Recommendations for depletion modelling of granivorous birds
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Table 3.3. Number of references containing information on the plant taxa eaten by farmland birds, and the sampling methods used to estimate dietary composition.
3.1.1 ranking plant taxa which are important for all bird species Rankings were determined by the total number of bird species for which a particular plant taxon was important. A plant was considered important if it made up 5% or more of the vegetable component of the diet (from percent of items, percent occurrence or percent biomass). This is a relatively simple method and is comparable with previous attempts at ranking the importance of food items for farmland birds (e.g. Wilson, Arroyo & Clark 1996; Boatman 2001). As discussed by these authors this approach can, however, lead to some bias as the methods are not directly comparable. For example low numbers of heavier food items may have a higher ranking when considering percantage biomass compared to percentage of items. Nevetheless all approaches were used as it was considered important that the highest number of studies were included thereby giving breadth to the analysis. The rankings of weed families, genera, and species are presented below (Table 3.4 A-C). Rankings for weeds are listed below from highest (rank 1) to lowest importance (Table 3.4A). These were determined separately for adult diet in the breeding and non-breeding seasons and for chicks based upon the total number of bird species. An overall ranking for each weed was also calculated by averaging the rankings from the three bird categories (adults – breeding, adults – non-breeding, and chicks). Fewer bird species are included in the assessment of chick preferences because most birds feed their chicks an insectivorous diet. The most important plant families overall were Poaceae (average ranking of 2.0), Polygonaceae (2.3), Caryophyllaceae (3.3), Cruciferae (3.3), Chenopodiaceae (4.5+), Compositae (6.3) and Labiatae (8.5+). These rankings are almost identical to those of Boatman (2001) who classified Poaceae, Polygonaceae, Caryophyllaceae, Chenopodiaceae and Cruciferae (ranked in that order) as very important foods and Compositae, Labiatae as important. These groups were also identified by Wilson et al. (1996) as being amongst the most important. It is worth noting, however, that the separate rankings for adults and chicks in the current study revealed that Chenopodiaceae and Labiatae were important food items for adults, but not for chicks (see Table 3.4A). When plant genus was considered there were relatively few important ones within each family (Table 3.4B) and the rankings changed slightly. For adults in the breeding season Stellaria (Caryophyllaceae) and Polygonum (Polygonaceae) were important to many more species than the other genera. These were also the highest ranked ones for non-breeding adults along with Chenopodium and Poaceae. For chicks Sinapsis, Poa, Stellaria and Polygonum were the highest ranked. Of the individual weed species Stellaria media (chickweed) was the highest ranked for adults in the breeding season along with Poaceae and Polygonum aviculare (knotgrass) (Table 3.4C). Other weed species were only important to two or less bird species. Non-breeding adults has different preferences with Poaceae, Chenopodium album (fat hen), Stellaria media and Polygonum persicaria (redshank) being the highest ranked. Stellaria media and Polygonum persicaria were ranked highest for chicks. Although a large number of weed species were identified as being important for 1-2 bird species, in reality most birds would consume whatever was available provided it was not toxic in any way in order to ensure the maximum calories were consumed for the minumum of foraging effort. As a consequence in many studies the diet may reflect availability rather than preference. This may only be overcome by conducting dietary studies in a wide range of locations, over several years combined with measurements of availability. Table 3.4. Rankings of weed taxa based on the number of bird species for which they are important (>5% of the plant component of the diet). Rankings were determined at three different taxonomic levels - A: Plant families, B: Plant genera, C: Plant species. Weeds are ranked from highest (1) to lowest importance. A: Plant families
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