Extended Application of a Marked-Nest Census Method to Examine Seasonal Changes in Habitat Use by Chimpanzees

The Kalinzu Forest Reserve in Uganda comprises various types of vegetation, including mixed mature forest, Parinari-dominated mature and secondary forest, and Musanga-dominated secondary forest. We used a marked-nest census method to examine seasonal changes in chimpanzees' use of the different vegetation types. We made 10 parallel line-transects in the study area; they were 5-km long and 500-m apart. During the first 3–4 walks along the transects, we marked all existing nests. We then conducted 10 main censuses of all transects at 15-day intervals, over a total period of about 5 months. In each main census, we recorded all unmarked nests visible from the transects and marked them. When we saw a nest, we searched for neighboring nests of the same age class 30 m of each other, in order to estimate the size and position of nest groups. To improve the accuracy of the estimation of nest density in each census period, we excluded nests that consisted only of brown leaves and corrected the number of nests observed by allowing for the proportion of newly-built nests that would still have green leaves at the next main census. We estimated the population density of chimpanzees in the study area both by the number of individual nests and by the number of nest groups; the two methods gave similar results. We found differences in number of chimpanzees that used different vegetation types in different fruiting seasons, and differences in nest group size related to the different fruiting seasons.