On starlings and farming: population decline, foraging strategies, cost of reproduction and breeding success

Abstract: The European Starling Sturnus vulgaris as many other farmland bird species has been declining during the last decades. The cause of these declines is generally believed to be changes with in agricultural methods affecting breeding and survival of birds. For starlings in Southern Sweden the amount of pasture in the landscape surrounding breeding colonies is the key factor. We found a positive relationship between the amount of pasture and breeding success as well as breeding densities. One probable explanation for this is a positive relationship between the amount pasture and food availability. To test this we took soil samples which showed a higher abundance in pasture and other permanently grass grown areas than in cereal fields, starlings also have higher foraging efficiency on grass covered areas. In detailed studies with radio-telemetry we showed that pasture is the preferred foraging habitat, but when the amount of pasture decreases starlings also use margin habitats. In landscapes with low amount of pasture starlings flies longer distances per feeding trip and also spend more of the total foraging time flying. To reduce flying cost the combine long trips to pasture, with shorter to alternative habitats when pasture is scarce. The reason for this is that starlings find higher amounts of their preferred prey, caterpillars, in pasture than in other kinds habitats. When starlings do not forage in pasture they fly longer distances searching for caterpillars. Caterpillars seem to be high quality food for nestlings and we also found a positive effect on nestlings tarsus length of the proportion of caterpillars in the diet. We could not show any effects on male starling humoral immune response of a brood size manipulation. Neither did the amount of pasture affect immune capacity, but both the landscape category and brood size affected breeding success.