• chimpanzee;
  • nest count;
  • nesting-site selection;
  • behavioral ecology;
  • conservation

We investigated nesting behavior of non habituated chimpanzees populating the Nimba Mountains to document their abundance and their criterions of nesting-site selection. During a 19-month study we walked 80 km of transects and recces each month, and recorded 764 nests (mean group size = 2.23 nests) along with characteristics of vegetation structure and composition, topography, and seasonality. Population density estimated with two nest count methods ranged between 0.14 and 0.65 chimpanzee/km2. These values are lower than previous estimates, emphasizing the necessity of protecting remaining wild ape populations. Chimpanzees built nests in 108 tree species out of 437 identified, but 2.3% of total species comprised 52% of nests. Despite they preferred nesting in trees of 25–29 cm DBH and at a mean height of 8.02 m, we recorded an important proportion of terrestrial nests (8.2%) that may reflect a cultural trait of Nimba chimpanzees. A logistic model of nest presence formulated as a function of 12 habitat variables revealed preference for gallery and mountain forests rather than lowland forest, and old-growth forest rather than secondary forests. They nested more frequently in the study area during the dry season (December–April). The highest probability of observing nests was at 770 m altitude, particularly in steep locations (mean ground declivity = 15.54%). Several of the reported nest characteristics combined with the existence of two geographically separated clusters of nest, suggest that the study area constitutes the non-overlapping peripheral areas of two distinct communities. This nest-based study led us to findings on the behavioral ecology of Nimba chimpanzees, which constitute crucial knowledge to implement efficient and purpose-built conservation. Am. J. Primatol. 76:999–1010, 2014. © 2014 Wiley Periodicals, Inc.