Report on Chimpanzee Nest Survey – Preliminary Results

Thurston C. Hicks


With the current decline of chimpanzee populations across Africa due to the bushmeat trade crisis and other factors (Butynksi, 2001), it has become increasingly important to achieve more accurate information about their presence and population density across their range. Currently, very little is known about chimpanzee numbers in northern Democratic Republic of Congo (DRC), or the prospects for survival of the chimpanzees known to live there. The chimpanzee study site near Bili, DRC provides us with an ideal opportunity to census the apes, and then extrapolate those numbers throughout the greater region.

During the 2004-2005 field season, between the months of March and July 2005, we conducted an extensive survey of the forests and savannas to the NW of Bili. The primary goal was to get an estimate of the density of chimpanzees (Pan troglodytes schweinfurthii). Secondary goals were to gather evidence of other large mammals in this area, as well as of human presence. As of this date, a preliminary analysis of chimpanzee densities has been calculated using the DISTANCE program, which are presented below.


Between March and July 2005, between the end of the dry season and the first part of the rainy season, 160 km of line transects were cut over an area of 467.32 km sq. There were three E-W transects each separated from one another by approx. 4 km. The E border of the three transects was the road connecting the village of Bili to of Pangali (just N of Badai). The W border of the three transects was found approx. 50 km to the W, in deep forest. The transects cut through cultivated fields, savanna, woodland mosaic, and swamp and primary forest.

A separate team consisting of T. Hicks and four experienced Azande observers then walked the transects looking for chimpanzee nests as well as other signs or sounds of the apes. Signs of other large mammal species and also humans were counted as well. When chimpanzee nests were spotted from the transect, details such as age of nest, tree species of nest tree, perpendicular distance of the nest from the trail, and whether it was a ground or a tree nest were recorded. After spotting all nests visible from the transect (and marching ahead 20 m from the last nest to be seen), the team controlled approx. 20 m on each side of the transect to look for nests not seen from the transect.

Vital to the determination of ape densities from nest counts is an estimation of nest decay rates. To achieve this, we monitored throughout the year the decomposition of 13 tree nests and 10 ground nests (all found fresh). This percentage assumes that ground nests make up approx. 18% of fresh non-transect ground nests discovered (Thurston C. Hicks, unpublished data). Chimpanzee nests in these forests take on average 176.4 days to decay into barely-visible nest ‘skeletons’, which is much longer than the 113.6 day decay rate of chimpanzee nests in Gabon (Tutin & Fernandez, 1984).

Back in the United States, all nests which were seen from the transect were entered into the DISTANCE 5.0 program. DISTANCE calculates densities of adult chimpanzees from nests found on transect data, as long as nest decay rate is known making the assumption that the weaned adults make one fresh nest per night. Results are presented below. It should be stressed that these are preliminary results, and that a more comprehensive analysis will be carried out soon.


Total transect distance = 160 km

Total surveyed area = 467.32 km sq.

Transect 1 = 50.47 km at 266 degrees W (Transect 1E = 9.92 km; Transect 1W = 40.55 km)

Transect 2 = 53.92 km at 268 degrees W (Transect 2E = 13.58 km; Transect 2W = 40.34 km)

Transect 3 = 55.55 km at 268 degrees W (Transect 3E = 15.28 km; Transect 3W = 40.27 km)

Nest decay rate for tree nests to reach cat. 5 (n = 13) = 197 days

Nest decay rate for ground nests to reach cat. 5 (n = 10) = 83 days

Nest decay rate (assuming that ground nests account for 18% of nests) = 176.4 days.


This is an analysis of chimpanzee densities across the entire 467.32 km sq. study region.

Density of weaned chimpanzee individuals per km sq. = 0.6 (278 weaned individuals).

Using Tutin and Fernandez's decay rate for Gabon, of 113.6 days, would give us a higher density of 0.93 ind. per km sq. (432 weaned individuals).


Subset of all transects, this is the 142.16 km sq. region to W of confluence of Bo and Gangu Rivers. It is the farthest region from human settlements. Here we did 49.57 km of transects (T1W = 16.52 km, T2W = 16.49 km, T3W = 16.56 km).

Density of weaned chimpanzee individuals per km sq. = 1.2 (175 weaned individuals).

Using Tutin and Fernandez's decay rate for Gabon, of 113.6 days, would give us a higher density of 1.92 ind. per km sq. (272 weaned individuals).


Subset of all transects, this is the 325.16 km sq. region to E of confluence of Bo and Gangu Rivers. This region is closer to the human settlements, and its E boundary is the main road. Here we did 110.37 km of transects (T1 = 33.95 km, T2 = 37.43 km, T3 = 38.99 km).

Density of weaned chimpanzee individuals per km sq. = 0.35 (114 weaned individuals).

Using Tutin and Fernandez's decay rate for Gabon, of 113.6 days, would give us a higher density of 0.55 ind. per km sq. (177 weaned individuals).


The preliminary results of the transect data indicate a surprisingly high density of chimpanzees in the forest and savannas to the NW of Bili. Overall density across the 467.32 km sq. area surveyed is 0.6 weaned chimpanzees per km sq., which is a higher density than found in Gabon (0.32 per km sq.) (Tutin and Fernandez, 1984), Lope (0.58 per km sq.)(White, 1994), or Ngotto (0.29 per km sq.)(Brugiere, 2000), but lower than in Odzala in The Republic of Congo (2.74 per km sq.) (Bermejo, 1995) and Ndoki 2 in CAR (2.65 per km sq.)(Yamigiwa, 1999).

At the confluence of the Bo and the Gangu Rivers, the extensive savanna mosaic peters out and there is a large stretch of more uniform primary forest. W of this point almost no evidence of human presence was recorded, but much evidence of elephants, leopards, and other big mammals. The chimpanzees here, when encountered, did not flee us and appeared to be ‘naïve’, and I would estimate that they have not been seriously hunted by humans in at least the life spans of the adult males. When this 142.16 km sq. area was analyzed separately as a subset of the total area, the density of weaned chimpanzees is much higher: 1.2 per km sq.(175 weaned individuals). This would be an excellent place for a future chimpanzee study site.

I also analyzed a 325.16 km sq. region to the E of confluence of Bo and Gangu Rivers. This subset of the total study area is closer to and abuts the human settlements, and its E boundary is the main road. Elephant evidence disappears, and the chimpanzees are for the most part fearful of human observers, fleeing in panic whenever we contacted them. This indicates that they have been hunted in the recent past. The density of weaned chimpanzees is much lower than that to the W, 0.35 per km sq. (114 weaned individuals).

Chimpanzee nest sites were found exclusively in primary and gallery forests, as well as in old overgrown human fields; however, we gathered much evidence that the chimpanzees do use and travel through the savannas and savanna woodlands as well.

For all analyses, if Tutin & Fernandez’s nest decay rate is used, then Bili densities are considerably higher.


Chimpanzees are numerous in the area, and their nest sites come to within 400 m of the main road connecting Bili to the Badai and Pangali villages. Local people claim that the chimpanzees regularly cross the road and steal sugar cane from their fields.

Due to the absence of commercial logging, mineral exploitation, and, with the exception of elephants, the commercial bushmeat trade, the Bili area presents an ideal opportunity to install a successful conservation/research program. Future work will involve fine-tuning these density estimates and comparing statistically the chimpanzee densities in human-settled versus unsettled areas. Using satellite images will allow us to extrapolate the results of our work across much larger areas of Northern DRC with similar habitat types and human population density, to determine the status of the chimpanzee in this region. An effort will be made to determine densities of elephants, leopards, buffalo, and other large mammals as well.


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