Fitness consequences of diet choice in waterbirds of agricultural and natural landscapes
Final Report Abstract
European waterfowl populations, e.g. geese and swans, have been dramatically increasing probably due to the year-round availability of high quality food in agricultural habitats. We investigated the effects of habitat and diet choice on fitness in greylag geese as one of the most abundant goose species in Europe. To this end, we first characterized potential feeding habitats by stable isotope pattern. Second, we determined individual habitat choice and diet choice by isotopic composition of tissues grown in two distinct periods within the annual cycle that potentially differ in the requirements for food quality and site safety: the early growth period of goslings and the postbreeding moult period of adults. And third, we searched for general pattern in habitat use in four greylag goose populations in the UK, The Netherlands, Sweden and Germany. Finally, we related habitat quality to fitness relevant parameters, e.g. annual reproduction success. Habitats: We used stable carbon and stable nitrogen isotope ratios in potential food plants to discriminate between habitat types (freshwater and terrestrial habitats) and agricultural practises (terrestrial habitats only). Plants from freshwater habitats had significantly higher dC-values than plants from terrestrial sites; no differences had been found for dN values. Moreover, plants from habitats with or without a currently extensive agricultural practise did not differ in dN patterns compared to plants from currently high-intensity farming sites. This may result from isotopic composition in soil persisting after changes in the farming regime. Therefore, we concluded that the isotopic nitrogen composition (dN) is not suitable for analysing diet selection on a small scale, whereas dC pattern does allow studying habitat selection on a broad scale. Habitat/diet selection in geese: We used carbon stable isotope ratios in wing/tail coverts, which are grown during particular life-history periods, i.e. offspring raising and moult, to determine whether the geese were feeding in terrestrial or freshwater habitats. During offspring growth, stable carbon isotope ratios (δC) showed low variation within and between populations indicating uniform habitat choice with an average proportion of terrestrial food between 87% (D) and 96% (GB). However, feathers grown during postbreeding moult had higher δC values in three populations and similar δC in the German population in comparison to feathers of goslings. Such higher δC are typical for freshwater plants, and thus, adult greylags preferred feeding in wetland habitats over feeding in terrestrial sites alone. The proportion.of freshwater plants in the diet increased to 11% in British geese, 20% in Swedish birds and 23% in bird moulting in the Netherlands. Fitness relevant parameters: Diet quality directly affected juvenile survival: in habitats wilh high food quality, e.g. intensive agriculture, gosling survived better than in habitats with low food quality, e.g. (semi)natural grassland or grassland with extensive farming. Conclusion: Our study showed that greylags from Scandinavia, central Europe and Great Britain generally relied on terrestrial habitats for feeding during offspring growth and moult. However, the degree of terrestrial site use changed within the annual cycle with freshwater habitats becoming more important during post-breeding moult.
