Ornithological fieldwork

In total, fieldwork was done in the Bossematié area, which includes the Forêt Classée and the surrounding farmland, but not the Beki Forest (appr. 640 sq. km as shown in Fig. 2), during thirteen months between April 1995 and August 1997. Observations were made at 52 days of the dry season (December 1995 - February 1996, December 1996), but 129 days were spend in the field during the wet season (April-July 1995, March 1996, September-October 1996, June-August 1997). Birds were observed inside the Forêt Classée on 161 days and in the surrounding farmland on 38 days, see Waltert et al. 1999 and appendix A.

5.2.1 Preliminary surveys

Because adequate knowledge of bird behaviour and vocalisations is a pre-condition for successful studies on tropical avifaunas (e.g. Terborgh et al. 1990), the first six months (April - July 1995 and December-January 1995/1996) may be regarded as an intensive preparatory phase. During this time, mist-netting and line transect censuses were undertaken in various

parts of the Bossematié area and along the 16 permanent transects of the Forêt Classée (Fig.

3).

Species were identified using Serle et al. (1977) and Mackworth-Praed & Grant (1970, 1973).

In addition, information from Brown et al. (1982) Keith et al. (1992), Urban et al. (1986, 1997), Fry et al. (1988), and the publications of Fishpool et al. (1994a,b), Allport et al.

(1996), Fishpool (1999) and Demey (in prep.) were used. Voices were learned with the help of Chappuis (1974a,b, 1975, 1978, 1979, 1985) and playback-experiments attracting shy and skulking species were carried out. Mist-netted species which could not be identified in the field were photographed and compared with skin collections at the Royal Museum of Central Africa at Tervuren, Belgium. Identifications were confirmed by M. Louette, R. Demey and L.D.C.

Fishpool. Tape-recorded voices were kindly identified by C. Chappuis and R. Demey.

5.2.2 Standardised mist-netting

For several reasons, mist-netting is a major component of breeding bird censuses in tropical rainforest (Terborgh et al. 1990). It introduces the previously unfamiliar researcher to shy and skulking understorey species, it provides assessment of ecological density and allows to study population parameters. Furthermore it facilitates the comparison of data from different reasearchers, because the method is less observer-dependent than mapping by visual or acoustical means.

To reach a certain level of saturation and to improve the probability to obtain recaptures of individuals between sampling periods, mist-netting was restricted to two core areas of the study plot (as e.g. in Thiollay 1994b). The two vegetationally most distinct areas, the northern part of the control compartment and the most degraded part of the disturbed compartment, were selected. Mist-nets in the disturbed compartment included the swamp forest area and the adjacent part of the liberation thinning area (Fig. 9, Fig. 6). Successive mist-netting during a single sampling period covered approximately 32 ha of the study plot, 16 ha in each compartment. The procedure was carried out three times: at the end of the 1995/1996 dry season and during the wet seasons 1996 and 1997 (Tab. 2). A mist-net line (102 m) composed of seventeen six-meter, 19 mm-mesh mist-nets was used to trap birds. This line was installed at sixteen locations during each sampling period, eight lying in each compartment (Fig. 9). One mist-net sample consisted of the birds trapped with this line during 10 daylight hrs (102 m x 10 hrs = 1020 mhrs). The sixteen 102m locations were regularly spaced by the 100m-spaced trail system (Fig. 6). Mist-netting in the control compartment always preceded a similar effort (number of samples taken) in the disturbed compartment. The net line was installed by two persons keeping disturbance to a minimum. The setting procedure started at 13:00 and took two hrs with only slight deviations from this schedule. The mist-nets were kept open from 15:00 until 18:30, then put together to avoid entanglement of bats (Chiroptera) and opened the next day from 6:30 to 13:00, after which the line was removed. This relatively short period was chosen to produce only minimal levels of net shyness. Captured birds were ringed with uniquely numbered aluminium rings, flight feathers were checked for moult (scores from 0-5), reproductive condition (brood patch in sores from 1-5), sex and age (whenever possible) and

standard measurements (wing and tail length; weight using 100 g PESOLA spring balance).

Time and exact location (single six-meter net) of each capture were also noted.

Tab. 2: Distribution of standardised mist-netting activities in the 110 ha plot of the Bossematié Forest.

Each date represents a single sample (1020 mhrs). In each season, trapping was done at sixteen single locations, two additional samples (marked by asterisk) were taken at different locations but not included in the analysis.

Fig. 9: Location of the sixteen sampling sites (vertical bars) in the control and the disturbed compartment, where mist-netting has been carried out using a 102 m mist-net line.

5.2.3 Spot-mapping

In comparison with the temperate zone, birds of tropical forests have developed more complex breeding systems, more flexible territorial behaviour and often unknown or (seasonally or daily) limited activity periods (see Karr 1976, Terborgh et al. 1990). Therefore, adequate census methods and developed observer skills are needed to map tropical forest birds by visual and acoustical means. It has been shown that a lack of bird identification skills, limitation to one single methodology and inadequate plot sizes produce serious under-estimation of species richness and diversity (discussions e.g. in Terborgh 1990 or Thiollay 1994b). But compared to the Neotropics these problems seem to be of minor importance in Africa, where many rainforest species show territorial behavioural traits and are more easily and regularly detected by acoustical means than species with larger, ill-defined homeranges and more complex spacing systems (Karr 1976).

Spot mapping was only carried out at the end of the field work period when maximal skills for identifiying birds were attained. Because a sample plot of 110 ha could not be covered completely by one observer during one morning, the plot was divided into six separate sectors of 18.3 ha each (Fig. 10). One census of the complete plot was based on six consecutive visits.

In this way, six complete censuses (A to F) were undertaken (36 separate visits). Each visit started at 6:00 hrs and was completed until 9:30 hrs. In each sector, the trail system was walked at a slow pace and all observations were mapped (1: 2000 scale). The observations made during the six successive visits were combined to produce a spot map for each single species and for each census. Observations along the borders of adjacent 18.3 ha sectors were only included on the census maps, when they were spaced by more than 50 m. The spot-mapping period ranged from 9 June to 21 August 1997.

In lowland primary forest in Peru (Terborgh et al. 1990), territorial maps were obtained from numerous field visits of different observers collecting ornithological data from different parts of a study plot (97 ha), using different methods and with a different state of knowledge. These differences were compensated by the comparatively long time-span during which the study was carried out. However, when focusing on habitat selection patterns, it seems more appropriate to use data from a single observer and to combine data from separate visits of several smaller adjacent sectors to one complete census (e.g. Thiollay 1994b).

Fig. 10: Study plot showing the six 18.3 ha sectors (a to f), each one representing the sampling unit of a single visit.

Control

Liberation

thinning

Swamp forest Plantation