Capture, sampling, and measurements
All species except wheatears were caught using funnel traps. Wheatears were caught using mealworm baited spring traps. Blood was collected from the wing vein using Na- heparinized microcapillaries. All birds were bled within 10 minutes of capture (range= 1-9.30 minutes, mean= 8.09 minutes), and hence before any expected impacts of handling stress on immune parameters (Zylerberg et al. 2005). The plasma was separated from the blood by centrifugation within 20 minutes of sampling for species caught in funnel traps, and within 2 hours of sampling for the wheatears. The plasma was stored at -20 °C during the field season and at -50 °C later on.
The birds were ringed and measured after blood sampling. Body mass was measured to nearest
0.1g. Fat score was estimated on a scale of 0 (no fat) to 8 (furcular and abdomen bulging, and
breast covered with fat) based on methods described by Kaiser (1993). Muscle score was estimated on a scale of 0 (sharp sternum, muscles depressed) to 3 (sternum difficult to distinguish due to rounded muscles) based on methods described by Bairlein (1994). All the measurements were consistently carried out by a single person. The sex of the birds was determined based on the plumage for Wheatears and Chaffinch, and molecular sexing was performed for Song thrush and Dunnock. All procedures were approved by the Ministry of Energy, Agriculture, the Environment, Nature and Digitalization, Schleswig-Holstein, Germany (permit number V 242-37068/2016).