Recommendations for depletion modelling of granivorous birds
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3.1.3 are there similarities in dietary composition between bird species?Correspondence analysis was used to investigate whether there were similarities in dietary composition between bird species (CANOCO 4.02, GLW-CPRO 1997-1999). A list of standard weed taxa was defined in order to overcome differences in taxonomic resolution between studies. Cultivated plants were not included in the analysis, and bird species that had fewer than two weed taxa in their diets were also excluded from the analysis. The percentages of weed taxa in the diet were not used because of the difficulty in comparing studies that use different measures of proportion in the diet (i.e. % of items, % occurrence, % biomass). For each bird, the mean percentages of weed taxa were converted into three abundance categories: 0 = not present in the diet 1 = present (mean is 0-20% of the diet) 2 = abundant (mean is greater than 20% of the diet)
Figure 3.1. Correspondence analysis of the plant component of bird diet (adults and young) during the breeding season (31.2% variance explained). AB = adults in the breeding season, C = chicks. Chaff = chaffinch, gold = goldfinch, green = greenfinch, grey = grey partridge, house = house sparrow, stock = stock dove, tree = tree sparrow, turtle = turtle dove, wood = woodpigeon. The first ordination axis was associated with increasing amounts of Convolvulus arvensis (Convolvulaceae) in the diet. Turtle doves and woodpigeons (Columbidae) had very similar diets and appeared at the higher end of Axis 1, indicating that they have relatively high proportions of these weed taxa in their diets. Conversely, members of the Fringillidae (goldfinch, greenfinch, and linnet) appeared at the lower end of this axis, which indicates that they consume smaller amounts of Convolvulus arvensis. The second axis represented an increase in the proportions of Silene spp. (Caryophyllaceae), Galeopsis spp. (Labiatae), and Euphorbia exigua (Euphorbiaceae) in the diet. Grey partridge (Perdix perdix) adults and chicks appear at the upper end of this axis, and therefore had the highest proportions of these taxa in their diets. 3.1.3.2 Diet of adults during the non-breeding season The diet of adults during the non-breeding season was examined using correspondence analysis, and a plot of the first two ordination axes (explaining 34.3% of the variation in the data) is shown below (Figure 3.2). The first ordination axis was associated with increasing amounts of Sonchus spp., Hypochaeris radicata, Lactuca spp. (Compositae), Dipsacus fullonum (Dipsacaceae), and Pinus sp. (Pinaceae) in the diet, and the finches (goldfinch, greenfinch, and linnet) appeared at the higher end of this axis. The two partridges (grey and red-legged) appear to have similar diets, both appearing at the lower end of the first axis indicating they have low proportions of Compositae in their diets. The second ordination axis represented an increase in the amounts of common cleaver Galium aparine (Rubiaceae) and Valerianella sp. (Valerianaceae) in the diet. The house sparrow and cirl bunting (Emberiza cirlus) are separated from the other bird species along this axis and appear to consume high proportions of these weeds.  Figure 3.2. Correspondence analysis of the plant component of adult bird diet during the non-breeding season (34.3% variance explained).
3.2 List calorific value (and digestibility) of crop and weed seeds. Information on the calorific contents of plants and animals was compiled by Joe Crocker from the Central Science Laboratory. Data on farmland weeds were extracted from this, although there was limited information on the calorific and nutritional contents of weeds that are important in the diet of farmland birds (as identified in Section 3.1). The calorific values (kJ/g) of some important weed taxa are given below (Table 3.6), and the values for seeds and green material/leaves are listed separately (when available). If the plant was not identified to species level in dietary studies, the calorific value of a related or unspecified species is given instead. There was no calorific information on chickweed Stellaria media (Caryophyllaceae), redshank Polygonum persicaria (Polygonaceae) or charlock Sinapis arvensis (Cruciferae), which were among the most important foods for both adults and chicks (see Table 3.4C). Table 3.6. Calorific values (KJ/g) of selected weed species that are important in the diet of farmland birds. The values for seeds and leaves (green material) are listed separately (when available). The number of bird species for which they are important is also given (as in Table 3.4C).
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