The study area was in Troms County (68–70°N, 15–22°E), northern Norway, which together with Lofoten and Vesterålen is an important nesting area currently supporting approximately 1000 breeding pairs of WTE (Trond V. Johnsen, unpublished data). Samples of 35 WTE nestlings were collected in late June and early July in 2011 (n = 19) and 2012 (n = 16), respectively. A large number of nests were visited from late March to mid-May to record breeding activity and egglaying (n=83). Field observations of nests were performed froma distance with binoculars and telescope to minimize disturbance. The sampling took place a fewdays prior to the expected fledgingdate, i.e. when the chicks were approximately 8 weeks of age. The nestlings were removed from the nests during sampling in order to collect data. Prior to blood sampling, morphological measurements were collected: body mass (g) with a spring balance; the length of the wing (mm) with a ruler, and skull (mm) with a sliding calliper. Blood samples (4–10 ml) were taken from the brachial vein using heparin coated syringes. At the end of each sampling day the blood samples were centrifuged at 8000 rpm for 10 min, plasma and red blood cells (RBCs) were transferred to sterile Eppendorf tubes and stored at −20 °C until analyses in the lab. Plasma samples were subsequently used to assess organochlorine and perfluoroalkylated concentrations, SOD activity and immunoglobulin levels. Red blood cells were used to assess telomere length and determine the sex. The nestlings were sexed following an adapted protocol by the IPHC-DEPE, France, described by Helander et al. (2007). Briefly, DNA was extracted from RBCs using a commercial kit (NucleoSpin® Blood QuickPure,  Macherey Nagel, Germany) and the sections of the sex-linked chromo-helicase-DNA-binding gene (CHD-Wand CHD-Z) were amplified by polymerase chain reaction (PCR) using primers 2550 F/2718R [2550 F (5′-GTT ACT GAT TCG TCT ACG AGA-3′)/2718 R (5′-ATT GAA ATG ATC CAG TGC TTG-3′) (Fridolfsson and Ellegren, 1999)].