Journal of Food, Agriculture & Environment, Vol.7 (2), April 2009 80 7
Journal of Food, Agriculture & Environment Vol.7 (2) : 807-814. 2009
Science and Technology
Meri-Rastilantie 3 B, FI-00980
Why are birds more abundant on organic farms?
Ailsa J. McKenzie * and Mark J. Whittingham
School of Biology, Ridley Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
Received 8 September 2008, accepted 26 Februaty 2009.
Recent reviews have concluded that bird diversity is greater and abundance is around 50% higher on organic than on conventionally-managed farms.
Promoting organic farming could, therefore, enhance populations of farmland birds many of which have fallen dramatically in Europe over recent
decades. No attempt has been made, however, to quantify the importance of different aspects of the organic farming regime. We attempt a novel
approach to answering this question by using data from existing literature to quantify the relative contributions of the five main differences between
the farming systems. Though sample sizes are small, results suggest lack of pesticides and increased area of non-cropped habitats on organic farms
make a significant positive impact on farmland birds (22 and 15% increases in important bird parameters, respectively). In contrast increased
heterogeneity in cropping and fertiliser applications on organic farms may both be slightly detrimental to farmland birds when compared with
conventional farm methods. The evidence for spring-sowing is minimal and thus we can only speculate as to their effects. Our work is useful in two
ways: (i) we have shown that both heterogeneity in cropping and fertiliser applications are unlikely to underlie the reported increases of birds on
organic farms; (ii) we hope to encourage work in areas that plug knowledge gaps in the current story, e.g. effects of spring-sowing on birds.
Key words: Biodiversity, farmland bird diversity, farming systems, fertilisers, pesticides, non-cropped habitats.
Organic farming has been shown to benefit a wide range of farmland
biodiversity, including many insects and vascular plants, but it is
the effect on farmland bird diversity and abundance which has
perhaps received the most attention. Two recent reviews have
concluded that both abundance and diversity of many bird
species is higher (up to 50% 1) on farms managed organically than
on conventional farms 1, 2. Farmland birds have undergone huge
declines over the last 50 years, something which has largely been
attributed to agricultural intensification 3, 4. Less intensive farming
systems like organic farming could, therefore, be expected to help
ameliorate such declines.
However, the mechanisms by which bird abundance and diversity
are increased on organic farms remain largely unclear. While
organic farming is typified by a lack of most artificial inputs (both
pesticides and fertilisers), many other differences also exist
between the two systems including increased non-cropped
habitats, more spring-sowing of crops and higher levels of mixed
farming on organic farms, all of which have been shown
individually to affect farmland bird biodiversity. However, while
one preliminary study described, non-empirically, the possible
effects of each component part 5 and another has shown that
non-cropped habitats explain some of the increase in bird
abundances on organic farms 6, our study is the first to attempt a
quantitative estimate of the relative importance of these individual
components, something which may significantly aid future agri-
Our aim, therefore, is to review the literature in an attempt to
gain preliminary estimates of the individual importance of five
main differences between organic and conventional farming
systems on farmland bird populations. The reasons behind the
selection of the differences will also be discussed.
After a preliminary review of the relevant literature, five main
differences between organic and conventional systems which
impact upon farmland bird population parameters were selected
for further investigation 1) pesticides, 2) fertilisers, 3) non-cropped
habitat, 4) timing of crop sowing and 5) within-farm heterogeneity.
These five differences will from this point be referred to as the
“five predictors”. Reasons for selection of these predictors are
discussed in the Results section below.
Appropriate search terms were entered into “Web of Science®”
(www.wos.mimas.ac.uk) including ‘pesticides AND birds’;
‘fertilisers AND birds’; ‘mixed farming AND birds’; ‘spring-sown
AND birds’; ‘hedges AND birds’; ‘woodland edge AND birds’ as
well as combinations of the words ‘hedgerows’, ‘boundaries’ and
‘birds’. Resulting papers were then checked for relevance, with
preference being given to papers where an effect size of any of
the five predictors on bird abundance and/or vital rates was
reported. The number of examples directly linking abundance or
vital rates of a particular species with any of the five predictors
was limited (n = 10). There were many examples linking other factors
(e.g. foraging location, invertebrate abundance) with the predictors
but it proved difficult to relate this information to abundance or
vital rates. No effect sizes could be identified for either “fertilisers”
or “timing of crop sowing”, and in these cases qualitative data on
808 Journal of Food, Agriculture & Environment, Vol.7 (2), April 2009
the likely direction of the relationship was included.
Different strategies had to be employed to each paper to extract
the relevant information, as methods and measures varied greatly.
A full description of how this was carried out is outlined in
Selection criteria and background: Organic farming is typified
by the exclusion of most artificial pesticide inputs (fungicides,
herbicides, insecticides and growth regulators). Pesticides can
impact farmland bird populations in two main ways: 1) directly,
acting physiologically on adults, juveniles and/or eggs and 2)
indirectly through reduction in food resources (seeds and insects).
While direct effects have been widely documented, these mostly
concern the deleterious effect of organochlorines (e.g. DDT) on
sparrowhawks (Accipiter nisus) and other bird species
pre-1980s 7, 8. However, such directly toxic pesticides have now
been banned in many countries (e.g. in the UK for more than two
decades), and insufficient evidence exists regarding similar effects
of new generation pesticides on bird populations. Therefore, only
the indirect effects of pesticides on farmland bird populations
and vital rates will be considered further in this review.
Pesticides have major indirect effects on birds via the killing of
both invertebrates important for food and also agricultural weeds
which provide seed resources and also cover for invertebrates. It
is important to note that only data regarding pesticide effects on
broods was found, and effect size estimates do not include effects
of pesticides on adults through reduced food supply, something
which would almost certainly augment the results.
Several pieces of evidence support the negative relationship
between insecticide spraying and vital rates of farmland bird
populations. Probably the best example comes from a fully
replicated study of the grey partridge (Perdix perdix L.) 9. This
study showed that pesticide spraying affected the invertebrate
food of partridge chicks, which was correlated with chick survival,
and was the main cause of population decline. More recent
examples come from another farmland bird specialist, the
yellowhammer (Emberiza citrinella). Morris et al. 10 showed that
arable fields sprayed during the summer were used less frequently
than fields not sprayed during the summer by adult yellowhammers
foraging for food for their young. Hart et al. 11 showed
that the availability of arthropods was depressed up to 20
days after an insecticide spraying event and that this
negatively affected yellowhammer chick survival. Both herbicide
spraying and fungicide spraying have also been shown to be
negatively correlated with invertebrate populations 10 and weed
populations 12 and so these are also likely to negatively affect
farmland bird populations. It is clear, therefore, that the lack of
pesticides used on organic farms will positively affect invertebrate
and weed populations and that these lower trophic levels are
likely to linearly relate to higher trophic levels like farmland birds.
Calculations: Three examples were found where pesticide
spraying was related directly to brood size in bird species - Rands 9
reported a 66% increase in brood size for grey partridge and a 37%
increase for red-legged partridges (Alectoris rufa) in fields with
unsprayed vs. sprayed margins, while Boatman et al. 13 reported
a 17% increase for yellowhammers. Data from a fourth study on
chestnut-collared longspurs (Calcarius ornatus) 14 could be
included by assuming the same parameters as for yellowhammers 13,
giving an estimated 20% increase in brood size on organic farms.
The mean average effect size across these four papers was fairly
high - 35% higher brood size on unsprayed vs. sprayed fields
and/or field margins (although this figure is amended below). This
conclusion that pesticides exert a large effect on population change
is supported by earlier work showing grey partridge chick survival
is the most important demographic process driving population
An important caveat to these findings, however, is that organic
fields and margins are not directly comparable with the
“unsprayed” areas considered in the studies reviewed. While
organic farming does not permit the majority of chemical
applications, other management techniques are employed to
control insect and plant pests, such as mechanical weeding (known
as tining), something not accounted for in the simple “unsprayed
vs. sprayed” comparison. Organic fields will likely have less seed
and insect resources than an “unsprayed” conventional fields
due to the impact of tining and other management techniques.
Therefore, to counter this and improve our estimated effect size
we calculated the mean difference in insect and seed abundances
between organic and unsprayed systems from the literature and
used this as an adjustment factor for our calculations. For insect
abundances, only data concerning important bird food insects
were included (e.g. Heteroptera, Lepidopteran larvae) 16. Full
results and papers involved are listed in Appendix 2. The mean
difference in seed resources between the systems was calculated
as 36%, and for insect resources 41%. In other words 64% of the
increase in seed densities in organic systems is due to lack of
spraying, and 59% of the increase in insect densities.
We have adjusted down the calculated effect sizes for the effects
of pesticides on bird abundance and vital rates in accordance
with these results: a 36% reduction as an upper limit (based on
seed result) and 41% reduction as a lower limit (based on insect
result). This equates to a mean effect size of between 21 and 23%
on brood size for pesticide applications alone (Table 1).
Selection criteria and background: Fertilisers are likely to impact
farmland bird populations through changes in: a) weed and
therefore seed abundances; b) insect abundances; and c) habitat
structure. While organic and conventional crops both receive
fertiliser applications, the type of inputs used differs substantially
and this may alter their impact on a-c above. Only natural fertilising
compounds are permitted in organic agriculture (e.g. manure, lime,
clays), while artificial N-rich compounds are the fertiliser of choice
in conventional agriculture. While levels of application in the two
systems are often similar 17, it is widely recognised that the nitrogen
in “organic” fertiliser is less available than that in artificial fertilisers
making overall uptake lower 18. No evidence exists relating fertiliser
type directly to bird vital rates, but direction of relationship may
still be determined by considering the effect of the two types of
fertiliser on important farmland bird food resources and habitat
While many studies discuss the effect of fertilisers on weed and
insect abundances, these tend to be “fertiliser vs. no fertiliser”
comparisons rather than “organic vs. conventional fertiliser”
comparisons. Studies which do exist tend to concern only insects,
Journal of Food, Agriculture & Environment, Vol.7 (2), April 2009 80 9
and often not species of particular value to farmland birds (e.g.
pest or beneficial predator species). However, the majority of
these studies suggest that insect abundances will be lower in
organic systems, and that organic fertilisers appear to confer some
degree of increased insect resistance 19. In a recent multi-crop
experiment 20, for example, beneficial invertebrates (e.g. Carabidae,
Hemiptera) were more often in greater abundance in conventionally
fertilised crops than organically fertilised crops (12 vs. 6 instances).
Culliney and Pimentel 21 had a similar result with abundances of
flea beetles, alate aphids and caterpillars all significantly lower on
some Brassica crops (e.g. collards) fertilised with “organic”
fertilisers than on chemically fertilised plants.
These results tally with the finding that insectivorous birds are
generally more abundant on intensively managed grass fields than
on fields which receive lower amounts of fertiliser input or no
input at all 22, 23 something believed to be a result of the increase in
large below-ground invertebrate species which occurs with
increased fertiliser use 24, 25. Organic fertiliser in moderate amounts
can also benefit below-ground invertebrate populations 26.
The difference in effect on weed and therefore seed abundance
is less well documented. However, if we assume organic systems
contain less available nitrogen than conventional systems (an
over-simplification, but broadly accepted 17, 18) it could be expected
that organic systems will have lower weed/seed abundance than
conventional systems for a number of reasons: (1) increased
fertiliser use has been linked with enhanced growth of many weed
species 27, 28 and (2) dormancy in a number of weed species seeds
is broken by increased N application 29.
No information could be found relating fertiliser type to habitat
structure, but as increased inputs are likely to promote weed
density (as outlined above), structural complexity will likely
increase with increased fertiliser use, something which has been
shown to be detrimental to foraging ability in farmland birds 30, 31.
Therefore, while not quantifiable, the use of solely organic fertiliser
may have a small, but negative impact on farmland bird vital rates
by, on average, reducing food supplies in crops (both seeds and
3) Non-cropped habitats
Selection criteria and background: Differences in the availability
of non-crop habitats between paired organic and conventional
farms have been reported by various studies, with organic farms
often possessing higher, wider hedgerows 32, 33 and greater areas
of woodland 34. While an increase in such components is not a
requirement of organic conversion, farmers entering into such
schemes may have a more ‘wildlife-friendly’ attitude and already
hold greater proportions of these habitats than average. Whatever
the reason, organic farms tend to provide significantly greater
areas of non-cropped habitats than conventional farms and this
is likely to benefit farmland birds.
Taller and wider hedgerows have been positively associated
with a wide range of different farmland bird species 35, 36 and fields
bordering woodland are also positively associated with a range
of bird species found on farmland 37.
Calculations: Across four studies, an increase in hedge height
resulted in an average increase in bird abundance of 9% (hedge
height differences between organic and conventional systems
estimated using relationships given in Fuller et al. 33) (see Appendix
1 for a more detailed explanation of how this figure, and those
below, was derived). Hedge presence resulted in a mean increase
of 5% estimated from two studies, and increased woodland edge
resulted in an increase of 1% in bird abundance. It is important to
note that these changes are those recorded across all the species
in each study, thus some species will tend to avoid woodland
edges (e.g. yellowhammer 37), whereas other species prefer
woodlands as a breeding habitat (e.g. great tit 38). However, as we
are attempting to explain the observed result of increased bird
abundance on organic vs. conventional farms (across all bird
species) then these sorts of species-specific differences are
important to include in our study. Overall the increased non-
cropped habitats associated with organic farms (hedgerows and
woodland) resulted in an increase of 15% abundance (Table 1).
4) Timing of crop sowing
Selection criteria and background: Organic farms tend to carry
out more spring sowing than do conventional farms 39, with an
estimated 27% increase post-organic conversion 40, 41. Spring-sown
cereal crops are likely to impact farmland bird populations in two
ways. Firstly as they are sown in spring, plants remain short
enough during the breeding season for birds to use them for
nesting. Indeed this has been shown to be a very important
resource for skylarks (Alauda arvensis), a species which has been
in significant decline in recent decades. Wilson et al. 42 showed
that in intensively managed autumn-sown cereal fields skylarks
only made around one nesting attempt as opposed to two or three
Predictor No. studies Direction
Estimation of magnitude of effect
1. No pesticides used on
3 + 21-23% increase in brood size on organic farms resulting
from zero-spraying (after adjustment – see Appendix 2)
2. Organic fertilisers used
on organic farms
0 (-) Data only available for the effects on invertebrates and
plants. While limited studies suggest conventional farming
promotes invertebrates no studies showing a direct link with
bird populations were found
3. More non-crop habitat
on organic farms
7 + 15% increase in bird abundance on organic farms as a result
of more hedgerows and woodland
4. More spring sowing on
5 + Cannot be quantified as no predictive equations exist in the
literature but the relationship is likely to be positive
5. Increased habitat
2 - 4.5% decrease in farmland bird abundance as a result of
increased habitat heterogeneity on organic farms
Table 1. Estimates of the magnitude of the effects of each of the five predictors on farmland bird
* + indicates parameter improves bird vital rates, - indicates the opposite.
810 Journal of Food, Agriculture & Environment, Vol.7 (2), April 2009
in spring-sown cereals. Secondly, as the crops are harvested in
autumn, fields are often left as stubble over winter. Stubble fields
are an extremely important food resource for farmland birds over
winter, with many species selecting stubble fields over other types
of field available in the winter 43.
However, the proposed benefits of an increase in spring-sowing
on organic farms have three important caveats. First, higher levels
of spring sowing do not always mean higher levels of winter
stubble. Chamberlain et al. 44 found higher levels of bare till on
organic farms, but no difference in stubble abundance. Second,
whilst some stubble fields hold high densities of birds the large
majority of stubble fields in some studies contain very few birds
or none at all 31. Third, although organic wheat fields have been
shown to provide higher densities of seeds than conventionally
managed wheat fields, the use made by birds of the two field
types did not differ consistently 31. Some species preferred
conventional fields (e.g. yellowhammer, grey partridge, skylark),
whilst others preferred organic fields, e.g. linnet (Carduelis
cannabina) and reed bunting (Emberiza schoeniclus) 31.
Spring-sown cereals have also been shown to increase
invertebrate populations relative to winter-sown cereals (including
several groups important in the diet of farmland birds) 45, 46 but no
direct relationships could be found between invertebrates and
bird abundance or vital rates.
We could find no quantitative predictions of the importance of
spring sowing on bird abundance and/or vital rates. As outlined
above, the relationship is almost certainly a positive one - spring
sowing will promote farmland bird populations. However, owing
to the three main caveats discussed previously (organic farms do
not always possess more stubble, stubble on organic farms does
not necessarily possess more food resources, and much stubble
remains unused). However, it seems unlikely that this is the over-
riding factor in the increased bird abundances on organic farms.
5) Within-farm heterogeneity
Selection criteria and background: Organic farms, especially in
the lowlands, are more likely to be mixed farms than those managed
under conventional systems, i.e. be farms with both crop and
livestock production 39, 40. Livestock play an important role in the
supply of nutrients and this cannot be compensated for by artificial
fertilisers on organic farms. Thus organic farms often have greater
habitat heterogeneity than conventional farms. Such heterogeneity
is likely to benefit a variety of bird species by providing both
arable and grass areas within close proximity (i.e. at the farm scale),
something which is important, particularly during the breeding
season 47. Indeed, at the landscape scale Atkinson et al. 48 reported
higher abundances of farmland birds in mixed farming areas.
However, the devil may be in the detail. Increased habitat
heterogeneity typically equates to increased grassland area but
decreased arable area and this decrease may not be a positive.
Data from Shepherd et al. 40 indicates that the amount of arable
crops decreases by 15% post-organic conversion whilst the area
under grass management increases. Data from a paired study of
89 farms 33 also showed that the proportion of grassland was
almost double on organic farms (37.7%) compared with
conventional farms (17.2%).
Importantly data presented in Robinson et al. 47 shows that the
proportion of arable land is positively related to farmland bird
abundance across most species in their study. Thus although
mixed farming does increase under organic farming it seems likely
this will have a negative effect on farmland birds by decreasing
the amount of cereals on organic farms. A possible caveat here is
that there is some difference between management on farms
converted to organic in upland and lowland areas but we did not
find any evidence of this.
Calculations: Using data on changes in grassland area 40 (see
above) in conjunction with the equations in Robinson et al. 47
regarding the relationship between arable land area and farmland
bird abundance, a negative effect on farmland bird abundance of
around 4% was due to organic conversion. The Robinson et al. 47
study did, however, include mainly seed-eating species and as
such may not be entirely representative of the whole farmland
bird community. Data from Atkinson et al. 48 were then considered
as they cover a wider range of farmland birds. This showed that
less than 40% of species were associated with landscapes with
more than 50% grassland in them. Additional raw data provided
by Atkinson (pers. comm.) which was used to draw the graphs in
Appendix 1 of Atkinson et al. 48 allowed the quantification of the
change in bird abundance across 68 species caused by a drop of
arable land from 25% arable (average across UK farms 40) to 21.25%
arable (a 15% decline in arable land holding). This gave a decline
in abundance of around 5%.
In summary, whilst the increased amount of grassland due to
organic conversion may benefit some grassland species (such as
whinchat Saxicola rubetra L. and redshank Tringa totanus) and
disadvantage other granivorous species (such as grey partridge,
skylark and yellowhammer) the weight of evidence from both
Atkinson et al. 48 and Robinson et al. 47 suggests that the loss of
arable land post-conversion seems likely to have a negative impact
on farmland birds overall of around 4.5% (Table 1).
From the limited data available, two factors have large effects in
farmland birds: pesticides (21-23% increase on brood sizes) and
non-cropped habitats (15% increases in abundance) (Table 1).
Increased habitat heterogeneity appears to have a slight negative
impact on farmland bird populations (around 4%) as a result of a
reduction in the availability of arable land.
The remaining two predictors cannot be quantified, but a
direction of effect can at least be obtained. Increased amounts of
spring-sowing on organic farms are likely to substantially enhance
skylark populations in the breeding season and could potentially
provide more stubble fields in winter; therefore the direction of
effect is likely to be positive. The effect of differing fertiliser use
between the two farming system seems likely to only manifest
themselves in relatively small, but negative, differences in bird
food abundance but the information on this is limited. This result
is not in agreement with the conclusions from a previous review 5,
which suggested organic fertiliser to be beneficial to farmland
bird populations. However, the evidence listed in the previous
review is very general and does not consider potential differences
in effect between fertiliser types (i.e. organic vs. conventional).
Novelty and shortcomings of the review: This is the first study to
attempt to quantify the effect of independent aspects of the organic
farming regime on bird population parameters. Fuller 5 discussed
the predicted response of bird populations to four main
Journal of Food, Agriculture & Environment, Vol.7 (2), April 2009 81 1
management differences in the organic regime but no attempt was
made to quantify the importance of each response. Chamberlain
and Wilson 6 took a modelling approach to quantify the effect of
non-crop habitat on bird parameters but the use of this data in the
current review was limited as it considered only one parameter
The present study clearly has a number of shortcomings. Most
problems stem from the small number of papers available for
inclusion in the review, and the associated issues of sample and
publication bias. Results for pesticides are, for example, largely
based on examples from game bird species. However, as pesticides
act on bird populations by depressing food availability, it could
be argued that impacts are likely to be similar regardless of bird
species. Seasonality is also an issue. Insufficient data exist to
consider impacts of the five predictors on bird populations across
all seasons. Studies of pesticides, for example, concern mostly
breeding season data, spring-sowing studies consist both
breeding and winter season data and studies of habitat
heterogeneity are from both summer and winter. However, while
we acknowledge that our results would be improved by resolution
of this problem, we believe much of the data included concerns
the season during which most impact on bird populations will be
The calculations made for non-crop habitat based on abundance
should also be treated with some degree of caution. It is likely to
be unrealistic to assume the relationship between abundance and
actual density is a direct linear one.
The appropriateness of comparing different population
parameters also needs consideration. Pesticide effects have been
reported mainly on brood size, whereas increases in non-crop
habitat mostly relate to changes in abundance. Given abundance
incorporates both changes in reproductive output and survival it
cannot be directly compared with changes in brood size. Therefore
we cannot rank the estimated effect sizes - a 23% increase in
brood size is not necessarily “better” than a 15% increase in
abundance. However, the effect on both these parameters is large
and significant (e.g. studies listed in Table 1 found statistically
significant effects of pesticides on brood size and of non-cropped
habitats such as hedge presence and hedge height on bird
abundance), and therefore it is likely that both make a significant
contribution to the observed increases in farmland bird populations
on organic farms.
Our calculations suggest that neither increased mixed-cropping
patterns on organic farms nor the use of organic fertilisers are
likely to account for the increased abundance of birds on organic
farms. Instead pesticide reductions and increased non-cropped
habitats had the largest impacts on the increased farmland bird
abundance observed on organic farms. We do, however,
acknowledge that the effects of spring-sowing are also likely to
be beneficial but cannot be quantified due to a lack of available
data. Our study suggests gaps in the current knowledge base and
future work on birds and organic farms may benefit from focussing
on these gaps, e.g. the effect of spring-sowing on birds and studies
of pesticides on non-game birds.
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Methods employed to extract relevant data from papers
1. Pesticides: Data were obtained from three studies on the effect
of pesticides on bird vital rates. Rands 9 reported brood size to be
lower on sprayed than unsprayed field margins for both grey
partridge (4.70±0.35 vs. 7.81±0.60) and red-legged partridge
(3.23±0.26 vs.4.43±0.39). This equates to an increase in brood size
of 66% for grey partridge and 37% red-legged partridge as a result
of spraying alone (amended to 39-42% and 22-24% respectively
as described in Pesticide section of main paper and Appendix 2).
Boatman et al. 13 reported the probability of brood reduction in
yellowhammers to increase from 0.26 when no sprays were carried
out within 200 m of the nest to 0.82 when the entire area within 200
m of nest was sprayed (values taken from Table 4 (Model 1) 13).
The values in this model are for the brood to be reduced by at
least one chick, therefore we have been forced to make some
assumptions about the number of chicks involved in an
average scenario. Average clutch size of yellowhammers is 3.4
(n = 1607 - source BTO nest record scheme; http://www.bto.org)
therefore assuming the probability of brood reduction decreases
by 26% with no spraying, this equates to a mean loss of 0.26
chicks when no sprays are carried out within 200 m of the nest
(26% chance of losing one chick) to 0.82 chicks with spraying
within 200 m of the nest (82% chance of losing one chick) assuming
all chicks lost. The mean of these values is 1.23 chicks, which
equates to a 17% increase in brood size (amended value 10-11%).
This only includes a subset of yellowhammer nests (n = 64) 14
which were not predated.
Martin et al. 14 found hatching success of chestnut-collared
longspurs was significantly lower in sprayed plots than in control
plots following spraying (67 versus 87%). Assuming the same
parameters as for yellowhammers in the Boatman et al. 13 study
this would result in a 20% increase in brood size of this species on
organic farms (amended 12-13%).
2. Fertilisers: No quantitative data could be found regarding
fertilisers and bird vital rates.
3. Increased non-crop habitat: We used data from four studies
to estimate the effect of increased woodland edge on organic
farms. Whittingham et al. 37 reported the following relationship:
Logit (farmland bird) = 1.49 x presence of woodland edge on
field boundary. We used the mean relationship for each of ten
farmland bird species from 25 sites in England (Table 3 37). If a
woodland edge was present on a boundary there was a 0.12 higher
probability of an average ‘farmland bird’ having a territory on
that boundary than if there was not (calculated using an intercept
of zero and woodland edge set to absent = 0, or present = 1). We
used 0.12 as the proportion of boundaries next to woodland edges
on conventional farms and 0.17 (41% increase from 0.12) on organic
farms. These figures were derived from Gibson et al. 34 who report
a 41% increase in woodland edge on organic farms and the mean
number of boundaries next to woodland edge reported in
Whittingham et al. 37 as a baseline figure (i.e. 0.12). This gives an
increase of 1% greater abundance of birds on organic farms.
Boundary height had stronger and more consistent effects
across species than boundary width so we have simply used
boundary height as our one measure of hedgerow structure. We
estimated the difference in hedge height between conventional
and organic farms (1.6 vs. 1.9 m) using data from Fuller et al. 33 and
this information was used in the following equations relating hedge
height to bird abundance/occupancy:
1. Logit (farmland bird) = 1 + 1.78 x boundary height 37
Difference = 12%.
2. No. of species with territories = 1.99 + 0.61 x hedge height 49
Difference = 6%.
3. Bird abundance = 1.45 + 2.25 x log (hedge height) 35
Difference = 13%.
4. Log (bird abundance) = 0.033 + (0.009 x hedge height) 50
Difference = 5%.
Increased hedge abundance has been associated with increased
bird abundance and equations are available from two studies.
Fuller et al. 33 indicated that hedges were 30% more common on
organic farms than on conventional farms and this allows
estimations to be made in the same way as with hedge height
1. Logit (farmland bird) = 1+ (1.04 x hedge presence)37
Difference = 4%.
2. No. of passerine territories = 3.30 + 0.75 x amount of hedge
cover at 1 m 49 Difference = 6%.
4. Increased habitat heterogeneity: The impact of increased
heterogeneity on farmland birds was calculated using data relating
changes in the availability of arable land to bird abundance.
Robinson et al. 47 (Table 3, page 1063), report the average
significant relationship between farmland bird abundance and the
percentage of arable within a square varying from 0 to 100% as
Log (farmland bird) = 1.9 x % arable in 1 km square. Shepherd
et al. 40 estimated a 15% decrease in arable land post organic
conversion and this, when used in the Robinson et al. 47 equation,
gives around a 4.5% decrease in abundance per species on organic
farms as a result of decreased arable availability. However, it is
important to note that in squares with high proportions of arable,
increased arable was not shown to benefit farmland birds at all 47.
Predictive equations were also available in Appendix 1 of Atkinson
et al. 48 for a change in abundance for a larger number of species
than the Robinson et al. 47 paper and using the same 15% decrease
in arable land post organic conversion, this equates to around a
814 Journal of Food, Agriculture & Environment, Vol.7 (2), April 2009
Organic vs. Conventional Unsprayed vs. Sprayed
Study Measure Result Study Measure Result
McKenzie et al. (unpubl.) Seedbank (top 5 mm) 70% higher in organic Taylor et al.
Weed volume 62% higher in unspraye
Moorcroft et al.
Seedbank (top 5 mm) 39% higher in organic de Snoo
% Cover weeds 80% higher in unspraye
% Cover 44% higher in organic 71% higher in unspraye
Roschewitz et al.
Seedbank (top 10 cm) 64% higher in organic 89% higher in unspraye
% Cover weeds 80% higher in organic Weed biomass 75% higher in unspraye
Seed rain 19% higher in organic 82% higher in unspraye
Hyvönen et al.
Weed abundance 15% higher in organic
Mennalled et al.
Weed biomass 83% higher in organic
Petersen et al.
Weed abundance 28% higher in organic
Moreby et al.
% cover 46% higher in organic
Mean effect 49% Mean effect 77%
Table A. Data comparing seed resources on organic vs. conventional and unsprayed vs. sprayed fields and margins compiled from
nine papers. The difference in the mean effect sizes between the two systems was calculated as 36% - which means
64% of the increase in seed densities in organic systems is due to lack of spraying. This was then be used as an
adjustment factor for the results.
Organic vs. Conventional Unsprayed vs. Sprayed
Study Measure Result Study Measure Result
37% higher in organic Taylor et al.
Chick food insects 13% higher in unsprayed
Heteroptera 19% higher in organic Moreby et al.
Heteroptera 36% higher in unsprayed
Feber et al.
44% higher in organic de Snoo
Butterfly abundance 79% higher in unsprayed
Rundlöf & Smith
Butterflies 18% higher in organic 62% higher in unsprayed
Rands & Sotherton
Butterfly abundance 64% higher in unsprayed
Sotherton et al.
Chick food insects 56% higher in unsprayed
Mean effect 30% Mean effect 51%
Table B. Data comparing insect resources on organic vs. conventional and unsprayed vs. sprayed fields and margins compiled
from nine papers. The difference in the mean effect sizes between the two systems was calculated as 41% - which means
59% of the increase in insect densities in organic systems is due to lack of spraying. This was then be used as an
adjustment factor for the results.
5% decrease in abundance across 68 species on organic farms.
5. Increased spring sowing: No quantitative data could be found
regarding increased amounts of spring sowing and bird vital rates.
Full details of the data gathered to make amendments to pesticide
effect size results in the review as a result of organic data not
being directly comparable to data from “unsprayed” systems can
be seen in Tables A and B. The literature was searched to find
instances where seed and insect abundances in either organic vs.
conventional or unsprayed vs. sprayed systems were recorded. A
comparison of these values allowed the mean difference between
organic and unsprayed systems (for full details of this see main