 chicks
(44.9%) were saved from harvesting operations, i.e.
46.2% and 42.3% of Montagu’s and Hen Harrier,
respectively (Fig. 7). The difference between the two
species was not significant (χ21 = 1.8, P = 0.18),
but when considering only data from Crayeuse,
Montagu’s Harrier necessitated more intervention
than Hen Harrier (respectively 54.2% and 42.3%;
χ21 = 13.2, P = 0.0003), probably resulting from
species differences in breeding phenology. There were
significant differences in intervention rate between
areas for the Montagu’s Harrier (G = 27.8, P < 0.001)
with Barrois 2 requiring fewer interventions as a
consequence of delayed cereal growth, resulting
in both delayed harvesting and weaker nest-site
selection for barley (Fig. 7). Four interventions were
attempted at the egg stage, but all failed. Nests in
winter barley necessitated intervention in 97.6%
of cases (n = 211) for Montagu’s Harrier, and in
85% for Hen Harrier. In contrast, nests in wheat
showed a more variable pattern: intervention rate
varied between 17% and 55% (average = 34%) for
Montagu’s Harriers and 0% and 33% (19%) for
Hen Harriers. Between-year variations (Fig. 7) were
apparently more related to crop growth (presumably
mediated by weather in spring) than to laying date,
as was suggested by 1997, when a spring drought
delayed wheat growth, and Harriers strongly selected
barley for nesting because it was taller. All nests in
other crop types (rye-grass, alfalfa, peas, rape, etc.)
needed intervention (n = 21).
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
102
A. Millon et al.
Figure 7. Conservation effort according to year, study areas and species. Chicks fledged with intervention are compared to those
fledged without intervention.
DISCUSSION
Comparative breeding biology of
Hen and Montagu’s Harriers
Between our three study areas, the distribution and
breeding density of the Montagu’s Harrier was
homogeneous, whereas Hen Harriers were almost
absent from Barrois. The numbers of Hen Harriers
breeding in the seminatural habitat (i.e. forest) of
Barrois have strongly declined during the last 10 years
(our unpubl. data). Why Hen Harriers did not shift
to crops in this area is unknown. In Crayeuse as well
as in several other regions of France, the Hen Harrier
seems to have increased in both numbers and distribution (Tombal 1999).
Both harrier species showed a strong preference
for winter cereals, especially winter barley, despite
its low availability. Such crop selection was particularly pronounced in the Montagu’s Harrier during
spring droughts, such as in 1997, and was more
marked for earlier breeding pairs. Conversely, Hen
Harriers did not show such a temporal pattern in
habitat choice. Winter barley starts growing earlier
than wheat, the latter being especially short at the
time of harrier settlement, if there is no spring rain.
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
This may suggest that habitat choice in the Montagu’s
Harrier is critically dependent on crop height at
laying, while Hen Harriers may be less dependent on
vegetation height (see also Schipper 1978, Simmons
& Smith 1985, Redpath et al. 1998). Hen Harriers also
started laying significantly earlier than Montagu’s: the
difference was about eight days, as in some other areas
(11 days in central Spain: Arroyo 1996). Productivity
also decreased with laying date for both species.
We suggest that part of the difference in breeding
parameters between the two species may result from
difference in breeding phenology: Hen Harriers laid
more eggs and reared more fledglings than Montagu’s
Harriers, possibly or partly because they bred earlier.
Diet was very similar in the two species, with only
minor differences in frequency of occurrence or in
biomass, resulting in a large dietary overlap. Such
overlap has been found elsewhere (e.g. central Spain:
J.T. Garcia & B.E. Arroyo pers. comm.; Schipper
1973), though different prey were involved. Montagu’s
Harriers are also known to follow vole cyclical fluctuations in western France, with breeding density
and clutch size being highly variable and strongly
correlated with spring vole abundance (Butet &
Leroux 1993, Salamolard et al. 2000). This is equally
true for the Hen Harrier in some areas (Redpath
Comparative breeding biology in harriers
& Thirgood 1999, V. Bretagnolle & B.E. Arroyo
unpubl. data). Because voles were an important
prey in our harrier populations, we expected that
Montagu’s and Hen Harrier densities would vary
greatly between years. Surprisingly, we only detected
weak temporal variation in density of Montagu’s
Harrier, which was mainly due to Barrois areas. In
this species, slight annual variations were also detected
for clutch size, laying date and diet, but the clutch
size coefficient of variation among years was much
weaker than in western France (28% against 50%,
Salamolard et al. 2000). Three non-exclusive hypotheses may account for this low and unexpected
interannual variation. First, it is possible that vole
numbers are not cyclical in this area of eastern France
(however, vole abundance data are not available).
Secondly, our study might have been too short to
detect cycles. Thirdly, it is possible that vole densities
were too low in eastern France to allow harriers to
specialize there, and so they behave as generalist
predators. In support of the last hypothesis, there is
some evidence that harriers in eastern France are
limited by food supply: harrier breeding densities
were low in our study compared with other areas
(e.g. > 30 breeding pairs / 100 km2 in peak vole years:
Arroyo & Bretagnolle 2000). Moreover, first-year
female Montagu’s Harriers rarely bred (< 5%), very
few first-year male Hen Harriers bred, and no cases
of polygyny were observed in Hen Harriers. In areas
where food is abundant, these three parameters are
reversed (Balfour & Cadbury 1979, Salamolard et al.
2000, V. Bretagnolle, A.B.A. Leroux & B.E. Arroyo
unpubl. data).
Conservation and future of harriers
in cultivated areas
The harrier breeding population of the Champagne–
Ardenne region stands at about 250–500 pairs of
Montagu’s Harrier and 200–400 pairs of Hen Harrier
(our unpubl. data), and thus is numerically one of
the three most important harrier populations in
France (Salamolard et al. 1999). Today, virtually all
Montagu’s Harriers in Champagne–Ardenne breed
in cereal crops, whereas a significant proportion of
the Hen Harriers still nests in natural or seminatural
habitats (< 15%), especially young forest plantations,
fallows or marshes. The Marsh Harrier breeds in
reedbeds where it is strongly declining, and the first
instance of breeding in cereal crops was recorded in
1991. Since then, one to five Marsh Harrier pairs have
bred annually in cereals. Therefore, all species of
103
harriers in this region have shifted to some extent from
natural habitats to cereal fields, as recorded elsewhere in France (Leroux 1994, Salamolard et al. 1999).
Historically, the Montagu’s Harrier was apparently
the first of the western European harriers to colonize
crops. In eastern France, the shift to crops might
have followed the draining of its original breeding
habitats, i.e. marshes and wet meadows (Paris 1906,
Erard & Spitz 1964) and this may have occurred
as early as the beginning of the twentieth century
(Frionnet 1925). It is likely, however, that the main
shift occurred in the 1950s and 1960s, when modification of agricultural practices drastically altered
the landscape (Pain & Pienkowski 1997). In Europe,
the proportion of Montagu’s Harriers nesting in cereals
increases westward (Arroyo 1995). Polish harriers
have only recently begun to shift from marshes to
crops (Krogulec & Leroux 1994), while > 80% of the
Russian harrier population still breeds in natural
and seminatural habitats, at least in the 1980s (Flint
et al. 1984).
The late breeding phenology of Montagu’s Harrier,
and the fact that almost all the breeding population
nests in crops, make their future entirely dependent
on conservation programmes. Without intervention,
between 50% and 90% of the nests would be destroyed
each year by harvesting (see also Arroyo & Bretagnolle
2000). Farming intensification has allowed earlier
crop harvesting, and harriers (particularly Montagu’s
Harriers) are therefore of great conservation concern
in Europe. Currently, more than 1000 nests are protected each year in both Spain and France. Harrier
population trends are not known in these countries
(Arroyo & Pinilla 1996, Salamolard et al. 1999), but
trends observed at smaller scales suggest stability or
decline (e.g. Berthemy et al. 1983, Armouet 1994,
Krogulec & Leroux 1994, Albert 1987–1998, but
see Fève 1994).
Recent changes in agricultural practices, e.g. ploughing of permanent cover or extensive use of rodenticides, are suspected to affect vole cycles. Montagu’s
Harrier population dynamics differed strongly between
eastern and western France, but also slightly between
Crayeuse and Barrois. Harriers depended more on
voles in Barrois than in Crayeuse, and yearly density
variations were higher in Barrois than in Crayeuse. Thus,
harrier populations may exhibit different dynamics
in western France, Crayeuse and Barrois. We suggest
that these differences reflect a gradient of agricultural intensification from western France, Barrois
to Crayeuse, along which vole fluctuations become
weaker and finally disappear. Direct persecution of
© 2002 British Ornithologists’ Union, Ibis, 144, 94 – 105
104
A. Millon et al.
chicks and adults may further affect populations,
although the scale of its impact is unknown in eastern
France (see Etheridge et al. 1997 for data on Scottish
harriers). As a last potential threat, the recent increase
of Hen Harriers in cultivated areas may pose a
problem in the long term for the slightly smaller
Montagu’s Harrier. The two species seem to overlap
much in their habitat, general breeding biology and
diet. This may lead to interspecific competition,
although we did not detect interspecific agonistic
interactions. The Hen Harrier, being more sedentary,
should respond better than the long-distance migrant
Montagu’s Harrier in cereal environments, since it is
less constrained by breeding phenology, by vegetation height for nesting, and by harvesting dates.
We are particularly grateful to Pascal Albert and Serge
Paris for their considerable help with fieldwork, as well as
S. Bellenoue, S. Gaillard, S. Garet, J.-P. & E. Girardot,
R. Guichon, G. Leveau, O. & M.-C. Paris, C. Putois and J.-C.
& M.-H. Rocquet for helping in protection effort. To all of
them we express our gratitude for their kindness, and
for allowing us to use their data. B.E. Arroyo, A. Leroux,
S. Redpath, J.-M. Thiollay and two anonymous referees
have commented on previous drafts of this paper and
made many constructive comments which greatly improved
its content. Finally we thank F. Cézilly, E. Chaffot and
S. Challan-Belval for facilities at Dijon University and
D. Crozier (DDAF-Aube) for providing data on land use.
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Received 2 February 2000; revision accepted 5 April 2001
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