We conducted fieldwork from April to June in 2013 and 2014 near the village of Goritsa (42°55′N, 27°49′E), East Bulgaria (Fig. 1). The study site is located in deciduous oak forest with dominant species of Hungarian oak (Quercus frainetto) and Austrian oak (Q. cerris). Breeding adult semi-collared flycatchers were caught while feeding nestlings or during incubation in the nest boxes, or with mist-nets set-up close to the nest boxes. In total, 110 individuals were sampled, including 13 sampled during both years. We collected a blood sample (10–50 μl) by puncturing the brachial vein. Samples were stored in SET buffer (0.015 M NaCl, 0.05 M Tris, 0.001 M EDTA, pH 8.0) at ambient temperature in the field and later at 20 °C. Blood films were prepared on glass slides and were air dried within 5–15 s after their preparation. Smears were fixed in absolute methanol for 4 min on the day of their preparation. In the laboratory, the blood films were stained in a 10% working solution of a commercially purchased stock solution of Giemsa's stain, pH 7.0–7.2, at 18–20 °C for 60 min