Differences in mist-net data between sampling periods

Standardised mist-netting was done during three separate sampling periods of which two (32 days) were in rainy season conditions and one (16 days) were at the end of the dry season (Tab. 2). To obtain adequate sample sizes for some analyses, it was necessary to pool data from sampling periods. Since a different distribution pattern can occur between seasons, in the following, community level differences between sampling periods are discussed.

Individual numbers

The median of individuals captured per sample (102m net line, open for 10 daylight hrs) did not differ significantly between the seasons (Friedman test, F²=1.63, D.F.=2, p=0.44), neither for the control (Friedman test, F²=4.19, D.F.=2, p=0.12) nor for the disturbed compartment (Friedman test, F²=0.06, D.F.=2, p=0.97). The average number of individuals per sample in control during February-March 1996 was c. 3.5 individuals lower than during the wet season 1997, but this difference was not significant (Wilcoxon test, Z=-1.61, two-tailed p=0.11) (Tab.

15).

Tab. 15: Total bird individuals (recaptures excluded) and medians of samples (102 m net line, 10 daylight hrs) in a control, a disturbed compartment and of both combined, by sampling period. n= number of samples obtained in each sampling period.

Sampling period Total Individuals Median and range

Control

(n=8)

Disturbed (n=8)

Combined (n=16) Dry season 1996 255 18 (14 - 23) 22.5 (17 - 31) 20.5 (14 - 31) Wet season 1996 297 21.5 (17 - 32) 22.5 (11 - 30) 22.5 (11 - 32) Wet season 1997 331 24 (16 - 37) 24 (18 - 40) 24.0 (16 - 40)

The higher absolute numbers of individuals captured during the wet seasons 1996 and 1997 in comparison to the dry season 1996 are mainly caused by two common species. These are Yellow-whiskered Greenbul Andropadus latirostris (increase from 27 to 59 individuals), and Green Hylia Hylia prasina (from sixteen to 25 individuals). The White-throated Greenbul Phyllastrephus albigularis increased from thirteen individuals (dry season 1996) to 28 individuals (wet season 1996), but decreased again to 19 individuals (wet season 1997).

Numbers of several other species increased as well: e.g. in the Collared Sunbird Anthreptes collaris from three individuals in both dry and wet season 1996 to sixteen in the wet season 1997, or in the Bluebill Spermophaga haematina from two individuals in the dry season 1996 to eight (wet season 1996) and six (wet season 1997). In contrary to this overall pattern (higher numbers in the last two seasons), there was a slight decrease in numbers for the Olive Sunbird Nectarinia olivacea in 1996 from the dry to the wet season from 49 to 37, but numbers increased again to 46 in the wet season 1997. In addition, Grey-headed Bristlebill Bleda canicapilla individuals decreased from 35 and 33 (dry and wet season 1996) to 23 in the wet season 1997.

However, all these differences were not very pronounced and after excluding the two species Andropadus latirostris and Green Hylia Hylia prasina from the sample, differences in overall

numbers of individuals between the dry season and the two wet seasons were no longer significant (F²=1.42, D.F.=1, p=0.23 or F²=1.63, D.F.=1, p=0.20, respectively).

Diversity and abundance-rank patterns

Species-abundance patterns from both control and disturbed compartment and from all sampling periods did not differ from a (truncated) log-normal (F² Goodness of fit tests, all p>

0.45) nor from a log-series distribution (F² Goodness of fit tests, all p> 0.10, Fig. 21). The two most abundant species Yellow-whiskered Greenbul Andropadus latirostris and the Olive Sunbird Nectarinia olivacea were captured in more than 35 individuals (37 to 59) in each sampling period except for the period February-March 1996, when only 27 Andropadus latirostris - individuals were captured and Grey-headed Bristlebill Bleda canicapilla (35 individuals) was ranked as second most abundant species. In each season, seven species were represented in at least ten individuals. The number of „rare“ species (less than five individuals) was highest in the dry season 1996 (25 species), but still high (18 and 21) during both wet season sampling periods. Diversity parameters differed slightly between seasons (Tab. 16). The number of species observed after 16 samples was highest in the wet season 1996 (40 species), intermediate in the wet season 1997 (38 species) and lowest in the dry season 1996 (36 species). Shannon, Simpson and Fisher’s alpha diversity indices were lower in the dry season compared to both wet season sampling periods, but differences were small. The expected number of species cumulatively captured at n individuals was higher for one species in the wet season 1996 than in dry season 1996 or wet season 1997. (Tab. 16, Fig. 22). The estimator of total species richness (ACE) was highest in the wet season 1996 (Tab. 16). In that period, the local species pool was estimated at 49 species, while in the wet season 1997 this value was 48 and in the dry season 45. However, all these differences in species richness were not caused by the presence or absence of dominant species. None of the species caught in considerable numbers in one season was found to be absent in the others.

Tab. 16: Number of species, species diversity indices and richness estimators of the mist-net bird community from different sampling periods (standard deviations in brackets). *Values computed with the help of EstimaeS (Colwell 1997), accumulation level n=16 samples per period. ** Values computed using RAREFACT (Krebs 1989). E[n] = expected number of species at n individuals.

Sampling period

A comparison of the community composition indicates an overall high similarity between seasons (Tab. 17). The two wet seasons 1996 and 1997 were very similar in terms of species composition (Morisita Horn=0.94, 41 species shared), but the similarity of the dry season 1996

to the wet season 1996 (Morisita Horn=0.92) and to the wet season 1997 (Morisita Horn=0.89) was also high. This, and the high number of 37 shared species indicated only slight turnover in the composition of the community between the sampling periods.

Tab. 17: Observed number of species shared and Morisita Horn sample similarity index for mist-net data from different sampling periods (after Colwell 1997).

Morisita Horn Shared species Dry 1996 Wet 1996 Wet 1997

Dry 1996 37 37

Wet 1996 0.92 41 Wet 1997 0.89 0.94

The computation of Morisita sample similarity indices for the control and the disturbed compartment separately, indicated that the bird communities in control were slightly more homogenous between the seasons than those from the disturbed compartment (Tab. 18).

However, the differences were not pronounced and all values ranged between 0.83 and 0.95.

The actual similarity in species composition between the seasons is expected to be even higher than the calculated indices suggest, as neither of the different seasons’ cumulative species curves did reach complete saturation (Fig. 22)

Tab. 18: Morisita Horn sample similarity index for mist-net data from different sampling periods, given for the bird communities in the control and the disturbed compartment of the study plot (after Colwell 1997).

Control Disturbed Dry 1996 Wet 1996 Wet 1997 Dry 1996 0.92 0.83 Wet 1996 0.88 0.86 Wet 1997 0.90 0.95

Habitat preference patterns on species level

Because of low numbers of individuals, seasonal differences in habitat selection of single species can hardly be assessed. However, for a small set of species, it was possible to calculate habitat preference patterns. In each season, at least ten species were captured in six or more individuals, which is the minimum for the Fisher’s exact test.

There was no indication for marked habitat shifts between the sampling periods. In all seasons, significantly more Little Greenbul Andropadus virens individuals were captured in mist-nets installed in the disturbed than in the control compartment (Fisher’s exact test, all p<0.05).

Another species with higher individual numbers in the disturbed mist-nets was the Collared Sunbird Anthreptes collaris. However, significant differences (Fisher’s exact test, p=0.008) were only found in the wet season 1997 when sixteen individuals were captured, the species was too rare in both previous sampling periods (three individuals) to carry out statistical tests.

Similarly, the Green Hylia Hylia prasina was significantly more abundant in the disturbed compartment during the wet season 1996 (F²=4.26, D.F.=1, p=0.039), but no significant differences appeared during the other sampling periods.

Species with a significantly lower number of individuals in the disturbed compartment were White-throated Greenbul Phyllastrephus albigularis and the Grey-headed Bristlebill Bleda canicapilla. Differences were visible in all seasons, but significant on the 5% level only in the wet season 1996 for Phyllastrephus (F²=9,14, D.F.=1, p=0.003) and in the wet season 1997 for Bleda (F²=5.26, D.F.=1, p=0.02).

There was no indication for major habitat shifts between sampling periods for any other species.

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