The Use of Nest Counts to Estimate Bonobo Occurrence and Density

All great apes make nests, and nest counts can be used to estimate populations where animals are difficult to detect, and where large areas must be covered. Researchers have used nest counts to develop landscape estimates of chimpanzees and gorillas, including nation-wide surveys of apes in Gabon (Tutin and Fernandez 1984), Cote d'Ivoire (Marchesi et al. 1995) and Uganda (Plumptre et al. 2003). We also confirm their utility for large-scale bonobo surveys.

Apes live in groups and often nest together, so many nest counts use nest groups as the observational unit with line transect measures to the center of the group. Counts of nest groups assume that each one represents a single nesting event, that separate nest groups can be distinguished, and that all nests in a group are constructed at the same time. Early in the Salonga survey, we found that some aggregations of bonobo nests observed in the field represented different nesting events in close spatial proximity. As nests aged, it was impossible to distinguish one nesting event from another, especially if different nesting events were separated by short periods of time. We opted to use the individual nests instead of nest groups as the observational unit.

The Phase III surveys confirmed that bonobos consistently use at least some nesting zones over time, and that nest site fidelity can be very specific. In 9 of 15 pair-wise comparisons from one time period to the next, bonobo nesting events occurred on the same transect within the monitoring zone and in all but two cases, within the same 250m of transect line. Mohneke (2004), Fruth (1995) and Fruth and Hohmann (1993) observed in Lomako and Lui Kotal that nesting is concentrated in selected areas of a bonobo community's home range, and that nesting events accumulate in the same locations. They also opted to count individual nests instead of nest groups in population surveys. Len Thomas (personal communication, 2005) reports that the potential bias due to non-independence of observations in counting individual nests from clusters is small, and also recommends counts of nests rather than nest groups on line transects where there is inability to distinguish the groups.

The phenomenon of repeated nesting in limited areas is not restricted to bonobos. Furuichi et al. (2001) compared counts of individual nests versus nest groups for chimpanzees in Uganda and justified the use of individual nests counts. While it has rarely been evaluated, it is likely that at least some of the large aggregations of nests reported for bonobos, and possibly chimpanzees, represent multiple nesting events in the same location instead of a large single nesting event (Kuroda 1979).

We observed that bonobos sometimes refurbished and reused older nests, especially in repeatedly used nesting areas. Therefore, estimates of daily nest production rates should be given with estimates of percentage reuse of old nests. This allows for the calculation of a correction factor in estimating bonobo densities from nests estimates (Plumptre and Reynolds 1997). If nest production rates are not corrected for reuse, estimates of bonobo density from nest counts might be biased downward.

Our surveys used standing crop nest counts. A second nest count method, using marked nests, requires repeated visits of the same transects. This has the advantage that it does not require independent estimates of nest decay (Plumptre and Reynolds 1996). However, the method is not feasible for large surveys where each sampling unit is visited once. It may not be feasible where nest accumulation rates are very low. In one study comparing marked and standing crop nest counts, the marked nest methods led to a higher density estimate (Mohneke 2004). It is not known if this is likely to be a consistent trend.

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