11.
Work needed to extend the range of bird species that can be covered by
deterministic models
11.1
Introduction
The deterministic (empirically-based) modelling approach requires information on the
relationships between: a) food abundance and breeding parameters and (b) breeding
performance measures and population status, for a range of farmland bird species. It is known
that this information exists for the grey partridge (Perdix perdix). Further work has recently
been completed on a number of other farmland bird species. A review of this work is therefore
needed in order to identify the data available, and assess it in terms of its usefulness for the
model.
11.2
Aim
To produce a matrix of declining farmland bird species and datasets describing food availability,
annual productivity and population change, to assess the suitability of the datasets for the
analytical framework proposed under Objective 10, and to indicate the studies required to fill
the gaps.
11.3
Methods
11.3.1 LITERATURE SEARCH
Published papers were derived from previous reviews on the indirect effects of pesticides on
birds (McKay, unpublished) and the impact of herbicides on weeds (Marshall et al., 2001). In
addition, Biosis was searched to find any papers published subsequent to these two reviews.
Additional information from the ‘grey area’ was obtained from the consortium’s knowledge of
recent and on-going studies. Bird species considered were the 21 identified by the consortium at
the start of the project.
11.3.2 EVALUATION OF DATA SETS
Each study was examined to determine the type of data collected, the sampling method used and
the parameters included in any population model.
11.4
Results
11.4.1 DATASETS AVAILABLE
Table 11.1 lists the species being considered in this report, and for each, the studies which have
attempted to measure food availability and breeding performance (intensive autecological
studies) or model population breeding parameters and population size (demographic studies).
Table 11.1
Farmland bird species and published papers relating breeding performance to either food availability or population size. The food availability,
productivity and population parameters used in each study are listed.
Species
Reference
Food availability
Chaffinch
(Fringilla coelebs)
Cirl bunting
(Emberiza cirlus)
Collared dove
(Streptopelia decaocto)
Corn bunting
Siriwardena et al. (2000)
(Miliaria calandra)
Aebischer & Ward (1997)
Siriwardena et al. (2000)
Annual productivity
Population change
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
None found
None found
Brickle et al. (2000)
invertebrate
abundance
densities of food items
nest survival and weight of chicks
density of nesting birds
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
Goldfinch
(Carduelis carduelis)
Siriwardena et al. (2000)
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
Greenfinch
(Carduelis chloris)
Siriwardena et al. (2000)
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
Grey partridge
(Perdix perdix)
Green (1984)
Potts & Aebischer (1991;
1994)
House sparrow
(Passer domesticus)
Freeman & Crick (2002)
breeding performance
population changes
since 1975
Linnet
(Carduelis cannabina)
Siriwardena et al. (2000)
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
egg and nestling loss, clutch size, brood size and
fledglings per attempt
population decline,
1975-1986
population
size
1962-1995
Quail (C. coturnix)
Moorcroft
&
(2000)
None found
Wilson
densities of food items
fluctuation in the food
supply
chick survival
chick mortality rate
annual variation in
population density
Table 11.1 continued.
Species
Red-legged partridge
(Alectoris rufa)
Reed bunting
(Emberiza schoeniclus)
Rook
(Corvus frugeligus)
Skylark
(Alauda arvensis)
Stock dove
(Columba oenas)
Stone curlew
(Burhinus oedicnemus)
Tree sparrow
(Passer montanus)
Turtle dove
(Streptopelia turtur)
Reference
Food availability
Annual productivity
Green (1984)
densities of food items
chick survival
Siriwardena et al. (2000)
Population change
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
clutch size, brood size and post-hatching survival
rate
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
Siriwardena et al. (2000)
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
Browne & Aebischer
(2001)
Siriwardena et al. (2000)
number of young fledged per pair
annual population
change
population decline
None found
Chamberlain & Crick
(1999)
Siriwardena et al. (2000)
Siriwardena et al. (2000)
population decline
population decline
None found
None found
Woodpigeon
(Columba palumbus)
Yellowhammer
(Emberiza citrinella)
None found
Yellow wagtail
(Motacilla flava)
None found
Siriwardena et al. (2000)
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
clutch and brood sizes, chick:egg ratio and daily
nest failure rates
population decline
11.4.2 EVALUATION OF DATASETS
11.4.2.1
Corn bunting
Brickle et al. (2000) sampled the abundance of four of the main corn bunting chick food
invertebrates: Opiliones, Lepidoptera larvae, Symphyta larvae and Orthoptera. These
invertebrates were found to account for the majority (62%) of chick diet (Brickle & Harper
2000). Invertebrates were sampled by sweep netting, but it was noted that this did not sample an
important component of the diet – large lycosid spiders, and resource constraints prevented
additional techniques being used. Invertebrate abundance was measured at foraging locations
within the territory around each nest. The measures of breeding success used were clutch and
brood sizes, chick weight and nest survival. The authors were able to regress food availability
against residual mean chick weight (slope = 0.11 ± 0.005, t24 = 2.21, P = 0.036). In conclusion,
this appears to be a very useful data set where effort has been made to ensure that the chick food
sampled was that available to the foraging birds, and to identify possible biases and sources of
error.
Aebischer & Ward (1997) examined the diet of nestling corn buntings in their study area by
faecal analysis. Data on invertebrate abundance (three taxa known to be important in the diet:
large beetles, caterpillars and spiders/harvestmen) was obtained from previous sampling within
the study area, but was not directed towards individual territories or nests. Abundance measures
were averaged over five years. Sampling was by D-Vac of spring- and autumn-sown cereals.
The efficiency by which this method samples the three target taxa is not discussed, but is likely
to be low for the larger and more mobile species. Breeding success was not measured directly,
but singing male corn bunting density was measured. The authors were able to regress
components of chick food (density of food items) against corn bunting density. In conclusion,
this study is more broad-based than the previous one and as such is of less use, particularly as
no direct measurement was made of breeding performance.
Siriwardena et al. (2000) modelled breeding performance (BTO nest records data) and
population decline (BTO Common Birds Census data) and found no evidence of a relationship
(but see comments in Discussion section).
11.4.2.2
Grey partridge
Green (1984) investigated the diet of grey partridge chicks using faecal analysis and found it to
be composed of various invertebrate taxa and grass spikelets. He then sampled the abundance of
these foods during the main hatching periods, within the foraging areas of the birds (which were
radio-tracked). D-vac sampling, sweep netting and ‘quadrat weed samples’, were used. Chick
survival rate was measured. The authors were able to correlate chick survival rate with the
abundance of commonly eaten arthropod prey. In conclusion this is a useful dataset as some
attempt has been made to sample the invertebrate and plant food items available to foraging
birds and a sample size of 13 farms was used.
Potts & Aebischer (1991), in an extension to work first presented in Potts (1986), used a wider
variety and range of data on the abundance of grey partridge chick food in the GCT Sussex
study site, in their model of the population dynamics of the grey partridge. Field-by field
measures of the abundance of insect food from 1970 were used, as were results of intensive
studies of the effects of pesticides on this food from 1973. Their insect abundance index was
obtained by summing the densities of five insect taxa: small beetles, sawfly and other
caterpillars, leaf beetles and weevils, plant bugs and leaf hoppers, and aphids. The sampling
methods used are described in Green (1984). They recalculate relationships between chick
survival and the index of insect abundance, for five farms in the Sussex study area, and for 17
radio-tracked broods. In conclusion, this is a useful extension to the work reported in (Green,
1984), but mainly uses the same dataset.
Potts & Aebischer (1994) construct a population model based on data from the GCT Sussex
study site as well as from two sites in Norfolk, and demographic data from 36 populations from
nine countries and the national UK data from the GCT’s National Game Census. They use this
model to show that annual fluctuations in the population can be attributed to annual variations in
chick survival rate.
11.4.2.3
House sparrow
In a recently completed study investigating the decline of starlings and house sparrows in Great
Britain, Freeman & Crick (2002) model the effects on the population of changes in breeding
performance measured using the BTO Nest Record Scheme data, and changes in adult and first
year survival, measured from ringing recoveries. They found that the best fit to the data was
found when egg-stage failure rates alone were permitted to vary. This most accurately
reproduced the full extent of the population decline in the early 1980s. However, reasonable
approximates of the decline were obtained whether egg- or chick-stage failure rates or hatching
success were allowed to vary in the model.
11.4.2.4
Linnet
Siriwardena et al. (2000) modelled the effects of clutch size, brood size, chick:egg ratio and
daily nest failure rates on the population size. Breeding performance data was obtained from the
BTO nest records database and abundance data from the Common Birds Census. They found
that a fall in fledgling production per breeding attempt, driven primarily by increased nest
failure rates during the egg period, probably affected abundance, and that this change could
have driven the principle population decline (1975-86) for this species.
BTO Nest Record and Ringing Schemes data were used by Moorcroft & Wilson (2000) to
model the relationship between breeding productivity (first-egg dates, clutch size, nest survival,
partial losses of eggs and nestlings, brood size at fledging) and population size. They concluded
that the most likely demographic mechanisms underlying recent population trends are changes
in the number of nesting attempts made by females and changes in post-fledging survival rate.
11.4.2.5
Red-legged partridge
As for grey partridge, Green (1984) investigated chick diet composition by faecal analysis,
identified foraging areas by radio-tracking broods and sampled chick foods by D-vac sampling,
sweep netting and ‘quadrat weed samples’. Chick survival rate was measured. The survival of
red-legged partridge chicks was positively correlated with the density of Coleoptera and grass
spikelets. In conclusion this is a useful dataset as some attempt has been made to sample the
invertebrate and plant food items available to foraging birds and a sample size of 14 farms was
used.
11.4.2.6
Skylark
Chamberlain & Crick (1999) modelled changes in reproductive performance (from Nest
Records Scheme data) and declines in the number of breeding skylarks (from CBC data). They
found that reproductive performance per nest, in terms of clutch size, brood size and posthatching survival rate, showed a general improvement between 1975 – 1994, the period in
which the number of breeding skylarks declined. They concluded that changes in reproductive
performance per nesting attempt were probably not responsible for the decline in numbers, and
inferred that number of breeding attempts per pair per season, reductions in the proportion of
birds breeding, or increased mortality outside of the breeding season may be responsible (but
see comments in Discussion section)..
Siriwardena et al. (2000) modelled breeding performance (BTO nest records data) and
population decline (BTO Common Birds Census data) and found no evidence of a relationship.
11.4.2.6
Turtle dove
Browne & Aebischer (2001) examined the annual abundance of breeding turtle doves in the UK
(BTO data) and compared it to breeding parameters measured in the 1950/60s and in the 1990s
(GCT data). The earlier data had been collected at a study site near Newmarket,
Cambridgeshire, and the 1990s data was collected by the GCT at study sites in Suffolk and
Lincolnshire. Browne & Aebischer’s calculations are quite basic, and the information available
in their paper is limited. In conclusion, this dataset is probably not very useful due to the small
amount of breeding performance data collected.
Siriwardena et al. (2000) modelled breeding performance (BTO nest records data) and
population decline (BTO Common Birds Census data) and found no evidence of a relationship
(but see comments in Discussion section).
11.4.2.7
Chaffinch, goldfinch, greenfinch, reed bunting, stock dove, tree sparrow and
yellowhammer
For these species, Siriwardena et al. (2000) also modelled breeding performance (BTO nest
records data) and population decline (BTO Common Birds Census data) and found no evidence
of a relationship (but see comments in Discussion section).
11.5
Discussion
Few studies have examined relationships between chick food availability and productivity. The
only species among those being considered in this report, for which suitable relationships for the
modelling approach outlined in section 10 have been published, are two galliforms (grey and
red-legged partridges) and one passerine (corn bunting). Studies of other species have been
carried out in which both food abundance and productivity have been assessed, but not related
to each other e.g. skylark (Poulsen et al., 1998 ); yellowhammer (Stoate et al., 1998; Morris et
al., 2002). It is hoped that examples of such relationships may be established from data
collected during the ongoing Defra-funded study on “assessing the indirect effects of pesticides
on birds” (PN0925).
Measurement of breeding performance is not always straightforward. In the examples modelled
in the previous chapter (grey partridge and corn bunting), there was a significant relationship
between chick food availability and chick survival. However, absence of such a relationship
does not necessarily mean that there is no effect, nor that altering pesticide use may not result in
a change in the population. Chick condition may be affected (e.g. Morris et al., 2002), which
may influence post-fledging survival. Post-fledging survival has been little studied to date
because of the technical difficulties involved. Chick quality is also difficult to measure, and
brood reduction (i.e. loss of one or more chick) may affect the performance of remaining chicks,
thus masking any measurable effects on individual chick performance. Alternatively, parents
may have to work harder to feed their young under conditions of low food availability, which
may affect their own condition and hence chances of survival over the following winter.
The ideal measure of productivity is the number of young produced per pair (or per female) per
year. Grey partridges and most corn buntings are single-brooded. However, most other species
in Table11.1 are multi-brooded. The best available data set measuring breeding output for most
species is the British Trust for Ornithology’s Nest Record database. However, this is based on
individual nests and does not provide an assessment of annual productivity. Lack of data on
total annual productivity does not invalidate the application of the empirically-based modelling
approach, but if there is an interaction between outcomes of first and second (or later) broods,
this could confuse the interpretation of results based on single nests. For example, failure of the
first nest may lead to an early second nest, which may produce young with a better chance of
survival than young from a later second nest.
An alternative scenario is that effects of external factors such as pesticides may act differentially
on broods produced at different times. For example, Morris et al. (2002) found that
yellowhammer foraging density was much lower in fields which had received an insecticide in
summer early in the season than later in the season, when unripe grain provided an alternative
source of food for nestlings. Chick condition was inversely related to numbers of summer
insecticides applied.
Studies relating productivity to population change are available for a greater range of species,
however these are not always ideal for incorporation into the modelling approach adopted here.
The most wide ranging are the demographic studies undertaken by Siriwardena et al. (2000).
These cover a number of years during which populations declined, and for several species
including turtle dove, skylark, tree sparrow, yellowhammer, and corn bunting, indicate that
breeding performance per attempt was higher while populations declined. However, these
models do not allow for density dependence, which could explain this apparently counterintuitive result. In a declining population, it is likely that sub-optimal habitats will be vacated
first, which may improve resource availability on average for the remaining birds, thus
potentially increasing productivity. It would be advantageous to carry out further analyses of
the BTO’s extensive datasets which incorporate density dependence in the models. However,
the modelling approach used in section 10 only estimates the population change from one year
to the next, thus approximating to a population in equilibrium and discounting most of the
additional factors which influence outcomes over longer periods.
11.6
Conclusion
There is a paucity of studies relating productivity to food availability. More studies relating
productivity to population change are available, but studies based on long runs of data which do
not incorporate density dependence may not give reliable predictive relationships for inclusion
in the empirically-based model.
11.7
References
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Sussex in relation to crop type and invertebrate abundance. The Ecology and
Conservation of Corn Buntings (eds. P.F. Donald & N.J. Aebischer), pp. 124-138. UK
Nature Conservation no 13. Nature Conservation Committee, Peterborough.
Brickle, N.W. & Harper, D.G.C. (2000) Diet of nestling corn buntings Milaria calandra in
southern England examined by compositional analysis of faeces. Bird Study, 46, 319329.
Brickle, N.W., Harper, D. G. C., Aebischer, N. J., & Cockayne, S. H. (2000) Effects of
agricultural intensification on the breeding success of corn buntings Miliaria calandra.
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Browne, S.J. & Aebischer, N.J. (2001) The role of agricultural intensification in the decline of
the Turtle Dove Stretopelia turtur. English Nature Research Report No. 421.
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Green, R.E. (1984) The feeding ecology and survival of partridge chicks (Alectoris rufa and
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Potts G.R. & Aebischer N.J. (1994) Population dynamics of the grey partridge. Perdix perdix.
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in the breeding performance of seed eating birds in determining their population trends
on farms. Journal of Applied Ecology, 37, 128-148.
Stoate, C., Moreby, S.J. & Szczur, J. (1998) breeding ecology of farmland Yellowhammers
Emberiza citrinella. Bird study 45, 109-121