Recommendations for depletion modelling of granivorous birds
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APPENDIX 3 Rank weed and crop seeds (+ crop leaves) according to the proportion in the diet for each bird species.Dietary information was collected for 22 farmland bird species, 12 of which are of high conservation concern (Red List species) and three of medium conservation concern (Amber List species) (from Gregory et al. 2002) (see Table 3.1). An extensive literature review was carried out and a total of 100 references from the United Kingdom and other European countries were consulted (Table 3.1). This was achieved through a combination of database searching and cross referencing using the bird species and diet as the main search terms. Data from foreign studies was used to increase the breadth of dietary information for each bird species, although there may be some differences in the plant species available and the habitats. Papers were not included if the habitat in which the study was conducted differed substantially to that found in western Europe. Only papers presenting quantitative data were included. Table 3.1. Farmland bird species for which dietary information was collected and their conservation status. Conservation status is divided into three categories (see Gregory et al. 2002): ** = Red List species, * = Amber List species, - = species which are not threatened. The number of references (United Kingdom and Foreign) used to rank plant and invertebrate taxa for each bird species is also given.
Seasonal differences in diet were investigated and dietary information was divided into breeding and non-breeding seasons based on the known breeding period in the United Kingdom (Buxton, Crocker & Pascual 1998, RSPB website 2002). Rankings were determined separately for:
Rankings were calculated separately for the plant and invertebrate components of the diet, however the ratio of plant to animal material reflects the relative contributions of each to a bird’s nutrition and is important for assessing the overall importance of seeds versus invertebrates in the diet. Birds were classified into phytophagous, insectivorous or mixed species based on the ratio of plant to invertebrate components in the diet as reported in the literature (Table 3.2). The ratio was determined by the mean proportion of the plant component (i.e. percent of items and/or percent biomass) from a number of studies. If the plant component comprised more than 75% of the diet then the bird is phytophagous (seed or leaf-eating), and if the plant component comprised less than 25% it is considered insectivorous (insect-eating). The remaining birds are those that consume a mixed diet of seeds and invertebrates (plant proportion of 25-75%) (see Table 3.2). It was not possible to separate cultivated species from weeds, as this was not always identified in studies. Many adult birds show seasonal changes in the composition of their diet, with the proportion of invertebrate material being highest during the breeding season (e.g. quail, reed bunting, rook, skylark, tree sparrow). Adults and young often have quite different diets, and chicks usually consume a higher proportion of invertebrate material than adults, with the exception of the greenfinch and linnet (which are granivorous), and members of the Columbidae i.e. collared dove, stock dove, turtle dove, and woodpigeon (which are fed crop milk initially). For some species, chicks consume mainly invertebrates shortly after hatching and then switch to a diet of seeds later in the breeding season (e.g. goldfinch, grey partridge). To investigate the importance of different plants only studies that listed plant taxa to family level or lower (i.e. genus or species) were used in the analysis. The following information was extracted from each reference (where possible):
Some studies covered more than one bird species and used more than one method to assess diet. Analysis of crop and/or stomach contents was the most commonly used method for investigating the composition of plant taxa in the diet (Table 3.3). Rankings of weed taxa were calculated at three taxonomic levels (i.e. Family, Genus and Species) and by two different methods:
Table 3.2. Ratio of plant to invertebrate components in the diet of farmland birds. Birds were classified into three dietary types: phytophagous (P), insectivorous (I), or mixed (M). NB. The plant component includes cultivated species. If the exact proportions of plant and invertebrate components were not known (indicated by -), the bird was classified based on the qualitative descriptions given in the literature.
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