Birds were ringed and, date, recovery location, body mass, wing length (W), tarsus length (T), skull length (S), and four bill morphometric measurements were recorded. Bill measurements were culmen (C, from the base of forehead feathers in centre of nasal tube to distant part of the curve of the hooked bill), bill length at nostril (BL, from centre of dorsomedial part of tube to distant part of the curve of the hooked bill), bill depth (BD, from the base of forehead feathers to ventral surface of lower mandible), and bill depth at nostril (BDN, from the base of nasal tube at nostrils to ventral surface of bill). The biometrics were taken by the same person (BR) using a spring balance (nearest 5 g), a rule (precise to 1 mm), and an electronic calliper (nearest 0.01 mm). The presence of down in the head and belly was assessed in an ordinal scale (0 = absence, 1 = presence of down, and 2 = entirely covered by down), and the sum of the two values (head and belly) was used as a down index (DI), ranging from 0 (down absence) to 4 (head and belly entirely covered by down). Birds were assigned to one of the three treatments: GPS-tagged, tape-labelled, and control birds. GPS birds were tagged with customized GPS-GSM devices designed and provided by DigitAnimal (www.digitanimal.com, Móstoles, Spain). Each device was put in a heat-shrink tube for waterproofing, and its final size was 28 × 55 × 18 mm. The device weighed 23 g. Devices were attached to the mid-dorsal feathers of birds with TESA tape. GPSs were programmed to record a position every 30 seconds from approximately one hour after sunset on the day of deployment. We attached a 4-5 cm long stripe of TESA tape to the mid-dorsal feathers of tape-labelled birds. Then, we wrote a code number with a black permanent ink pen. Control birds were not GPS nor tape-labelled, but they were handled as long as the other treatment birds, and they were also measured and banded to allow identification.

Peer reviewed