Field sampling
To document the morphology and diet of black wheatears we carried out captures throughout the study area, during the entire breeding season from April to August of 2014 to 2016, using spring traps baited with mealworms (Tenebrio molitor ). Birds were removed from the traps immediately after being captured, placed in a cotton bag, and afterwards ringed and measured. Birds were retained for less than 15 minutes and all procedures were made with the required permits from national authorities. We made a total of 143 captures, but for this study we only considered the first capture of adult individuals, i.e. 2nd calendar year or more identified following Svensson (1992), totalling 110 adult black wheatears, 79 males and 31 females. For each individual, a number of morphometric measures were taken following Svensson (1992): maximum cord wing length; 3rd primary length; tail length; tarsus length; bill length, depth and width at the distal edge of the nostril; and body mass. Wing, 3rd primary and tail were measured using a ruler to the nearest 0.5 mm, tarsus and bill measurements were made with a calliper to the nearest 0.1 mm, and body mass with a digital balance to the nearest 0.1g. All measures were taken by LPS and when feathers were not fully developed (i.e. moulting birds) the measurements affected were not recorded (Table S1).
Droppings for molecular analysis were collected from bird handling bags, or directly from small rocks used to disguise the bottom of the spring traps (McInnes et al., 2017; Oehm, Juen, Nagiller, Neuhauser, & Traugott, 2011). Bags were soaked in 10% bleach for 1 hour and then washed between each use to minimize contamination. From the 93 droppings thus collected, 62 from males and 31 from females, three were obtained from birds that defecated inside the traps but were not captured. Droppings were stored in 2ml tubes with 98% ethanol at 4ÂșC until laboratory analysis (da Silva et al., 2019a).