Field and lab procedures
Nightjars are attracted to roads at night (Jackson, 2003), so road transects are one of the most commonly used methods for nightjar monitoring (Camacho, 2013b; De Felipe et al., 2019). From late March to late October, we conducted nocturnal surveys for red-necked nightjars on a regular (3-7 days) basis along a 24-km road transect by driving a car at slow speed (30 km h-1). Nightjars found on roads were captured using a torch and a butterfly net (Jackson, 1984) and marked with numbered metal rings (if not ringed already) for individual recognition. Birds were sexed and aged as first-year (juvenile), second-year (approx. 1 year old), or older using plumage characteristics (Camacho, 2013a; Forero et al., 1995), and measured according to the standard protocol for long-term monitoring of nightjars (see details in Hidalgo-Rodríguez et al., 2021). Many individuals’ ages are known because they were first captured as juveniles (Camacho et al., 2022).
To estimate the frequency of damage on limbs, we carefully examined the feet and toes of all captured individuals, noted anomalies (e.g. breaks in skin, and full or partial amputations; Figure 1a) and checked for remains of the attacker (e.g. mandibles or full head; Figure 1b-d). The remains found during the three years of systematic record were removed from the bird and preserved in 96º ethanol for taxonomic identification in the laboratory. The remains, specifically a left mandible found in 2021 and a full head found in 2023, were closely examined under a 80x stereomicroscope to identify the attacker to the genus or species level (Borowiec, 2016; Fisher & Bolton, 2016).
We calculated the proportion of juveniles and adults missing feet or toes (or in the process to lose them, as determined from the severity of damage) as a measure of the incidence of damage, and used the Fisher’s exact test as implemented in R (https://cran.r-project.org/) to test the null hypothesis that the incidence of damage does not differ between age classes. Non-systematic data collected from birds trapped during ringing operations in previous years (2009-2020) provided additional useful information about the nature of damage.