Griffon Vultures scavenging at night: trophic niche expansion to reduce intraspecific competition?

Vultures feed exclusively on carrion and so are the only obligate scavengers among extant vertebrates (Ruxton and Houston 2004). Because the occurrence of carrion is unpredictable, vultures soar over large areas during daylight searching for carrion with their keen eyesight (Ruxton and Houston 2004). Although some data exist on vultures departing or arriving at roosts at dusk or dawn, respectively (Nathan et al. 2012), foraging outside daylight hours has only been reported in American Black Vultures (Coragyps atratus; Charette et al. 2011). We document here a new foraging behavior of Griffon Vultures Gyps fulvus, scavenging at night (Fig. 1), which has not been described in Old World vultures and adds to the repertory of unexpected behaviors reported for the species. At least 14 different Griffon Vultures were recorded feeding on two naturally dead cows Bos taurus (~400–500 kg each) during five nights in summer 2014 in alpine pastures of northwestern Spain (Fig. 1; Appendix S1). Vultures were recorded scavenging a mean of ~3 h per night (range: 33 min–6 h; i.e., between sunset, 22:08, and sunrise, 06:43, local time UTC+2) by motion-triggered remote cameras (Fig. 1a, c). Griffon Vultures scavenged at night only at these two carcasses out of 93 carcasses of wild and domestic ungulates monitored in the study area during 2011–2014 (i.e., 2%; Appendix S1; Mateo-Tom as et al. 2017). Vultures fed on the remaining carcasses exclusively during the day (i.e., mean 12:50 hours; Rayleigh and Rao tests, P < 0.001; N = 91 carcasses). The two carcasses here reported appeared in the afternoon (mean SE: 17:26 hours 1 h 51 min); these times were not significantly different from those of other ungulate carcasses in the area (13:50 hours 40 min, Kolmogorov–Smirnov test for fitting a lognormal distribution, P = 0.062; N = 20 carcasses). Vultures arrived at both carcass types at approximately the same time after the animals died (i.e., 25.9 0.6 h vs. 40.7 10.1 h; P = 0.184). Because Vultures arrive at carcasses first and in higher numbers than other scavengers, they outcompete other scavengers (Ruxton and Houston 2004) through exploitation competition and thus consume most of the carrion. Some individual Griffon Vultures exclude others at carcasses through aggressive displays or attacks, thus also displaying high levels of intraspecific competition (Bos e and Sarrazin