6.
Sampling and data recording techniques applicable to questions concerning
farmland bird food resources.
6.1
Introduction
Relatively few studies on farmland bird ecology aim to measure food availability. As discussed
under Objective 1, availability is a function of the abundance and accessibility of this food, as
well as the foraging behaviour of the bird species (food and habitat preferences and response to
changes in food availability). In practice, many studies measure relative (rather than absolute)
food abundance. Although this undoubtedly simplifies matters (the methods used need not be
efficient), the sampling method(s) utilised can influence the reliability of the data obtained.
The method used is generally governed by the diet of the species of bird being investigated.
Most sampling methods are very selective, and have a wide range of effectiveness (Hutto 1990;
Tones et al. 2000). Sampling can thus over-represent, under-represent, or not represent at all,
the food abundance in an area. In addition, each method has its associated assumptions and
limitations, and each will differ in terms of repeatability and hence reliability of the estimates
obtained.
If data from different studies are to be compared, or indeed combined in order to build the
models being considered in this project, some degree of standardisation (of both methods and
measures of abundance) seems necessary. Bird species may be separable into guilds sharing
common diet elements, and so may be investigated using similar techniques, although one
single method is unlikely to be suitable for all circumstances. For example, methods of
measuring arthropod abundance are likely to be more or less efficient for different species
depending on mobility and size. Different methods, also, are likely to differ in terms of
repeatability.
6.2
Aim
To: (i) review the literature on sampling methods used to estimate density or biomass of bird
food/prey items; (ii) assess repeatability of the estimates obtained using the different methods;
(iii) assess to what extent the methods measure food availability to the relevant species; (iv)
assess to what extent it is necessary to standardise the methodology for assessment of bird food
resources, in order to include such data into the proposed models; and (v) make
recommendations on the methods that should be used.
6.3
Review of sampling methods used to measure food availability
6.3.1
ARTHROPODS
The standard techniques for measuring the absolute and relative abundance of arthropods are
listed in Southwood (1978), which remains the definitive text on the subject. Powell, Walton &
Jervis (1996) list some of the most commonly used techniques and describe their limitations.
The techniques relevant to the estimation of the abundance of insects are shown in Table 6.1.
Calibration, where indicated, is necessary for the estimation of absolute abundance.
1
Table 6.1
Field-sampling methods for estimating insect abundance (derived from Powell,
Walton & Jervis, 1996).
Method
Description
Comments
Interception Device to intercept arthropods Provide data on relative (rather than
traps
moving on the ground (e.g. absolute) abundance, except fenced
containers used in pitfall trapping, pitfall traps. Locomotor activity
water traps and pan traps for should be taken into account (cannot
molluscs) or the air (e.g. nets used as detect immobile species, some species
malaise traps, and sticky traps).
aggregate in traps), can sample over a
period of time, make simultaneous
comparisons between habitats and
monitor night-active species.
Vacuum net Suction
devise
that
collects Calibration necessary, efficiency
arthropods from vegetation and depends on height and density of
ground. Fan draws air through tube vegetation and varies with weather
via a net, sucking small arthropods conditions, as well as varying with the
onto the net from the vegetation design of the device (e.g. size of
enclosed by the sampling head, e.g. sampling head and power of motor
D-Vac (Dietrick, 1961) or vortex which drives the fan). A snap shot of
draws arthropods into collecting pot what is present at time of sampling.
eg. Vortis sampler.
Provides estimate of density. Suction
power limits size of insect that can be
sampled.
Sweep net
A fine-meshed cone-shaped net Difficult to estimate absolute rather
mounted on a rigid frame commonly than relative abundance, only efficient
used for collecting arthropods from for groups active in the vegetation
vegetation.
Sampling
involves canopy, sampling variability caused
sweeping the net rapidly through the by many factors, inc. person operating
vegetation so that insects are net
dislodged and caught
Knock-down The dislodgment of insects from their Calibration necessary, whole plant
substratum (usually vegetation) by may need to be enclosed, may not
mechanical or chemical means, sample all species with the same
causing them to fall and be collected efficiency (very active flyers may
in a tray or funnel, e.g. ‘beating’
escape)
Visual count Of in situ individuals or their Labour-intensive, insects need to be
artefacts in a defined area or length conspicuous, efficiency varies (most
of time
efficient for conspicuous species),
betweenobserver
variation,
detectability must not vary
MarkLive-trapping
a
sample
of Species-specific, depends on a
releaseindividuals, marking and releasing number of assumptions, need to use
recapture
them, followed by re-sampling the appropriate methods of calculation.
population
Very labour intensive. Only for
insects that can be marked.
Attraction
Using visual (e.g. colours) or Difficult to define area of influence,
olfactory stimuli
insect responses may change with
time, different species may not
respond to the same stimuli
Emergence
Enclosures are placed on the soil Specialist method to measure the
traps
surface, colour or pitfall traps added, abundance of pupating insects in the
and left to collect emerging adults.
soil, e.g. sawflies and beetles.
Soil cores
Cylinder of soil is removed and Restricted to soil meso- and microarthropods extracted using funnel.
arthropods. Time-consuming.
2
Sunderland et al. (1995) review the principle methods available for the estimation of
invertebrate predator densities (which include many listed in Table 6.1) and discuss the relative
advantages, disadvantages and limitations of each. They conclude that there is no single method
that is suitable for all circumstances, and recommend methods according to the types of
predators being assessed, the habitat and the scale of the investigation. Similarly, the methods
used to estimate the abundance of arthropod bird foods should be appropriate to the species of
arthropod being measured, and to the foraging habitat and behaviour of the birds being
considered. Most of the studies described in this chapter select techniques primarily on this
basis, rather than with regard to standardisation of methodology. This has resulted in a wide
range of techniques being used to estimate the abundance of invertebrate food for farmland
birds and may reduce the potential for between-study comparisons.
Stewart & Wright (1995) list a variety of vacuum nets (suction samplers) which have been
designed for extracting insects and other arthropods from natural vegetation and agricultural
crops. Macleod et al. (1994) describe a modified petrol-driven leaf-collecting device, and
compare its efficiency to that of the ‘Thornhill vacuum sampler’ (Thornhill, 1978), and Arnold
(1994) describes a lighter and easier to use device, the ‘Vortis’ suction sampler. The Game
Conservancy Trust (GCT) have generally used a Dietrick vacuum net (D-Vac) suction trap,
which is similar to the Thornhill device (Dietrick, 1961), although Stewart and Wright (1995)
show that their ‘Blow & Vac’ device achieves a greater air velocity and hence improved
efficiency of extraction, although it has a smaller sampling head. A study of D-Vac efficiency
found variable capture rates of between 43% and 70% for ground-dwelling linyphiid spiders,
compared to capture rates of 84% to 92% for linyphiids that lived higher in the crop
(Sunderland & Topping 1995). Vegetation density and species microhabitat preferences
influenced capture rates. Mommertz et al. (1996) concluded that short-term D-Vac sampling is
not appropriate for Carabidae, Staphylinidae and Lycosidae, because relatively large individuals
are under-estimated. In grassland, Standen (2000) found that both pitfall traps and D-Vac/sweep
netting were required to adequately sample species richness. This was because many species
were “method-unique”.
The published literature on farmland bird ecology was then examined, and studies in which bird
food availability or abundance was measured were reviewed. Using a D-Vac, Ewald. &
Aebischer (1999) took five ten-second sub-samples, each of 0.092 m2 along a diagonal transect
into a field. Most invertebrates were identified to the family level, some to genus or species.
Holland et al. (2002) used a similar method to estimate invertebrate chick food for farmland
birds, as did Moreby et al. (1994) to compare the abundance of invertebrates between
conventional and organic winter wheat fields. The D-Vac method was also used by Moreby &
Southway (2002) to measure the availability of invertebrate groups important in the diet of
nestling farmland birds, in order to investigate crop preferences in arable farmland, and by
Moreby & Southway (1999) to investigate the influence of herbicides on autumn food available
to birds.
In a study to assess the indirect effects of pesticides on birds, Morris (unpublished draft report to
DEFRA) used a petrol-motor leaf suction device (Stihl BG 75) equipped with a 10 cm diameter
suction-hose, rather than a D-Vac to sample invertebrate food for yellowhammers Emberiza
citrinella , because the latter required two operators. He found that catches depended on weather
conditions and the height and density of vegetation. Therefore invertebrate populations from
patches with different vegetation structure could not be compared. However, he argues that the
results provide a reasonable indication of relative prey accessibility (which is a component of
prey availability – see Objective 1). He also used a sweep net (a kite net – as suggested by
Evans, 2001) to sample aerial invertebrate food for swallows Hirundo rustica, and a 10 x 10 cm
soil corer to sample abundance and biomass of earthworm food for lapwings Vanellus vanellus.
In a study of the effects of spraying strips along crop edges on non-target insects, de Snoo & de
Leeuw (1996) estimated insect abundance by sweep netting. In each 100m strip, 10 sub-samples
3
were taken in six sweeps of a sweep net with a diameter of 35 cm. The total area sampled was
20 m2 per 100m. Sampling took place 1.5 m from the field edge. For mobile insects, additional
visual observations were made.
In his study of grey partridge Perdix perdix and red-legged partridge Alectoris rufa chick
ecology, Green (1984) took two sets of five 0.1 m2 D-Vac samples, 5 and 50 m from each field
boundary. In cereal fields, fifty sweeps were made with a 1.0 x 0.4 m sweep net, 5 m from the
field boundary. Each sweep covered c.1 m and a pace was taken between each. All sweep
netting was done by the same person. In an experiment to test whether pesticides reduced the
survival of grey partridge chicks, Rands (1985) measured insect food availability using a sweep
net, taking 50 sweeps per field edge.
Hill (1985) measured insect food supply of pheasant Phasianus colchicus chicks by taking five
D-Vac samples and five 50 m sweep net samples within the brood range during their second
week. Between-year variation in insect abundance was estimated from D-Vac sampling data.
Beintema et al., (1991) measured prey abundance for breeding wading birds by sweep netting
and pitfall trapping. Ten pitfalls were used, in two rows of five, at 10 m intervals. At 12 points
between the pitfalls, a sample was taken with the sweep net, each time making three sideways
sweeps when walking. A suction apparatus, ‘based on a small fan, sucking air through a narrow
nozzle’ was also used to remove as many insects as possible from a 50 x 50 cm quadrant.
In a study if the breeding ecology of farmland yellowhammers, Stoate et al. (1998) used both
the D-Vac method and sweep netting to sample ground-dwelling invertebrates. Five 0.5 m-2 Dvac samples and five sweep samples, each of ten sweeps, were taken at each sampling location.
Sweep netting was also used in a study by Brickle et al. (2000) to sample the abundance of corn
bunting Miliaria calandra chick foods around nests in the GCT Sussex study site in 1996-97.
Twenty sweeps were made during the first week in July in each habitat block. For large fields
the mean of up to three samples was used. Barker et al. (1999) used sweep netting and
emergence traps to estimate sawfly abundance in grassland and arable fields, and in cultivated
vs. uncultivated experimental plots. Møller (2001) estimated swallow food abundance on farms
with and without dairy cattle using sweep netting. Nine transects of 20 m in each 40 ° direction
from the farms were sampled, on pasture.
6.3.2
OTHER INVERTEBRATES
Non-arthropod invertebrate food of farmland birds include earthworms and molluscs (snails and
slugs). These can also be sampled using the methods listed in Table 6.1, with the addition of soil
sampling and extraction for sub-surface invertebrates and chemical extraction techniques for
earthworms (Edwards & Bohlen, 1972).
Tucker (1992) measured soil macro-invertebrate densities as an estimate of availability to
surface picking or bill probing species of farmland birds. He took 15 soil cores per field, each
core being 120mm diameter, 50mm deep, extracted with a sharpened steel cylinder. Cores were
frozen and stored for up to 3 months. After thawing, invertebrates were extracted by washing
and flotation and sorted into broad taxonomic groups and biomass estimated from wet weight.
The invertebrate species measured using this method included Lumbricids, Molluscs,
Coleopterans, Myriapods and Diptera larvae.
In a study of habitat use of lapwings in rough grazing and arable areas in Scotland, Galbraith
(1989) used soil cores and pitfall traps to sample surface-living and subsurface-living
invertebrates. Each soil sample was 20 cm square and 10 cm deep, and was hand-sorted into
earthworms, leatherjackets and other (mainly beetles and their larvae). Pitfall traps were plastic
beakers (circular in cross-section, 8.5 cm deep and 7 cm in neck diameter) containing 5%
4
formalin solution and buried up to their rims. Sheldon et al. (2002) used soil cores of 10 cm
diameter and 10 cm depth to estimate earthworm food for foraging lapwings, in different crop
types. Ten samples were taken from the central area of each field and hand-sorted. Village &
Westwood (1994) measured earthworm abundance for foraging lapwings by formalin extraction
at eight 50 cm x 50 cm quadrats per field on 22 occasions from October through January.
In an investigation of stone curlew habitat selection, Green et al. (2000) measured the
abundance of earthworms by applying a solution of formaldehyde to an area within a 0.25 m2
wire quadrat frame. For or five quadrats within a 30 m diameter area were treated at each site.
They also measured invertebrates using pitfall traps (five traps in a line at intervals of 5 m at
each site). The traps were 10 cm in diameter, 15 cm deep, rim flush with the soil surface,
containing a preservative.
Table 6.2 lists the most common methods used in the above studies on farmland birds. For each
method, the taxa sampled are listed, and comments made on ease of use and processing time.
Table 6.2
Methods used to sample the invertebrate food of farmland birds.
Method
Examples of taxa sampled
Ease of use and time to
process
Quick and easy method using
light and simple equipment.
Time-consuming to sort and
identify samples.
Sweep nets
Asilidae, Cantharidae, Chrysomelidae,
Chrysopidae, Cicadellidae, Coleoptera,
Curculionidae, Dolichopodidae, Elateridae,
Empididae,
Heterocera,
Heteroptera,
Nitidulidae, Scatophagidae, Symphyta
D-vac
Acari,
Araneida,
Auchenorrhyncha,
Chrysomelidae, Cicadellidae, Coleoptera,
Curculionidae,
Diptera,
Hemiptera,
Heteroptera, Hymenoptera, Lepidoptera
larvae, Symphyta, Tenthredinidae larvae,
Thysanoptera
Generally
requires
two
operators. Heavy, noisy and
with
safety
implications
(petrol
motor).
Timeconsuming to sort and identify
samples.
Vortis sampler
As for D-vac
Lighter than D-vac, but has
smaller sampling head and
hence poorer repeatability
Soil cores
Lumbricids, Mollusca, Myriapoda, larvae Quick and easy to sample,
of Coleoptera, Tipulidae and Diptera
time-consuming to extract,
sort and identify samples.
Pitfall traps
Araneida, Auchenorrhyncha, Brachycera Time consuming to set up,
&
Cyclorrhapha,
,
Coleoptera, quick and easy to sample.
Hymenoptera, Lumbricidae, Mollusca, Time-consuming to sort and
Nematocera, Sternorrhyncha,
identify samples.
Visual count
Apidae,
Coccinellidae,
Crambinae, Time-consuming
and
Stratiomyidae, Syrphidae, Tipulidae
dependent on skill of observer
Emergence traps
Hymenoptera larvae
Quick and easy to sample.
Takes space and time for
emergence.
5
6.3.3
WEEDS
Methods of estimating plant cover and numbers are straight forward, and have been well
reviewed in numerous textbooks, e.g. Greig-Smith (1983) and Moore and Chapman (1986). The
main methods are summarised in Goldsmith (1991) and those useful for measuring abundance
listed:
density (plants often spread vegetatively making this measure less useful except for certain
species, such as bulbs, orchids, annuals and trees),
cover (proportion of ground covered by a species, often recorded with pins of very narrow
diameter located at random, or estimated by eye and placed in a range, e.g. the Domin 1 to 10
range),
biomass or yield (accurate but destructive),
basal area (appropriate to trees or tussocky plants),
frequency (the proportion of quadrats which contain a particular species, tends to combine
abundance with distribution).
It has recently been appreciated that the structure of vegetation may affect food availability to
farmland birds (see Objective 1). Additional methods to measure vegetation structure include
the following:
drop disc, in which a disc of standard weight and diameter is dropped on the vegetation from a
height of 1 m down a vertically held ruler, in order to provide an indication of leaf and stem
density within the sward canopy,
point quadrat, in which pins are slotted within a frame placed randomly within a plot, and the
number of vegetation contacts made at each height interval recorded, enabling the 3-D structure
of the sward to be determined, and
graduated board, in which estimates of the proportion of the board obscured when viewed
from 1 m are made at different heights, in order to build up a profile of vegetation density.
Quadrat size and sampling pattern (random, systematic or stratified) should also be appropriate
to the species and habitats being considered.
The GCT estimates the overall general abundance of broad-leaved weeds and grass weeds by
scoring them from zero (none present) to five (complete infestation of the crop by weeds). In the
case of weed occurrence (presence/absence) most weeds are identified to species level (Ewald
& Aebischer, 1999; Moreby et al., 1994; Moreby & Southway, 1999)
Green (1984) took ten 0.1 m2 quadrat weed samples, 5 and 50 m from each field boundary. He
gives no further details, but results are presented as ‘quadrat samples of Poa and Agrostis
spikelets 1.0 m-2’.
Watson & Rae (1997), in a study of corn buntings in north-east Scotland, scored weeds on a
scale from 0 (none) to 5 (>75% of the ground covered).
In surveys of cereal weeds undertaken from June - August, occurrence and abundance were
estimated visually on a crude scale, while walking through fields and scanning from vantage
points (Froud-Williams & Chancellor, 1982; Chancellor & Froud-Williams, 1984).
6.3.4
SEEDS
There is an extensive literature on seed banks and seed rain, however, very little in relation to
food availability for seed-eating farmland birds. Seeds on plants, seeds on the soil surface and
seeds buried within the soil can be measured, depending on the foraging behaviour of the
species being considered.
6
Seeds on plants can be estimated from numbers of flowers, or by relationships to plant weight
(Wilson et al., 1988). Diaz & Telleria (1994) measured seed availability by cutting all seed
bearing plants within a 20 cm x 20 cm quadrat, and counting all seeds and fruit in the
laboratory. Sample points were at 18 m intervals along a transect. Two contiguous soil samples
3.8 cm x 3.8 cm, 1 cm deep, were also taken 15 cm away from the quadrat in the transect
direction, and seeds extracted and counted. Seeds were identified to species level and average
seed composition and energy value determined from the literature. Average energy abundance
per unit area was then calculated and expressed as kJ 10 ha-1.
Pascual et al. (1999a) and McKay et al. (1999) estimated the density of exposed seed available
to woodpigeons Columba palumbus on winter cereal fields by counting the number of visible
seeds in 0.25m2 quadrats. Twenty sampling points were chosen at random in a diagonal line
across the main field and 10 points along each of the two headlands walked in a zigzag pattern.
In the RSPB Skylark Alauda arvensis Project (Donald et al., 2001) ten soil samples per field
were taken and analysed to provide a measure of seed density within the top 5 mm of the soil
surface. Similarly, in a project to measure the indirect effects of pesticides on farmland birds,
Hart et al. (2002) estimated availability of seed to skylarks and yellowhammers by taking 5 mm
deep, 20 cm x 20 cm soil scrapes. Five soil scrapes were taken at each sampling point and the
number of seeds counted. Two sampling points in the centre and two at the edge of each field
were used to make an estimate of seed density.
Robinson & Sutherland (1997) quantified seed density and related it to density of skylarks. In
each field, 10 samples were taken at randomly selected positions. For each sample, eight 5 cm
soil cores were taken to a depth of 3 mm at random positions within a 50 cm x 50 cm quadrat.
Seeds were then separated and identified to species level with the aid of a binocular microscope.
Draycott et al., (1998) measured weed seed and waste cereal availability to pheasants, in arable
fields on 16 farms in southern and eastern England. Sampling involved randomly throwing a
0.25m2 quadrat within 20 m of the field boundary and scraping the top 1 cm soil and seed
bearing vegetation into a plastic bag (after Klute et al., 1997).
Wakeham-Dawson & Aebischer (1998) measured availability of weed seeds to foraging
skylarks by placing two 0.25 m2 quadrats 50 m from the field margin and 25 m apart in each
field, and sucking up the seeds in two passes of a Halfords 12 V car vacuum cleaner, pressing
the head down to the ground to allow all seeds to be collected.
Robinson & Sutherland (1999) measured seed density and related this to habitat preferences of
seed-eating birds. Seed density in the upper layer of soil was estimated by taking samples at ten
random positions in each field (in the last two weeks of November and the last two weeks of
March). Each sample consisted of eight individual cores (each 5 cm in diameter) 6 mm in depth
bulked to give a total area of 0.016 m2, seeds extracted by washing through a stack of sieves,
and hand-counted. Marshall & Vickery (2000) used a ‘cyclone sampler’ to take surface soil
samples to measure seed availability on stubble fields. Seeds were subsequently extracted from
soil samples by eye.
Table 6.3 gives the frequency by which the different methods have been used in the studies on
farmland birds reviewed above. In summary, a wide variety of techniques have been used to
estimate the abundance of farmland bird foods. The choice of method has mainly been based on
knowledge of the diet and foraging behaviour of the species concerned, but also on time
available, practicality and cost. For each method that has been used, the sampling regime also
varies widely between studies. Few studies justify the sampling regime used, and few provide
7
data (e.g. the distribution and number of sampling points per field, depth of soil core taken,
etc.). Some studies do not describe it adequately.
Table 6.3 The frequency with which methods for estimating the abundance of bird food
resources have been used in the studies reviewed in this section.
Bird food resource estimated
Method
Invertebrate abundance
Sweep netting
Abundance of weeds
Abundance of seeds
6.4
Number of studies (out of 35)
using method
11
D-Vac
8
Soil corer
4
Other suction device
2
Pitfall traps
3
Soil extraction
2
Visual count
1
Emergence traps
1
Visual estimation of % cover
6
Biomass
1
Density (grass spikelets)
1
Soil sample/core
6
Suction apparatus
2
Visual count
2
Repeatability
Most studies comparing sampling methods or devices have tended to concentrate on their
relative efficiencies (e.g. Stewart & Wright, 1995) rather than on their repeatability. Also, the
repeatability of destructive methods, such as the various vacuum sampling methods for seeds,
will be difficult to separate from spatial variability without undertaking carefully designed
experiments. Perhaps for this reason, few studies on terrestrial arthropods have attempted to
investigate repeatability of the methods used to measure food availability. The seed bank is
highly variable in space and time, in addition to any variability due to the method used.
Nevertheless, a few studies give an indication of the repeatability of the methods which have
been used.
Marshall & Vickery (2000) investigated the efficiencies of four different methods of seed
sampling: visual counts, brush sweeping (using a stiff plastic brush and dustpan), cyclonesampling (using a prototype petrol-driven vortex vacuum sampler) and Aquavac sampling
(using a car battery operated vacuum sampler). Seeds were sampled on wet soils in December
and in a designed experiment in which known densities of rape and grass seeds were sown on
dry soil in March. Overall, the cyclone sampler gave the best seed recoveries, while sweep
8
sampling consistently underestimated seeds. Cyclone samples varied from 15 to 100 seeds,
equivalent to 167 to 1111 seeds m-2. This extent of variation suggests that, at first sight, the
repeatability of the cyclone technique does not appear to be very satisfactory. In addition, the
process is time-consuming, as seeds must be separated from soil in the samples back in the
laboratory.
The efficiency, and hence repeatability, by which seeds visible on the soil surface are counted is
likely to be high and dependent on the care and time taken by the observer. However, a good
estimation of the mean density of exposed seeds on fields will depend on the spatial variability
and the number and size of quadrats per field. For example, Pascual et al. (1999a), using 20
sampling points and 0.25 m2 quadrats, estimated means ranging from 2.9 to 10.58 wheat seeds
per quadrat and standard errors around 2. That is, 95% of samples were within about 4 seeds of
the mean.
Holland et al. (2002) measured total invertebrate food availability for farmland birds. Using the
D-Vac method, they estimated Chick Food Index (CFI) for grey partridge and chick food
availability (CF) for yellowhammers at study sites in Hampshire and Lincolnshire. They
estimated variability between and within fields and found significant differences between crops
and between the edge and centre of fields. For a mean total number of insects of 1.72 for the
edge and 1.63 for the centre of fields, the pooled standard error was 0.02, and for a mean CFI
ranging from 0.07 for spring barley to 0.14 for winter wheat, the pooled standard error was 0.05.
These data suggest that 95% of D-vac samples will fall within about 0.1 of means ranging from
about 0.1 to 2. That is, repeatability of the D-vac method appears satisfactory.
6.5
Availability or abundance?
Only items known to be present in the diet of the bird species being studied should be measured
when estimating food availability. Diet composition can be investigated by observation of
foraging birds (e.g. corn buntings: Gillings & Watts, 1997), from crop or gizzard contents of
some species (e.g. woodpigeons: McKay et al. 1999), faecal analysis (e.g. yellowhammers:
Moreby & Stoate 2001), and bolus analysis of neck-ringed chicks (e.g. corn buntings:
Gliemann, 1973). For invertebrates, methods appropriate to the species in the diet should be
selected, e.g. sweep sampling for mobile species or the D-Vac method for surface active
species. For example, Holland et al. (2002) used the D-Vac technique to estimate general
invertebrate food availability for farmland birds and availability specific to grey partridge and
yellowhammers from knowledge of their diet. If invertebrate availability, as opposed to
abundance, is to be measured, samples should be taken at the time of year, time of day, weather
conditions and in the habitat and microhabitats in which the birds are known to forage.
It should be appreciated that methods for assessing diet composition also have associated
problems. Larger items in the diet are more easily seen and identified than smaller items in
observation studies, some items may not be retained in the crop in crop content studies, and
some may be completely digested and not be detectable in faecal analyses. All three methods,
therefore, are usually qualitative rather than quantitative, without very careful interpretation of
the data.
Choice of sampling site is also crucial if availability rather than overall abundance is to be
measured. For example, Brickle et al. (2000) investigated habitat use by corn buntings
collecting food for nestlings by comparing the proportion of foraging visits to each habitat with
the proportional availability of habitats around the nest. However, they then sampled chick food
in every discrete habitat block within the study area using sweep netting. In contrast, Green
(1984) sampled the abundance of grey and red-legged partridge chick foods during the main
hatching periods, within the foraging areas of the birds (which were radio-tracked) and Hill
(1985) sampled pheasant chick insect food within the home range of radio-tracked broods.
9
It is not clear how seed is available to seed-feeding birds. Seed shed onto the surface is available
to ground feeders, but if seed is covered by soil, is it unavailable? The seeds of many weed
species are very small and possibly not actively sought. Availability of different seed sources is
likely to be bird species-specific (Marshall & Vickery 2000). The timing of seed sampling is
also crucial if availability rather than abundance is to be measured. Seed density must be
measured at the time of year, and in the particular habitat, that birds forage.
In general, seed not visible on the soil surface is assumed to be unavailable to most seed-feeding
birds. Marshall & Vickery (2000) made this assumption and so only estimated seeds on plants
and on the soil surface. Similarly, McKay et al. (1999) and Pascual et al. (1999a) estimated the
density of winter cereal seed visible on the surface of newly drilled fields. Pascual et al. (1999a
and 1999b) found that agricultural methods which resulted in more seed being buried, such as
sowing seed deeper, and rolling and harrowing, reduced the availability of seed to woodpigeons
in pen trials. If it is true that most birds do not dig for buried seed, then the methods which rely
on removing the top layer of soil (which includes vacuum sampling techniques under some
conditions) may be overestimating the availability of seeds to birds.
6.6
Standardisation
Farmland birds can be separated into guilds based on diet and foraging behaviour. Both diet and
foraging behaviour are reviewed in Buxton, Crocker & Pascual (1998), the CSL ‘bird bible’.
This information was used to separate the species considered in this report into six categories
(Table 6.4). Species feeding only on the wing and only by probing the soil, are not considered in
this project and are excluded from the table. For each category, a method for estimating
availability was selected, based as far as possible on information on repeatability and efficiency.
Because there are few studies comparing different methods, and assessing their efficiencies and
repeatability, this should be regarded as an informed suggestion based on current knowledge. A
considerable amount of further work is required before a definitive list of standard methods can
be put forward.
6.7
Discussion
The standard techniques for measuring the abundance of plants and arthropods are listed, and
summaries given of their known efficiencies and limitations. The techniques which have been
used in studies of farmland bird food resources are reviewed with a view towards
standardisation. A wide variety of methods are found to have been used, governed mainly by the
species of bird being considered and its foraging behaviour and habitat. Other considerations
include cost, staff resources available and time. The methods vary in terms of their efficiency
and probably repeatability, though few data are available on the latter. Also, different methods
are affected in different ways by external factors such as vegetation structure and climatic
conditions. It is therefore very difficult to compare the results of different studies, and to
combine their data when constructing models.
Generally, methods measure relative rather than absolute availability (Southwood, 1978) and
this becomes important when attempting to compare the amount of food available to the amount
of food needed to survive or reproduce (see Objective 1). More work is needed, therefore, on
the efficiencies of the different methods in a range of different habitats, and hence on the
calibration coefficients needed to convert relative to absolute abundance for each device used.
Farmland bird species can be divided into guilds based on their diet and foraging behaviour.
When this was done for the species being considered in this report, we were able to suggest
standard methods for assessing food availability. The method selected for each guild was based
on the frequency by which the various methods have been used in previous studies, as well as
10
efficiency and any data on repeatability. Cost and ease of use have also been taken into
consideration, although these are regarded as being of secondary importance.
6.8
Further work
Further work is needed on the repeatability and efficiency of most methods reviewed in this
Section, in particular the D-Vac method and sweep netting.
The D-Vac device, despite being the most popular method for sampling surface arthropods, is
expensive, heavy, cumbersome to use and has safety implications (the operator walks about
with a heavy, noisy petrol-driven motor strapped to his/her back). Further work is therefore
needed to improve upon its design, while still maintaining a comparable efficiency of
extraction.
11
Table 6.4 Categories of farmland birds based on diet and foraging behaviour, used to select standard methods for the measurement of food availability.
Species considered in the present study are placed into these categories based on their winter and spring/summer diets.
w= winter, s=spring/summer
Diet
Seeds
Foraging behaviour
Mainly forages on
ground
Seeds
Forages on ground
and by disturbing
surface
Forages on ground,
disturbs surface and
searches plants and
trees
Seeds, plants
and/or fruit
Invertebrates
Invertebrates
Invertebrates
Mainly forages on
ground and low
vegetation
Forages on surface
and below ground,
e.g. by digging
Caught on the ground
and on the wing
Species
Chaffinch (w)
Collared dove
Corn bunting (w)
Grey partridge (w)
Rook (w)
Stock dove
Turtle dove
Woodpigeon
Yellowhammer (w)
Red-legged partridge (w+s)
Skylark (w)
Quail (w+s)
Goldfinch
Greenfinch
House sparrow (w)
Linnet
Reed bunting (w)
Tree sparrow (w)
Yellowhammer (s)
Grey partridge (s)
Stone curlew
Quail (s)
Chaffinch (s)
Red-legged partridge (s)
Rook (s)
Tree sparrow (s) Skylark (s)
Corn bunting (s)
House sparrow (s)
Reed bunting (s)
Yellowhammer (s)
Yellow wagtail
Fringilla coelebs
Streptopelia decaocto
Miliaria calandra
Perdix perdix
Corvus frugilegus
Collumba oenas
Streptopelia turtur
Columba palumbus
Emberiza citrinella
Alectoris rufa
Alauda arvensis
Coturnix coturnix
Carduelis carduelis
Carduelis chloris
Passer domesticus
Carduelis cannabina
Emberiza schoeniclus
Passer montanus
Emberiza citrinella
Perdix perdix
Burhinus oedicnemus
Coturnix coturnix
Fringilla coelebs
Alectoris rufa
Corvus frugilegus
Passer montanu
Alauda arvensis s
Miliaria calandra
Passer domesticus
Emberiza schoeniclus
Emberiza citrinella
Motacilla flava
12
Recommended standard method
Count of surface seed using 0.25 m2 quadrats
(c.f. Pascual et al. 1999a)
Removal of top 1 cm soil, followed by
separation and counting of seed (c.f. Robinson
& Sutherland 1999).
Removal of top 1 cm of soil and vegetation,
separation and counting of seed and fruit and
weighing of plants (cf. Diaz & Telleria 1994)
For trees, count seed or fruit in situ.
D-Vac (cf. Ewald & Aebischer 1999).
Removal of vegetation and 1-5 cm of soil
surface (depth depending on bird species),
separation of inverts into groups and weighing
(cf. Tucker, 1992).
D-Vac + sweep net (cf. Brickle et al., 2000)
6.9
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