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E�ects of agricultural intensi®cation on the breeding
success of corn buntings Miliaria calandra
NICK W. BRICKLE*{{ , DAVID G.C. HARPER*x, NICHOLAS
J. AEBISCHER{ and SIMON H. COCKAYNE*{*School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK; and {The Game
Conservancy Trust, Fordingbridge, Hampshire SP6 1EF, UK
Summary
1. Corn buntings Miliaria calandra have declined steeply in Britain and north-wes-
tern Europe since the mid-1970s; changes in farming practice are believed to have
been partly responsible.
2. We studied nesting corn buntings on the South Downs in west Sussex between
1995 and 1997 to examine the possible e�ects of agricultural intensi®cation on
breeding success. The abundance of invertebrates around individual nests was
sampled by sweep-netting in July.
3. Corn buntings provisioning nestlings foraged in grassy margins more than any
other habitat relative to their availability within the maximum foraging range. The
other habitats used more than expected were spring-sown barley, unintensi®ed
grass and set-aside. Those used less than expected included winter-sown wheat and
intensively managed grassland. The invertebrates most commonly fed to chicks
were more abundant in foraging areas than elsewhere. Their density was negatively
correlated with the number of insecticide applications both when cereal ®elds only
were considered and when all foraging habitats were included.
4. The lower the abundance of chick-food invertebrates close to nests, the greater
the distance from the nest at which parents foraged, and the longer such trips were
in duration. The weights of nestlings, corrected for age using tarsus length, were
positively correlated with the abundance of chick-food invertebrates.
5. The probability of nest survival was negatively correlated with the abundance of
chick-food invertebrates close to the nest, apparently as a result of an increased
risk of predation.
6. Agricultural intensi®cation in Britain, including the increased use of pesticides,
has led to a widespread decrease in the availability of chick-food invertebrates on
lowland farmland. If our results are typical of corn buntings in an arable environ-
ment, this decrease correlates with reduced breeding success. Depending on the
mortality rates for ¯edged chicks and older birds, this reduction may have contrib-
uted to the corn buntings' decline and may hamper recovery.
7. Farming practices that increase invertebrate availability ought to bene®t breed-
ing corn buntings. Large-scale measures such as set-aside and the spring-sowing of
cereals (especially if undersown with grass) depend heavily on overall agricultural
policy. Small-scale initiatives might therefore be more feasible; these include the
provision of grassy margins or beetle banks and selective spraying of headlands.
Key-words: cereals, farmland birds, nest survival, pesticides, population declines.
Journal of Applied Ecology (2000) 37, 742±755
xCorrespondence:David Harper (e-mail [email protected]).
{Present address: Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2DL, UK.
Journal of
Applied Ecology
2000, 37,
742±755
# 2000 British
Ecological Society
Introduction
Since the mid-1970s, corn buntings Miliaria calandra
(L.) have declined sharply in numbers over much of
their European range (Tucker & Heath 1994; Hage-
meijer & Blair 1997), including Britain (Siriwardena
et al. 1998) where they have become one of the 36
species of greatest conservation concern (Gibbons
et al. 1996). Their plight is shared by many other
farmland species, leading to the hypothesis that
changing agricultural practices over the period of
the decline have been harmful (Potts 1991; Fuller
et al. 1995). Most of the changes in farming since
the mid-1970s have been related to intensi®cation.
On tilled land the main changes have been a dra-
matic increase in the use of pesticides and inorganic
fertilizers (Church & Leech 1983; Davis, Garthwaite
& Thomas 1991; Campbell et al. 1997), a switch
from spring- to autumn-sowing of most cereals
(O'Connor & Shrubb 1986; Evans 1997), and the
simpli®cation of crop rotations with the loss of
grass leys (Shrubb 1997). In pastoral areas, silage
has replaced hay as the most common grass crop
and stocking rates on pasture have increased
(O'Connor & Shrubb 1986). During the period of
intensi®cation, the polarization of farms (and
regions) into predominantly arable or predomi-
nantly pastoral has increased. The consequent
reduction in habitat diversity has been exacerbated
by the conversion of considerable areas of pre-
viously non-productive land, such as hedgerows, to
productive agricultural use (O'Connor & Shrubb
1986).
The decline of the corn bunting has frequently
been attributed to reduced over-winter survival, pos-
sibly caused by the decreased availability of stubble
®elds (Donald, Wilson & Shepherd 1994; Donald &
Evans 1994, 1995; Siriwardena et al. 1999). For
example, Shrubb (1997) highlighted the loss of tradi-
tional rotations, suggesting that undersown stubbles
had been a critical source of winter food. Unfortu-
nately there are no data on changes in survival or
components of ®tness likely to in¯uence survival,
such as body condition. Although nest record
scheme data suggest that the success of breeding
attempts increased in parts of Britain during the
population decline (Siriwardena et al. 1999), pro-
blems during the breeding season may have contrib-
uted in other areas (Aebischer & Ward 1997;
Donald 1997). The collapse of the corn bunting
population in Schleswig-Holstein, Germany, corre-
lated with increased chick starvation (Busche 1989).
Most of the invertebrates eaten by nestling corn
buntings in southern England (Aebischer & Ward
1997; Brickle & Harper 1999) have declined in abun-
dance on lowland farmland in Britain (Aebischer
1991; Campbell et al. 1997; Donald 1998). Reduced
availability of invertebrate food for chicks has been
implicated in the decline of the grey partridge Perdix
perdix (L.) (Potts 1986) and skylark Alauda arvensis
(L.) (Wilson & Browne 1993; Poulsen, Sotherton &
Aebischer 1998), which overlap considerably in diet
with corn buntings.
Aebischer & Ward (1997) found that the density
of breeding corn buntings on the South Downs,
southern England, was higher in crops associated
with traditional low-input farming than in ones typi-
cal of intensive agriculture. It was also positively
correlated with the density of Lepidoptera and Sym-
phyta larvae in cereal ®elds. We extended their
study by identifying the habitats from which adults
collected food for their chicks and examining the
relationships between pesticide use, abundance of
common chick-food items, foraging behaviour and
breeding success.
Methods
STUDY AREA
Between 1995 and 1997, breeding corn buntings
were studied on farmland on the South Downs,
north of Worthing, west Sussex. The study area of
10 km2 was part of that described by Potts (1986)
and Aebischer & Ward (1997). Just over half was
covered by tillage (winter-sown wheat, 35% of study
area on average; spring-sown barley, 12%; brassicas,
4%) and just under a third was grass (intensively
managed grass, 31%; unintensi®ed chalk grassland,
1%). Field margins provided another 1% of grassy
and rank vegetation, and about 9% of the area was
set-aside (all non-rotational; for the history of set-
aside in Britain, see MAFF 1999). The remaining
7% of the area consisted of habitats of little or no
importance to foraging corn buntings, such as
woodland, scrub, buildings and roads.
NEST RECORDS
Singing males were mapped and their territories sys-
tematically watched for at least 1 h at intervals of no
more than 3 days from mid-April until mid-August.
Nests were found by watching the adults, and nest
locations were marked at a distance using stones or
knots of grass. We visited nests as little as practical
(never on successive days) and care was taken when
visiting nests to leave no obvious trail. Analyses
involving clutch or brood size included only nests
for which the exact number of eggs or chicks was
known. Partial loss of eggs was extremely rare.
Brood size was that recorded when chicks were seen
for the ®rst time. Daily survival probabilities of
nests (see below) were used to compare breeding
success, to avoid biasing the analysis towards suc-
cessful nests. Nests were considered to be successful
at the incubation stage if one or more eggs hatched,
while they were considered successful at the nestling
stage if one or more chicks ¯edged. Whenever possi-
743N.W. Brickle
et al.
# 2000 British
Ecological Society
Journal of Applied
Ecology, 37,
742±755
ble, the species of predator destroying a nest was
identi®ed from ®eld signs (Brown, Lawrence & Pope
1984; Brown et al. 1987). Owing to their short legs
and large size, European badgers Meles meles (L.)
left obvious `paths' to and from the nest and ¯at-
tened the surrounding area, which often contained
other badger signs.
About 5±7 days after hatching, chicks were
removed to about 15m from the nest, weighed to
the nearest 0�1 g using a spring balance (Salter Super
Samson, 50 g; Salter Weighing Products, 3620 Cen-
tral Ave, Minneapolis, MN, USA), and their tarsus
length measured to the nearest 0�1mm using dial
callipers (method A of Svensson 1992). Mean chick
weight of each brood was corrected approximately
for age di�erences using mean tarsus length as a
covariate in the analysis. Food shortage has a much
greater impact on nestling weight than on the
growth of the tarsus (Konarzewski et al. 1996).
Although recently independent males are about
30% heavier than females (Harper 1995), sexual
dimorphism among nestlings of 5±7 days of age is
slight, with males and females growing at similar
rates (D.G.C. Harper, unpublished data). We esti-
mated that the mean condition index (weight
divided by tarsus length) of male nestlings would be
only about 8% higher than that of females.
FORAGING TRIPS
Foraging distance and duration were determined by
watching foraging adults; means were calculated for
each nest. Observations were made 1±6 days after
hatching between 08:00 and 15:00 GMT (foraging
trips become shorter late in the day; Gillings &
Watts 1997). Foraging distance was recorded as the
maximum straight-line distance from the nest
reached on a foraging trip, estimated from 1 : 10 000
maps. The duration of a trip was the time in seconds
between leaving and returning to the nest.
FORAGING HABITATS
Habitat use by birds collecting food for nestlings
was investigated by comparing the proportion of
foraging visits to each habitat with the proportional
availability around the nest. In order to calculate
availability of habitats, a circle with a radius of 346
m was used, corresponding to the maximum
observed foraging distance. We chose this distance
because we wanted to compare habitat use with a
biologically meaningful measure of the habitats
available to a foraging parent (rather than of the
most heavily used area). Eight habitat types were
de®ned: winter-sown wheat; spring-sown barley;
intensively managed grass (both rotational and non-
rotational); unintensi®ed grass (chalk downland, not
grazed); brassicas (oilseed rape, kale); grassy margin
(tussocky grass margins, often deliberately provided
for nesting game birds, usually under 3m wide); set-
aside; other (all other habitat types, such as hedges,
tracks, buildings, bare ground, etc.).
For each ®eld, information on the use of herbi-
cides, foliar fungicides and insecticides was obtained
from the farmers. To investigate the relationship
between foraging and pesticide use, the mean num-
bers of applications of each pesticide group (insecti-
cide, fungicide, herbicide) were compared between
foraging and non-foraging areas within 346m of the
nest. Only areas that were sprayed at least once with
a pesticide were used. This included all cereal ®elds;
if no foraging visits were made to a cereal ®eld, the
nest was excluded from the analysis. Unsprayed
areas were excluded from the analysis in this way
because they were heterogeneous. For example, con-
crete tracks, intensively managed grass and uninten-
si®ed grassland all received no pesticide applications
but di�ered markedly in food availability (see
below, Fig. 2) for other reasons. Including these
habitats could spuriously mask any relationship
between pesticide use and foraging location.
INDEX OF INVERTEBRATE ABUNDANCE
Sweep-netting was used in 1996 and 1997 to sample
the abundance of four of the main chick-food inver-
tebrates: Opiliones, Lepidoptera larvae, Symphyta
larvae and Orthoptera. These invertebrates were
known to be major components of chick diet on our
study site (each group was fed to more than 50% of
39 broods and contributed at least 10% of 720
invertebrate items found in faeces; Brickle & Harper
1999). Together these four groups accounted for
62% of the dietary items. One important group,
Araneae, was excluded despite being fed to 80% of
broods and making up 19% of invertebrates in
faeces. Remains in faecal samples revealed that nest-
lings were usually fed lycosid spiders much larger
than the small linyphiid spiders that were numerous
in the sweep-net samples. Large ground-living inver-
tebrates like lycosids are poorly sampled by sweep-
netting (Southwood 1978) and suction sampling
(Richmond & Graham 1969). Constraints of time
and personnel prevented the use of pitfall trapping
(Southwood 1978). Thus we had no meaningful esti-
mate of spider availability to corn buntings. It is
hard to see how excluding spiders could have gener-
ated artefactual results; the density of spiders col-
lected by suction sampling was negatively correlated
with that of corn bunting territories (Aebischer &
Ward 1997).
The e�ciency of sweep-netting varies between
habitats (Southwood 1978), but as most of those on
the study site were dominated by grasses, containing
few herbs, this bias was probably small. Sweep-net-
ting has been widely used in dietary studies of grass-
land birds (Robel et al. 1995). As the vertical
distribution of invertebrates can vary diurnally and
744Farming practices
and corn buntings
# 2000 British
Ecological Society
Journal of Applied
Ecology, 37,
742±755
with the weather (Southwood 1978), sampling was
carried out only in the morning and on clear dry
days.
Twenty sweeps made during the ®rst week of
July, at the height of the breeding season, consti-
tuted one sample. Samples were taken in every dis-
crete habitat block within the study area. These
habitat blocks were determined by their apparent
uniformity and the geography of the farms, and
included such features as individual ®elds, grass
margins, track sides, abandoned dew ponds, unin-
tensi®ed grass banks and so on. For large ®elds
(greater than about 10 ha), the mean of up to three
samples was used. Samples were transferred to plas-
tic bags and stored in a freezer. To identify contents,
the sample was placed in a tray of dilute industrial
methylated spirits and sorted under a binocular
microscope. There were few hedgerows or wood-lots
on the study site (less than 3% total area). Those
within the foraging range of a nest were assigned a
zero invertebrate count as birds were never seen to
forage in trees or bushes.
In order to assess the e�ect of pesticide use on
food availability, the sampled abundance of the four
invertebrate groups was compared with pesticide use
for each discrete habitat block. As with the earlier
analysis of pesticide use, problems arose due to the
diverse nature of unsprayed habitats. In this case,
however, the analysis was ®rst restricted to areas
that were sprayed at least once and then repeated
using all foraging habitats (cereal ®elds, set-aside,
grassy margins and unintensi®ed grass; see the
Results). Including all unsprayed areas was unsuita-
ble for the reasons outlined above.
To investigate the relationship between foraging
location and food availability, the abundance of the
four invertebrate groups (weighted by the area of
each habitat block) was compared for the foraging
and non-foraging areas within 346m of each nest.
In this way the sampled abundance of invertebrates
around each nest was used to calculate a value for
food abundance for that nest. The invertebrate
groups were combined to allow for individual par-
ents responding to locally high abundance of a par-
ticular group.
When comparing breeding success with food
availability, the area within 346m of the nest was
inappropriate because large, but variable, propor-
tions of it fell inside other territories. Females rarely
foraged in other territories, possibly because they
were harassed by males, including their mate, while
doing so. The weighted density of the four inverte-
brate groups within 115m of each nest (one-third of
the maximum observed foraging distance) was used
to reduce this problem. Densities for each habitat
block within 115m of the nest were weighted by the
area of that block within the circle described by this
radius. To test hypotheses about foraging distances,
the most appropriate measure was the abundance of
invertebrates close to the nest. Although a 115-m
limit was clearly arbitrary, it provided a biologically
more meaningful measure than using the maximum
foraging distance, and was chosen before analysis.
STATISTICS
Tests were two-tailed and used a signi®cance level of
P� 0�05. Unless stated otherwise, means are
expressed �1 SE. Analyses were made using SPSS
Version 7 (NorusÏ is 1996), Genstat Version 5 (Digby,
Galway & Lane 1989) and specially written rando-
mization programs (see the Acknowledgements).
Four variables were transformed in order to nor-
malize their distributions, which were skewed away
from zero. For invertebrate availability, the trans-
formation used was ln(x� 1) and for the means of
chick weight, foraging distance and foraging dura-
tion it was ln(x).
To compare the habitats used for foraging with
the availability of habitats around the nest, we
applied compositional analysis (Aitchison 1986).
This technique is required for all analyses of sets of
proportions summing to 1 and is described by
Aebischer, Robertson & Kenward (1993) and
Aebischer et al. (1993). Proportional use and pro-
portional availability of habitats were converted to
log-ratio di�erences (using nests as the units of ana-
lysis), which were then analysed by multivariate ana-
lysis of variance. The test statistic was Wilk's L(Kendall 1980), which varies between 0 and 1, with
larger values indicating that group means are more
similar. If the log-ratios did not meet the assump-
tions of normality required by MANOVA, the level
of signi®cance associated with L was determined by
reference to its rank among 999 values obtained
after data randomization (Manly 1997). If there
were missing values, Wilk's L and F statistics were
weighted using the method described by Aebischer,
Robertson & Kenward (1993). Where habitat use
was signi®cantly non-random, to identify which
habitats were used less or more than expected by
their availability, single sample t-tests were made on
the log-ratio di�erences obtained from all possible
combinations of habitat types, to test for signi®cant
departure from zero. Although for D habitat types,
D(Dÿ 1) tests were involved, they were not inde-
pendent (only those within a same row or same col-
umn were independent). Given that the tests were
carried out only when L indicated signi®cant depar-
ture from random use, we used standard signi®cance
levels for t by analogy with the protected least-sig-
ni®cant di�erence method (Carmer & Swanson
1973; Snedecor & Cochran 1980). For clarity, each
t-value was replaced by its sign and placed in a
matrix where the rows corresponded to habitat
types in the numerator and columns to ones in the
denominator (Aebischer, Robertson & Kenward
1993). A triple positive sign in this matrix indicated
745N.W. Brickle
et al.
# 2000 British
Ecological Society
Journal of Applied
Ecology, 37,
742±755
that the row habitat was used signi®cantly more
than the column habitat relative to their availability;
a triple negative sign indicated the opposite.
Daily failure probabilities of nests were calculated
using the May®eld method (May®eld 1961, 1975;
Johnson 1979; Hensler & Nichols 1981; Hensler
1985), using data from all 3 years. Nest survival
probabilities were compared with food availability
(1996±97 only) using an extension of the May®eld
method (Aebischer 1999), involving logistic regres-
sion to test for the e�ects of continuous variables
and factors (in our case, food availability and year,
respectively). Only nests visited at least twice could
be included. Where the exact date of failure was not
known, the mid-point between possible days was
used.
The relationships between foraging duration,
foraging distance and food availability, and between
clutch size, brood size and food availability, were
assessed by multiple regression after entering year in
the regression model to remove any year e�ect.
Similarly, in multiple regressions involving mean
chick weight, year and mean tarsus length were
entered ®rst to remove any e�ects of year or body
size (mainly due to age, see above). Variables could
be entered or removed from the model during analy-
sis. Normality tests (Shapiro±Wilk, Kolmogorov±
Smirnov) were performed on the residuals after each
regression. In no case was there a signi®cant depar-
ture from normality.
Results
FORAGING HABITATS
The foraging locations of birds feeding nestlings
were observed for 67 nests (seven in 1995, 32 in
1996, 28 in 1997). The mean number of foraging vis-
its observed for each nest was 4�0�2�2 (range 1±11)
and the maximum foraging distance was 346m. Few
adults were colour-ringed, but most nests appeared
to belong to monogamous pairs because we rarely
found two nests simultaneously on the same terri-
tory. At no nest did we suspect that more than one
bird was feeding the young; judging by their size
and behaviour, these adults were females (Cramp &
Perrins 1994).
Habitat use relative to availability did not vary
signi®cantly between years (Wilk's L� 0�762, P�0�178). Corn buntings did not forage in habitats at
random (Wilk's L� 0�122, P� 0�001). Grassy mar-
gins were the habitats used most relative to their
availability, followed by spring-sown barley, unin-
tensi®ed grass and set-aside (Table 1). These habitats
were all used signi®cantly more than winter-sown
wheat, brassicas, intensively managed grass and
other habitats (Fig. 1).
FORAGING HABITATS AND INVERTEBRATE
ABUNDANCE
The density of the four common chick-food inverte-
brates (combined) varied signi®cantly between habi-
tats, with no year e�ect or interaction (Fig. 2;
habitat, F7,260� 46�96, P<0�001; year, F1,260� 1�86,P� 0�174; interaction, F7,260� 1�38, P� 0�216).Highest densities were found in unintensi®ed grass-
land and ®eld margins, which contained at least
eight times as many invertebrates as the poorest
habitats (intensively managed grass, winter-sown
wheat and `other'). The rank order of habitat use
relative to availability (Table 1) was signi®cantly
correlated with mean food availability (Spearman
rank r� 0�786, n� 8, P� 0�021).
FORAGING AND PESTICIDE USE
Corn buntings foraged in cereal ®elds that, on aver-
age, had received fewer applications of herbicide,
fungicide and insecticide compared with sprayed
areas within 346m of the nest that were not used
(paired t-tests: herbicides, t28� 3�22, P� 0�003; fun-
Table 1. Summary of di�erences in the use of habitats relative to their availability within 346 m of 67 corn bunting nests.
Each entry in the matrix represents the results of a single-sample t-test. A positive sign indicates that the row habitat was
used more than the column habitat relative to availability; a negative sign means the opposite. If the sign is tripled, the dif-
ference was signi®cant at P� 0�05. Ranked habitat use relative to availability was calculated by summing positive signs in
each row (8 � most used habitat relative to availability). Sample sizes for each test varied between 9 and 55; NA indicates
that there were insu�cient data to conduct test
Habitat W Ba I U Br M S O Rank
Winter-sown wheat (W) ± ± ± � ± � ± ± ± ± ± 3�5Spring-sown barley (Ba) ��� ��� � ��� ± � ��� 7
Intensively managed grass (I) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 1
Unintensi®ed grass (U) � ± ��� NA ± � � 5�5Brassicas (Br) ± ± ± ± ��� NA ±± ± ± ± ± ± 2
Grassy margin (M) ��� � ��� � ��� ��� ��� 8
Set-aside (S) � ± ��� ± ��� ± ± ± ��� 5�5Other (O) � ± ± ± ��� ± ± ± ± ± ± ± ± 3�5
746Farming practices
and corn buntings
# 2000 British
Ecological Society
Journal of Applied
Ecology, 37,
742±755
gicides, t28� 2�15, P� 0�040; insecticides, t28� 2�58,P� 0�016; Fig. 3). All cereal ®elds were sprayed at
least once.
INVERTEBRATES AND PESTICIDE USE
Pesticide data were available for 53 sprayed cereal
®elds. The abundance of the four common chick-
food items (combined) was signi®cantly negatively
correlated with the number of insecticide applica-
tions (insecticide, B�SE�ÿ0�114�0�038, t52�ÿ2�965, P� 0�005, R2� 0�147) but similar trends for
the number of herbicide or fungicide applications
were non-signi®cant (herbicide, B�SE�ÿ0�035�0�029, t52�ÿ1�213, P� 0�231, R2� 0�028; fungicide,B�SE�ÿ0�011�0�021, t52�ÿ0�521, P� 0�604,R2� 0�005). When all foraging habitats were consid-
ered (n� 171 habitat blocks), the abundance of
chick-food items was signi®cantly negatively corre-
lated with the number of applications of all three
types of pesticide (insecticide, B�SE�ÿ0�259�0�040, t170�ÿ6�480, P<0�001, R2� 0�199; herbi-
cide, B�SE�ÿ0�091�0�022, t52�ÿ4�148, P<
0�001, R2� 0�092; fungicide, B�SE�ÿ0�150�0�023, t52�ÿ6�408, P<0�001, R2� 0�195).
Fig. 1. Mean percentage of foraging visits made to each of eight habitat categories (abbreviations in Table 1) plotted against
mean percentage availability within 346m of nest. If habitats were used for foraging in proportion to their availability, the
data would follow the straight line.
Fig. 2. Mean density (number per 20 sweeps) of chick-food
invertebrates (Orthoptera, Lepidoptera larvae, Symphyta
larvae, Opiliones) in eight habitat types (abbreviations in
Table 1); bars show 1 SE. Means with the same superscript
(a, b, c, d or e) are not signi®cantly di�erent at the P�0�05 level (Tukey tests).
Fig. 3. Mean number of pesticide applications in sprayed
foraging and non-foraging areas within 346m of nests (n�29 nests); bars show 1 SE.
747N.W. Brickle
et al.
# 2000 British
Ecological Society
Journal of Applied
Ecology, 37,
742±755
FORAGING AND INVERTEBRATE
ABUNDANCE
The four common chick-food items (combined) were
signi®cantly more abundant in sweep-net samples
from foraging areas than in those from non-foraging
areas within 346m of the nest (Fig. 4; paired t-test,
t59� 4�23, P<0�001). The mean weighted density in
foraging areas was 5�31�0�68 compared with 2�41�0�25 in non-foraging areas. This result held when
tested separately for Opiliones (paired t-test: t59�3�27, P� 0�002), Orthoptera (t59� 3�01, P� 0�004)and Symphyta larvae (t59� 5�11, P<0�001), but notfor Lepidoptera larvae (t59�ÿ0�95, P� 0�347).
FORAGING DISTANCE
Data from 60 nests were used in the analysis (32 in
1996, 28 in 1997); the overall mean foraging distance
was 117m. The distribution of mean foraging dis-
tances was heavily skewed, with most values less
than 115m (63%) and only 10% over 230m. Mean
foraging distance was negatively correlated with
food availability (Fig. 5; r59� 0�344, P� 0�007). No
signi®cant year e�ect was apparent.
FORAGING DURATION
Mean foraging duration was not signi®cantly corre-
lated with food availability once the e�ect of mean
foraging distance had been removed and there was
no signi®cant year e�ect. Further investigation was
made of the relationships between food availability
and the distance and duration of foraging trips by
examining every trip of known duration. Foraging
duration was compared with foraging distance and
food availability by ANCOVA with ln(foraging dura-
tion) as the dependent variable, ln(foraging distance)
as a covariate and nest entered as a factor. A further
covariate was added for the sampled availability of
invertebrates in the location of every visit. In this
way the foraging duration could be compared with
the density of invertebrates in the location of that
visit while controlling for the e�ects of nest and
foraging distance. The analysis was based on 96
foraging visits of known duration from 31 nests.
Foraging duration was not signi®cantly related to
food availability, but increased with foraging dis-
tance (food, F1,63� 3�41, P� 0�069; distance, F1,63�5�98, P� 0�017).
CLUTCH AND BROOD SIZES
Clutch size was known for 62 nests (eight in 1995,
25 in 1996, 29 in 1997) while brood size was known
for 49 nests (®ve in 1995, 20 in 1996 and 24 in
1997). Mean clutch size was 4�65�0�62 eggs (range
3±6) and mean brood size was 4�45�0�93 chicks
(2±6). No signi®cant relationships were found
between clutch size or brood size and food availabil-
ity (only 1996 and 1997 data used; partial correla-
tions controlling for year: r51� 0�057, P� 0�686 and
r41� 0�186, P� 0�206, respectively).
CHICK WEIGHT
Data from 27 nests were used (10 in 1996, 17 in
1997). The mean tarsus length was 21�9�2�2mm
(range of brood means 17�7±26�0mm), while the
mean weight was 22�6�3�8 g (range of brood
means 15�7±30�7 g). As expected, the mean weight
and mean tarsus length of broods were positively
correlated [ln(mean weight)� 1�615� 0�068 tarsus
length, F1,25� 94�07, P<0�001], con®rming that
Fig. 4. Mean weighted density of chick-food invertebrates
in foraging and non-foraging areas within 346m of nests
(n� 60); bars show 1 SE. Op, Opiliones; Or, Orthoptera;
L, Lepidoptera larvae, S, Symphyta larvae; C, all four
taxa.
Fig. 5. Mean foraging distance of corn buntings foraging
to provision nestlings [ln(x) transformed] compared with
the weighted density of invertebrate food within 115m of
the nest [ln(x� 1) transformed].
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weights should be adjusted for tarsus length before
further analysis.
Food availability was positively correlated with
chick weight after adjusting for tarsus length in a
multiple regression (Fig. 6; slope � 0�011�0�005,t24� 2�21, P� 0�036). Foraging distance was not sig-
ni®cantly correlated with chick weight after adjust-
ing for age (t24�ÿ0�05, P� 0�961). Foraging
duration was signi®cantly negatively correlated with
chick weight after adjusting for age (slope � ÿ0�103�0�042, t7�ÿ2�49, P� 0�042). No signi®cant year
e�ect was apparent in any of these analyses.
NEST SURVIVAL
Nest outcome varied signi®cantly between years
(Table 2; excluding abandoned nests and pooling the
two predation categories; w42� 20�83, P<0�001).
Partitioning the w2 value showed that the proportion
of nests ¯edging at least one chick did not vary sig-
ni®cantly between years (w22� 3�47, P� 0�176) but
that the relative importance of predation and farm-
ing operations as causes of failure did (w22� 17�35, P
<0�001). Inspection of the data suggested that this
was because of unusually high losses to farming
operations in 1995. Although most nest predators
were unidenti®ed (67%, n� 45) at least 20% were
badgers, which destroyed at least 18% of nests in
1997 but none in the other 2 years.
The May®eld estimate of daily failure probability
was 0�076�0�011 (66 nests) during incubation and
0�046�0�010 (71 nests) during the nestling period.
If incubation lasted 12 days and nestlings ¯edged at
9 days (Crick 1997), the overall survival rate
(excluding laying period and partial losses) was 0�25�0�05 (Hensler 1985). Clutch size and the subse-
quent brood size were known for 41 nests. Eight suf-
fered partial losses because 1±3 eggs failed to hatch.
No partial losses of chicks were recorded, although
the data were incomplete (see the Methods).
Daily failure probabilities were compared with
food availability for the incubation and nestling
stages separately, as the e�ect of food availability is
likely to be di�erent at each stage. Seven nests that
failed owing to mechanical farming practices or
desertion were excluded from the analysis. Only
nests that received two or more monitoring visits
could be used, but this included all nests (total of
89, 51 providing data on clutch survival and 63 for
broods). Nest survival at the egg stage was not sig-
ni®cantly related to food availability (w12� 0�024, P
� 0�876; goodness-of-®t, w472 � 52�26, P� 0�277) but
that at the nestling stage was (w12� 4�45, P� 0�035;
goodness-of-®t, w592 � 52�57, P� 0�710); survival rate
increased with invertebrate availability. There was
no signi®cant year e�ect in either analysis.
Discussion
FORAGING HABITATS
Corn buntings collecting food for their nestlings vis-
ited spring-sown barley, grassy margins, unintensi-
®ed grassland and set-aside more than any other
habitats relative to their availability within 346m of
the nest (Fig. 1 and Table 1). These were the four
habitats with the highest availability of the four
common chick-food items (Fig. 2). Three of these
Fig. 6. Residual mean chick weight [ln(x) transformed, cor-
rected for tarsus length using regression equation] against
the availability of invertebrate food within 115m of the
nest [ln(x� 1) transformed].
Table 2. Outcomes of corn bunting nests on the South Downs from 1995 to 1997
Outcome 1995 1996 1997 Total Total percentage
At least one chick ¯edged 12 28 20 60 50
Predation of eggs 2 12 14 28* 23
Predation of nestlings 2 5 10 17{ 14
Destroyed by farming 8 1 4 13 11
Abandoned 0 0 2 2 2
Total 24 46 50 120
*Badger (5), rodent (3), small mustelid (1), red fox Vulpes vulpes (L.) (1), unknown (18).
{Badger (4), corvid (1), unknown (12).
749N.W. Brickle
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habitats have become scarcer during the last 20
years. The exception is set-aside, which was intro-
duced in autumn 1988 and became common in
autumn 1992 when arable subsidies became condi-
tional on setting land aside (MAFF 1999). Set-aside
is more attractive than winter-sown cereals or grass-
land to a wide variety of birds, a trend which is
strongest for rotational set-aside (Henderson et al.
2000). Thus, our results may underestimate the
national importance of set-aside to corn buntings
because all the set-aside studied was non-rotational
with a rather uniform sward.
Spring-sown barley was used signi®cantly more
relative to availability than winter-sown wheat and
contained a higher density of the four common
chick-food items (Fig. 2). Choice of foraging sites is
likely to involve a trade-o� between food availabil-
ity and other factors such as exposure to predators
and the weather (Grubb & Greenwald 1982).
Because spring-sown barley and winter-sown wheat
are structurally similar while nestlings are being fed,
the di�erence in their relative use by foraging birds
seems most likely to re¯ect di�erences in food avail-
ability (but see Chamberlain et al. 1999). Even in
the absence of spraying, which tends to be more fre-
quent on autumn-sown crops, the density and diver-
sity of wild ¯ora is highest in spring-sown cereals
(Hald 1999).
Most previous studies of habitat use by corn
buntings have not treated the grassy margins of cer-
eal ®elds as a separate habitat. Mùller (1983), how-
ever, found that Danish territories typically included
a length of ®eld margin, ditch or roadside verge.
Nearly a third (29%) of foraging trips were to mar-
gins, which were the most used foraging habitat
relative to their availability in our study. They are
also important for grey partridges (McCrow 1980;
Green 1984; Rands 1985; Aebischer & Blake 1994),
which are similar to corn buntings in some aspects
of their ecology, including chick diet (Green 1984;
Potts 1986). As well as being important foraging
habitats themselves, margins can increase the abun-
dance of some invertebrates, such as ground beetles
and saw¯ies, in the adjoining crop (Thomas, Wrat-
ten & Sotherton 1991, 1992; Aebischer & Ward
1997) and often include song posts (EisloÈ �el 1997).
The fauna and ¯ora of ®eld margins have been
severely a�ected by increased pesticide and fertilizer
drift (Kleijn & Snoeijing 1997; Kleijn & Verbeek
2000). It is therefore unfortunate that Gillings &
Fuller (1998) were unable to examine data on the
loss of grassy margins before concluding that future
research `should concentrate on disentangling the
processes which a�ect recruitment and survival
within the ®eld rather than looking at factors out-
side the ®eld'. Our results suggest that grassy mar-
gins might be particularly valuable to corn buntings
in areas dominated by winter-sown wheat.
INVERTEBRATE ABUNDANCE, PESTICIDE
USE AND FORAGING
Corn buntings foraged in areas with a high average
abundance of certain invertebrates, particularly saw-
¯y larvae. A possible reason for this correlation not
holding for Lepidoptera larvae, despite their impor-
tance in nestling diet (Brickle & Harper 1999), could
be their diversity ranging from palatable to highly
distasteful species. We found that when corn bunt-
ings had a choice between sprayed areas, they for-
aged in those areas that had received on average
fewer pesticide applications, while the abundance of
preferred invertebrates in sprayed areas was nega-
tively correlated with the number of insecticide
applications (all three pesticide types when all fora-
ging habitats considered). Thus, heavily sprayed
areas were avoided probably because they contained
fewer invertebrates; saw¯y larvae are known to be
particularly susceptible to insecticides (Sotherton
1990). The ®rst analysis only included cereal ®elds,
but these covered 46% of the study area, held 69%
of nests (n� 120) and received 45% of foraging vis-
its. The greater use of less heavily sprayed ®elds
could therefore have been of real importance to the
birds.
Insecticides and a few fungicides can have direct
insecticidal activity, while herbicides can have an
indirect e�ect by killing insects' food plants (Rands
& Sotherton 1986). Lepidoptera and Symphyta are
known to be particularly vulnerable to the latter
e�ect (Rands & Sotherton 1986). Although we had
only su�cient data to test for relationships involving
the number of pesticide applications, other aspects
of pesticide use are also known to a�ect invertebrate
abundance. These include the e�cacy of the chemi-
cals involved, timing of application and dosage
(Ewald & Aebischer 1999). The negative impacts of
pesticide use on invertebrates have been demon-
strated in many studies (reviewed in Campbell et al.
1997), including several conducted on our study site
(Aebischer & Potts 1990; Aebischer 1990, 1991;
Ewald & Aebischer 1999).
Adults foraged closer to the nest when food was
abundant, suggesting that their habitat choice was
in¯uenced both by distance to a habitat patch and
food availability within it. This is consistent with
central place foraging theory (Orians & Pearson
1979). Our analyses of habitat use and food avail-
ability were limited to the area within the maximum
observed foraging distance. We were unable to test
a null model allowing for a decrease in habitat use
with distance from the nest (Rosenberg & McKelvey
1999) because our sample sizes were inadequate.
This raises the possibility that the habitats used
more for foraging than expected (those above the
line in Fig. 1) were simply more common close to
nests. Any such constraint on foraging location
seems to have been weak (Brickle & Harper 2000);
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for example, a third of nests were located in winter-
sown wheat which was used little when provisioning
chicks (Fig. 1).
On North Uist, 95% of foraging trips were within
100m of the nest for males and 120m for females
(Hartley & Shepherd 1997). Birds on the South
Downs fed farther from the nest, with more than a
third of mean foraging distances exceeding 115m.
Increased foraging distance will impose extra ener-
getic costs on the parent and may expose them to
additional risks of predation.
CLUTCH AND BROOD SIZES
Clutch sizes were higher than in other published
British studies (means from 3�82 to 4�00 eggs; Ryves
& Ryves 1934a,b; MacDonald 1965; Yom-Tov
1992; Gillings & Watts 1997; Hartley & Shepherd
1997), resembling those from central Europe (means
from 4�20 to 4�63 eggs; Cramp & Perrins 1994). The
mean at a downland site 20 km to the east was simi-
larly high (1985±97, 4�46�0�04 eggs, n� 424; D.
Harper & T. Collins, unpublished data). Our study
provided no indication of why clutches on the South
Downs were large because clutch size was not signif-
icantly correlated with food availability.
CHICK WEIGHTS
The mean weight of nestlings about 5±7 days after
hatching, when corrected for age using tarsus length,
was positively correlated with food availability.
Even if this was due in part to a correlation between
sex ratio and food availability, which would be a
surprise (Hartley et al. 1999), the female still had to
provide the food for sons to grow (there was no sig-
ni®cant correlation between food availability and
brood size, see above). We conclude that reduced
food availability restricted chick growth, which
although unsurprising is not always true (Brodmann
et al. 1997). Low chick weights may contribute to
increased predation risk, as reported for cirl bunt-
ings Emberiza cirlus (L.) (Evans et al. 1997). Hungry
broods often beg more loudly or for longer than
well-fed broods (Redondo & Castro 1992), an e�ect
which is exaggerated because the nest mates of hun-
gry chicks tend to match their begging behaviour
(Kacelnik et al. 1995). A planned investigation of
begging rate was abandoned because the extra dis-
turbance was considered too intrusive.
Chick weights could have longer term conse-
quences. For example, weight at ¯edging is posi-
tively correlated with future survival among many
passerines (Mock & Parker 1998). Corn bunting
chicks typically ¯edge about 9 days after hatching
(Crick 1997) and so there were only 2±4 days for
compensatory growth between their being weighed
and ¯edging.
Chick weight (corrected for tarsus length) was
negatively correlated with foraging duration, sug-
gesting that when parents are taking longer to col-
lect food, the chicks are receiving less food. We had
insu�cient nests with data on chick weight, food
availability, foraging distance and foraging duration
to construct a meaningful multiple regression model.
NEST OUTCOMES
At least one chick ¯edged from half of the nests
(Table 2). Most nest failures were the result of pre-
dation (74% of failures) or farming practices (22%).
The level of predation was much higher than
recorded by the nest record scheme (Crick et al.
1994) for corn bunting (32%), yellowhammer E.
citrinella (L.) (52%) and reed bunting E. schoeniclus
(L.) (63%). Although badgers usually eat few eggs
or chicks (Neal & Cheeseman 1996), in one year
they preyed upon nearly a ®fth of the nests in this
study. Signs of badger damage were so obvious that
it is unlikely that they were responsible for any of
the nests destroyed by unknown predators.
Crick et al. (1994) found that corn bunting nests
were more likely to be destroyed by farming opera-
tions than those of the other common farmland
buntings (43% of losses compared with 19% for yel-
lowhammer and 17% for reed bunting). They sug-
gested this was due to corn buntings' habits of
nesting in cereal ®elds and nesting late. In our study
the number of nests lost to farming operations var-
ied between years, from only 6% of failures in 1996
to 67% in 1995. Harvesting of cereals was particu-
larly early in 1995, starting in early rather than late
July. Hay was usually cut before the end of May.
The low number of nests that were abandoned
suggests that we did not disturb nests too severely.
It is possible, however, that we altered predation
rates, either by leaving visual and olfactory clues for
predators or by deterring them (May®eld 1975).
Mayer-Gross, Crick & Greenwood (1997) found,
however, that visits by experienced nest recorders
had very little e�ect on nest success.
DAILY SURVIVAL RATE OF NESTS
Survival rates of broods, but not clutches, were sig-
ni®cantly correlated with food abundance. These
analyses were based on nests that were either suc-
cessful or attacked by predators, as failures due to
mechanical farming practices or desertion were
excluded. Thus, lower food availability correlated
with a higher risk of predation at the chick stage.
The overall May®eld estimate of nest survival rate
(0�25) was much lower than that reported by Crick
(1997), who found that the probability of success for
nests in the nest record scheme from East Anglia
and the Midlands rose from 0�25 in the 1960s to
0�61 in the early 1990s. There are three possible
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explanations for this discrepancy. First, nest survival
on our study site might be unusually low. Our data
on brood size and daily survival rate suggest, how-
ever, that the number of chicks ¯edged per nest
(1�13, ignoring partial losses) was similar to that
(1�05) reported for monogamous females in North
Uist (Hartley & Shepherd 1997). Secondly, nest sur-
vival rates may have collapsed for some unknown
reason during the 1990s, as they did for reed bunt-
ings (Peach, Siriwardena & Gregory 1999). Finally,
the nest record scheme data may be unrepresenta-
tive. For example, corn buntings might have become
concentrated in breeding sites of higher quality as
the population declined (Rodenhouse, Sherry &
Holmes 1997). Crick (1997) felt that the absence of
correlations between habitat features and breeding
performance suggested that such a bias was unim-
portant. Strong site-dependent e�ects can, however,
be detected among blue tits Parus caeruleus (L.) in a
single wood (Dhondt, Kempenaers & Adriaensen
1992). Heterogeneity of the nest record scheme data
could generate apparent trends in breeding perfor-
mance because of changes in the populations
sampled, especially when sample sizes are as low as
they are for corn buntings. This problem is empha-
sized by the signi®cant regional variation in survival
rate for corn bunting nests monitored by the nest
record scheme between 1954 and 1989 (Aebischer
1999). It is also striking that nest record scheme
data suggest a trend for increasing nest success
among skylarks (Chamberlain & Crick 1998),
despite low breeding success in two detailed studies
(Wilson et al. 1997; Poulsen, Sotherton & Aebischer
1998; Chamberlain et al. 1999).
GENERALITY OF RESULTS
At ®rst sight, our study area was typical of much of
the breeding habitat occupied by corn buntings in
Britain. The invertebrates found in nestling faeces
(Aebischer & Ward 1997; Brickle & Harper 1999)
were similar to those reported by other studies
(Gliemann 1973; Hartley & Quicke 1994; Gillings &
Watts 1997). Most of these invertebrates have
declined in abundance over a wide geographical
scale during the last 25 years (Campbell et al. 1997).
Some unusual features of our study population,
however, merit discussion. All suggest that provid-
ing invertebrates for their chicks was harder for
corn buntings on the South Downs than for those in
some other populations. First, we found that nest-
lings were regularly fed grain (Brickle & Harper
1999), which has been suggested to be a suboptimal
food (Watson 1992; Hartley & Quicke 1994). Sec-
ondly, adults often foraged farther away from the
nest than those on North Uist (Hartley & Shepherd
1997) and we found that foraging distance was nega-
tively correlated with food availability. Thirdly, the
May®eld estimate of nest survival was lower than
estimates from the nest record scheme (Crick 1997),
and we found that nest survival was negatively cor-
related with food availability. The last two argu-
ments are weak because di�erences between
populations can have di�erent causes from those
within populations. A ®nal possible reason for
chicks being unusually hard to feed on our study
site was that males rarely, if ever, fed nestlings. The
impact of reduced food availability might be amelio-
rated in areas where males provide some food for
the chicks (Harper 1995; Gillings & Watts 1997).
Hartley & Shepherd (1997) concluded, however,
that `within the variation [0±22% of feeds provided
by male], male parental care did not greatly a�ect
¯edging success'.
CONCLUSIONS
Changes in farming practices have reduced the avail-
ability of most of the invertebrates fed to corn bunt-
ing chicks. For example, Symphyta and Lepidoptera
larvae are scarcer in cereal crops than in natural
grassland (Moreby & Aebischer 1992), highly sus-
ceptible to insecticides (Sotherton 1990) and indir-
ectly harmed by herbicides (Sotherton 1991). They
will also have been reduced by the loss of crop rota-
tions including ley grass (Aebischer 1990; Potts
1997). Although large-scale changes in farming prac-
tices seem unlikely, if organic farming continues to
spread (Wilson et al. 1997) it could provide some
respite for corn buntings as long as cereals form
part of the landscape. On the other hand, any future
loss of set-aside (MAFF 1999) will remove an
important foraging habitat. Our results suggest
some smaller-scale measures that would assist corn
buntings by increasing invertebrate availability in
small areas, which are readily used by corn bunt-
ings. Tussocky grass margins around cereal ®elds
seem to be very e�ective, typically supporting rela-
tively high numbers of invertebrates and increasing
invertebrate abundance in adjacent crops; selectively
sprayed headlands and beetle banks will have a simi-
lar e�ect of increasing invertebrate abundance in a
small area (Sotherton 1991; Thomas, Wratten &
Sotherton 1991, 1992). Undersowing of cereals, even
on a small scale, can increase the availability of saw-
¯ies (Symphyta) on a whole farm (Potts 1997).
This study suggests that agricultural intensi®ca-
tion can reduce the breeding success of corn bunt-
ings and could therefore have contributed to their
decline. Even if reductions in chick food did not
cause the decline, they seem likely to hamper popu-
lation recovery as suggested for cirl buntings (Evans
et al. 1997). There is an urgent need to test the
hypothesis of reduced over-winter survival, which
has thrived despite the absence of data (Donald,
Wilson & Shepherd 1994; Donald & Evans 1994,
1995; Crick 1997; Shrubb 1997). Estimates of post-
¯edging survival would also allow us to test whether
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observed breeding success was high enough to main-
tain a stable population.
Acknowledgements
This study was funded by English Nature (The Nat-
ure Conservancy Council for England) and the
Royal Society for the Protection of Birds, as part of
a larger project organized by The Game Conser-
vancy Trust. It would not have been possible with-
out the support of the Sussex farmers, who kindly
gave us access to their land and provided valuable
help and advice. Alastair Burn, Andy Evans and
Phil Grice advised throughout the study. Simon
Brickle wrote the randomization programs. Three
anonymous referees helped us to improve our ®rst
draft. We thank them all.
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Ecology, 37,
742±755
