The decision by an individual to move from one area to another is mediated by a number of factors, such as resource quality and availability, predation risk and local environmental conditions, all of which will influence its survival and reproductive output1,4. The challenge for conservationists is understanding how these individual decisions can affect population dynamics, home ranges and ultimately species’ survival1.
Home ranges of carnivores should overlap and in some cases envelop those of their prey species. Southern Ground-hornbills feed on a variety of prey, ranging from snakes, rabbits and birds to invertebrates12,18. Through tracking Southern Ground-hornbill movements, we were able to show that group home ranges during the early and late dry seasons were larger than in the wet season. As the Southern Ground-hornbill breeding season in South Africa coincides with the warm, wet summer months, prey availability, especially that of invertebrates, is expected to be higher20,21, suggesting that individuals would not need to travel as extensively to find sufficient food. Furthermore, in the late dry season, groups used between 76 and 115% of their home ranges. This was likely a result of having to increase their search for food and relaxation of the central place foraging required around the nest during the breeding season.
Previous research on Southern Ground-hornbill home ranges has recorded group densities ranging from one group per 4000 ha (communal areas in Zimbabwe22), to one group every 10,000 ha (KNP14), with one group in the Limpopo Valley having a home range close to 20,000 ha21. These results were obtained by direct observations of active nest sites or using VHF radio transmitters. In our study using GPS data, we showed that home range sizes of Southern Ground-hornbills within KNP vary considerably. Despite this, our results confirmed the findings of Theron et al.21 and Zoghby et al.20, demonstrating a restricted and contracted home range during the breeding season, when group movements are concentrated around the nest site (central place foraging). Presumably, breeding success would influence the extent of wet seasonal home range for Southern Ground-hornbills, with groups abandoning their central place foraging behaviour when nests fail. Wyness19 reported that of four Southern Ground-hornbill groups studied in the Association of Private Nature Reserves (APNR) adjacent to the KNP, the three that bred successfully in the year of their study showed a breeding season range reduction to between 24–36% of their non-breeding home range. The unsuccessful group used 70% of their home range during this time19. Surprisingly, the groups within the KNP did not show such a definitive pattern in home range size reduction associated with breeding success, although all groups that attempted breeding did show a wet seasonal home range reduction. Of the six Southern Ground-hornbill groups monitored in our study, four groups bred successfully, one group’s attempt failed (Ngotso Camp), and the breeding status for the third group (Shingwedzi) was unknown. The groups that bred successfully used 21–97% of their respective home ranges, with the unsuccessful group using 85% of their home range (See Table 1).
Southern Ground-hornbills are known to favour more open habitats for foraging20,23. Our results supported this, with groups selecting the open woodland and grassland habitat types year-round, following their availability within the landscape.
Although Southern Ground-hornbill seasonal territory size differed significantly amongst the groups, they all showed a decrease in the amount of low shrubland and an increase in the amount of grassland habitat used with increased territory size. Similarly, as seasonal territory sizes increased, the amount of low-medium woody cover (25–50%) decreased. Thus, when selecting an area for a reintroduction of Southern Ground-hornbill groups, the ratio of low-medium woody cover (low shrubland) to grassland, calculated based on the national land cover datasets available, should be taken into account, as this will likely influence the home range size and the number of groups that could be supported in an area.
Although an understanding of the changes and restrictions in territory size is important for the management of a species, the types of movements adopted within a population will influence the management actions needed for their conservation, such as ensuring connectivity or access to certain resources1. Conservation policy and management actions are less effective when interventions do not integrate both the spatial and temporal changes in habitat use and the scale of species movements1,3. The results from the first-passage time analysis of Southern Ground-hornbill movements showed that the different groups did not consistently demonstrate seasonal patterns in the scale at which they concentrated their foraging efforts. The mean distances travelled for all trajectory paths, classified as active foraging behaviour, were similar and lower in the late wet and early dry seasons compared with the late dry and early wet seasons. Movement between foraging resource patches or mean relocation distances were highest in the wet season months, with the maximum mean distances travelled during the early wet season and the start of the breeding period. Overall prey abundance for Southern Ground-hornbills is generally higher in the wetter months, resulting in a decrease in relocation distances. Our results support the theory that Southern Ground-hornbill wet season movements are most likely influenced by the need to travel to and from the nest site to provision prey to the incubating female and growing nestling. Once resources closer to the nest are depleted, the distances travelled to access additional habitats and prey would likely increase.
Southern Ground-hornbills seemingly prefer nest sites surrounded by more open woodland habitat24,25. Habitat structure and the diversity of habitat types within a 3 km radius around the nest site positively influenced Southern Ground-hornbill nesting success. An increase in the density of woody habitat surrounding the nest site, however, had a negative impact on Southern Ground-hornbill breeding success24, possibly owing to decreased foraging opportunities, an increased risk of predation or an increase in foraging effort beyond a value which is beneficial.
Habitat structure will likely promote or inhibit the types of movement that can occur in an area. The results from the multinomial regression (Table 5) indicate that the likelihood of a movement behaviour being classified as “foraging” within the open woodland, grassland and dense thicket habitat types was higher than the behaviour being attributed to “relocating”. This is to be expected for open woodland, and grassland habitats as these are both ideal open foraging habitats for Southern Ground-hornbills20,23 and are used year-round in proportion to their availability. Southern Ground-hornbills spend around 70% of their day walking12 and have been shown to travel distances of up to 10.6 km in a day20. Having to navigate through dense thicket vegetation in an area may increase the amount of time spent there, possibly accounting for why this habitat type is predicted to be used more for “foraging”-type behaviour as opposed to “relocating” behaviour. Travel through areas of low shrubland habitat was considered “relocating” behaviour, suggesting that within this habitat type, it is more profitable for Southern Ground-hornbills to move further, and the corresponding chance of finding food greater, than conducting area-restricted searches and spending longer periods concentrated in one patch.
When comparing movements between habitats allocated to “resting” as opposed to “foraging”, the time spent in all habitats was most likely as a result of “foraging”. As GPS locations were only recorded during the day, switching off at dusk (~ 18h00) when Southern Ground-hornbills would roost for the night, habitat preferences for “resting” movements may not have been recorded. Moreover, during the day, Southern Ground-hornbills may not be actively selecting for specific habitat types in which to roost or rest. They may simply be roosting or resting at a chosen site to escape the midday heat within the habitat type in which they were “foraging” or “relocating”.
We were unable to explore differences in movement relating to specific characteristics of the tagged bird (age, sex, helper versus breeder status, etc.) in our study. However, future research should consider study designs able to account for these potential differences, as García-Jiménez et al.26 showed that both the breeding season and sex of the individual influence displacement and distance travelled in Pyrenean Bearded Vultures (Gypaetus barbatus). They found that all individuals travelled more in the breeding season, with females having greater cumulative and maximum distances regardless of the season.
Source: Ecology - nature.com
