Movements of White-Headed and White-Backed Vultures

Vultures are the only obligate vertebrate scavengers, and as such provide crucial services as keystone species and support the health and function of ecosystems in which they live. African vultures are a diverse group, with nine species found throughout Sub-Saharan Africa, many with overlapping distributions. Unfortunately, African vultures are faced with numerous threats throughout their range that have led to significant population declines, some greater than 90%, in only three generations. Four of these species are currently listed as critically endangered, and three as endangered. Despite the significant perils faced by African vultures, there are still significant knowledge gaps and, until recently, very little was known about vultures in Mozambique, a large country that falls within the distribution of six of these species. Our research in Gorongosa National Park, Mozambique, focused on movement data collected from 10 White-backed Vultures (Gyps africanus) and 12 White-headed Vultures (Trigonoceps occipitalis), one of Africa’s rarest vulture species. We first focused on estimating White-headed Vulture monthly home ranges and core ranges with continuous-time movement models. We assessed the relationship between ranging behavior and extrinsic (environmental characteristics) and intrinsic (individual characteristics) predictor variables using Bayesian generalized linear mixed effects models. We also explored the degree of White-headed Vulture home range and core range overlap with Gorongosa National Park and its buffer zone. We found that breeding individuals had smaller home ranges and could maintain these into the non-breeding season or abandon them. These small breeding and non-breeding home ranges were representative of central place foraging and averaged 239 km2 and 131 km2, respectively, 80-90% smaller than the average 1180 km2 non-breeding, non-central place foraging home range. Home ranges of birds that used resources outside of the park and its buffer zone were approximately 2.5 times larger than of birds that stayed within park boundaries, suggesting an increase in search effort required to locate less abundant resources. Excursions outside of the park and its buffer zone were rare. Only 15 of the 149 monthly home ranges suggested that birds used resources outside the park; the remaining 134 monthly home ranges fell within 10 km of the edge of the park buffer zone. Additionally, we explored differences in White-headed Vulture and White-backed Vulture movement characteristics. We used Bayesian generalized linear models to determine the effect of species as a predictor for flight altitude, flight speed, onset of movement on two scales (>100 m and >1000 m), and onset of flight at altitude, and the effect of species and hour predictors on hourly activity levels. We found that White-headed Vultures flew at lower altitudes and slower speeds, and initiated movement and flight at altitude earlier than White-backed Vultures. All of these findings correspond with flight less reliant on strong thermals and suggest that the White-headed Vulture is more likely a pioneer than follower. These findings expand on our understanding of both space use by White-headed Vultures and their place within the avian scavenging guild. They also demonstrate the critical importance of protected areas for the survival of the White-headed, and probably other, African Vultures.