PART II: UNDERSTOREY BIRD COMMUNITY STRUCTURE, SPECIES RICHNESS
V.3. Results
V.3.4. Understorey bird community structure
Mist-netting produced a total of 1,528 captures. The number of captures was not significantly affected by habitat type (one-way ANOVA, F 3,20 = 1.06, p = 0.39): the number of captures per sampling station were highest in SF (70.5 ± 12.56; mean ± SD), slightly lower in AC (66.7 ± 22.7) and CF (65.0 ± 21.2), and lowest in NF (52.5 ± 15.8).
The number of captures was very highly and positively correlated with the number of individuals (§ V.3.1.) (rs = 0.98, p < 0.001); thus the pattern of captures number can be inferred to that of individual numbers.
V.3.4.2. Species abundance patterns
Species abundant patterns for all captured species did not differ from a (truncated) lognormal distribution (χ² Goodness of fit tests = 4.05, p < 0.99) nor from a logseries distribution (χ² Goodness of fit tests = 19.94, p < 1.00, see Fig. V.4A.). The four most abundant species, namely Olive Sunbird, Little Greenbul, Yellow-whiskered Greenbul and Forest Robin, were captured with more than 95 individuals (96 to 169) each. Fifty-two species, which accounted for less than five individuals, are classified as ‘rare’. Captured species abundance patterns in NF did not differ from a (truncated) lognormal (χ² Goodness of fit tests = 2.46, p < 0.99) nor from a logseries distribution (χ² Goodness of fit tests = 3.53, p < 1.00, see Fig. V.4B.). The five most abundant species in NF include Forest Robin, Pale-breasted Illadopsis, Lesser Bristlebill, Fire-crested Alethe and Olive Sunbird with respectively 42, 28, 28, 28 and 24 individuals. Sixteen species are classified as ‘rare’ in NF. In SF, abundance patterns fitted both to a (truncated) lognormal (χ² Goodness of fit tests = 2.42, p <1.00) and a logseries distribution (χ² Goodness of fit tests = 3.25, p < 0.99, see Fig. V.4C.). The six most abundant species are Yellow-whiskered Greenbul, Olive sunbird, Forest Robin, Little Greenbul, Fire-crested Alethe and Lesser Bristlebill with respectively 63, 42, 37, 26, 25 and 24. Twenty-four species are considered as ‘rare’ in SF. Mist-netted species abundance patterns in CF did not differ from a (truncated) logseries (χ² Goodness of fit tests = 36.11, p < 0.83); but it differed significantly from a lognormal distribution (χ² Goodness of fit tests = 57.88, p < 0.000, see Fig. V.4D.). Three species appeared to be the most abundant in CF namely Little Greenbul, Olive Sunbird and Yellow-whiskered Greenbul with respectively 82, 71 and 50 individuals.
Thirty-eight species are considered as ‘rare’ in CF. Captured species abundance patterns in
AC did not differ from a (truncated) lognormal (χ² Goodness of fit tests = 3.20, all p < 0.99) nor from a logseries distribution (χ² Goodness of fit tests = 1.79, all p < 1.00, see Fig. V.4E.).
The three most abundant species in AC are Little Greenbul, Olive Sunbird and Olive-bellied Sunbird with respectively 56, 32 and 30 individuals. Thirty-one species are classified as ‘rare’
in AC.
0 5 10 15 20 25
0 50 100 150 200
Number of individual
Species frequency (%)
A)
0 5 10 15 20 25 30 35
0 10 20 30 40 50
Number of individual
Species frequency (%)
B)
5 10 15 20 25 30
Species frequency (%)
C)
0 5 10 15 20 25 30 35 40 45
0 20 40 60 80 100
Number of Individual
Species frequency (%)
D)
0 5 10 15 20 25 30
0 20 40 60
Number of individual
Species frequency (%)
E)
Figure V.4.: Species-abundance distribution and rank-abundance plots for A) all mist-netted species, B) species captured in NF, C) species captured in SF, D) species captured in CF and E) species captured in AC. Expected distributions (logseries and lognormal curves) are included to rank-abundance plots.
It should be noted that the overall number of ‘rare’ species increases from natural habitats (NF and SF) to land use systems (CF and AC) i.e. from 16 in NF and 24 in SF to 38 in CF and 31 in AC. Also, many true forest species captured in great numbers in natural habitats are considered ‘rare’ in land use systems (see Table V.4.), suggesting that some individuals of these species are using degraded habitats temporally for their daily needs, outside their territories known to be somewhere in the nearby natural forests.
Table V.4.: Comparison of true forest understorey species abundance from natural habitats to land use systems.
Number of individuals Species
NF SF CF AC
Blue-headed crested Flycatcher 10 9 1 0
Lesser Bristlebill 28 24 0 1
Pale-breasted Illadopsis 28 9 1 0
Icterine Greenbul 22 10 2 0
Fire-crested Alethe 28 25 8 3
Red-tailed Bristlebill 9 8 2 0
Forest Robin 42 37 13 4
Red-tailed Greenbul 3 13 3 1
Notes: See Fig. V.1. for abbreviations of habitats.
V.3.4.3. Family level V.3.4.3.1. Overall
One thousand three hundred and seven (1307) individuals (recaptures are excluded) were mist-netted in all the four habitat types, belonging to 93 species from 24 families. The most abundant bird families were Pycnonotidae, Nectariniidae, Turdidae, Sylviidae, Estrildidae, Timaliidae, Monarchidae, Alcedinidae and Plastysteiridae with respectively 503, 219, 208, 82, 80, 53, 34, 30 and 25 individuals. ‘Rare’ families include Strigidae, Malaconotidae, Eurylaimidae, Dicruridae, Bucerotidae, Motacillidae, Muscicapidae, Meropidae and Accipitridae with less than five individuals each (see Fig. V.5A.).
Pycnonotidae, Nectariniidae, Sylviidae and Turdidae also appeared to be the most species-rich families with respectively 15, 10, 9 and 7 species. Other species-rich families include Estrildidae (six species), Alcedinidae (five species), Indicatoridae, Platysteiridae and Ploceidae all represented by four species each. Capitonidae, Columbidae, Monarchidae, Muscicapidae and Timaliidae were all represented by three species each. Accipitridae, Cuculidae, Meropidae and Picidae were all represented by two species each. Bucerotidae,
Figure V.5.: Percentages of (A) individuals (n= 1307) and (B) understorey bird species (n=
93) of different mist-netted families.
V.3.4.3.2. Comparison between habitats
A comparison of individual percentages for the six most abundant families revealed no clear overall defined pattern between the four habitat types (see Fig. V.6A.). It was not also clear for Pycnonotidae’s proportion with 16.9% in NF, 30.4% in SF, 31.6% in CF and 21.1% in AC. However, trends were visible in some families: the proportion of Turdidae decreases with increasing habitat modifications from 44.2% in NF and 39.4% in SF to 12.0% in CF and 4.3%
in AC; the same pattern was observed for Timaliidae, which tend to avoid disturbed habitats, with 66.0% in NF and 32.1% in SF to 1.9% in CF and 0.0% in AC. The proportion of Nectariniidae increases with increasing habitat modifications from 11.4% in NF and 20.1% in SF to 35.6% in CF and 32.9% in AC. The same pattern was observed for Sylviidae with 6.1%
in NF and 12.2% in SF to 24.4% in CF and 57.3% in AC. The Estrildidae percentages indicated that individuals of this family tend to be confined to very degraded habitats with 67.5% in AC to 10.0% in CF, 12.5% in SF and 10.0% in NF (see Fig. V.6A.).
A comparison of species percentages for the six species-rich families revealed that overall number of species increases with increasing habitat modifications from 20 species in NF and 25 species in SF to 29 species in CF and 36 species in AC (see Fig. V.6B.). The species proportion of Pycnonotidae decreases with increasing habitat modifications from 40.0% (8 species) in NF and 40.0% (10 species) in SF to 31.0% (9 species) in CF and 25.0% (9 species) in AC. Similar pattern was observed for Turdidae with a species proportion of 20.0% (4 species) in NF and 24.0% (6 species) in SF to 13.8% (4 species) in CF and 11.1% (4 species) in AC. The species proportion of Nectariniidae increases with increasing habitat modifications from 10.0% (2 species) in NF and 8.0% (2 species) in SF to 17.2% (5 species) in CF and 22.2% (8 species) in AC. Similar pattern was observed for Sylviidae with a species proportion of 15.0% (3 species) in NF and 12.0% (3 species) in SF to 20.7% (6 species) in CF and 19.4% (7 species) in AC (see Fig. V.6B.).
The overall numbers of captures as well as the numbers of individuals were significantly different between habitat types in six families (see above). Both parameters were also highly and positively correlated (see § V.3.4.1.). Estrildidae (six species in total) were significantly more abundant in AC than in other habitat types (one-way ANOVA, F3, 20 = 3.60, p < 0.03 for individuals). The abundance of Nectariniidae (ten species in total) was significantly higher in
Figure V.6.: Percentage of (A) individuals and (B) species of well-represented understorey bird families captured in the four habitat types.
Ploceidae (four species in total) were significantly more abundant in land use systems (AC and CF) than in natural habitats (SF and NF) (one-way ANOVA, F3, 20 = 3.88, p < 0.02 for individuals). In Sylviidae (nine species in total), highest abundance was found in AC; it was significantly different in other habitat types (one-way ANOVA, F3, 20 = 11.43, p < 0.001 for
individuals). Timaliidae (three species in total) showed significantly higher abundance in NF as compared to other habitat types (one-way ANOVA, F3, 20 = 17.88, p < 0.001 for individuals). The abundance of Turdidae (seven species in total) decreases with increasing habitat modifications: it was significantly higher in natural habitats (NF and SF) than in land use systems (CF and NF) (one-way ANOVA, F3, 20 = 28.64, p < 0.001 for individuals). All other families present in considerable numbers, namely Pycnonotidae, Monarchidae, Alcedinidae and Platysteiridae, were relatively equally abundant in the four habitat types.
V.3.4.4. Specie level
Out of the 93 species captured, twenty-five species showed significant responses to habitat type (ANOVAs, P < 0.05), i.e. just 26.9% of all species (see Appendix V.1.).
V.3.4.4.1. Species found in all habitat types
Out of the 93 species captured, twelve (i.e. 12.9%) were found using all four habitat types among which eight were insectivorous (White-bellied kingfisher Alcedo leucogaster, Lesser Bristlebill Bleda notata, Red-tailed Greenbul Criniger calurus, Forest Robin Stiphrornis erythrothorax, Fire-crested Alethe Alethe diademata, White-tailed Ant-Thrush Neocossyphus poensis, Green Hylia Hylia prasina, Red-bellied paradise Flycatcher Terpsiphone rufiventer), two omnivorous (Little Greenbul Andropadus virens, Yellow-whiskered Greenbul A.
latirostris), one Nectariniidae (Olive Sunbird Cyanomitra obscura) and one granivorous (Western Bluebill Spermophaga haematina) (see Appendix V.1.).
V.3.4.4.2. Species unaffected by habitat modifications
Among the twelve species captured in all habitat types, four, all insectivorous birds (White-bellied kingfisher Alcedo leucogaster, Fire-crested Alethe Alethe diademata, White-tailed Ant-Thrush Neocossyphus poensis, Green Hylia Hylia prasina) did not show significant differences in abundance between habitat types (One-way ANOVA, for all the four species, p
> 0.1) (see Appendix V.1.).
V.3.4.4.3. Species with significantly higher abundance in land use systems
Among the 25 understorey bird species with significant responses to habitat type, one granivorous (Blue-spotted Wood-dove Turfur afer), nine insectivorous (African Pigmy Kingfisher Ceyx pictus, Little grey Greenbul Andropadus gracilis, Baumann's Greenbul Phyllastrephus baumanni, Western Nicator Nicator chloris, Red-tailed Greenbul Criniger calurus, Chattering Cisticola Cisticola anonymus, Grey-backed Camaroptera Camaroptera brachyura, Green Crombec Sylvietta virens, Black-necked Weaver Ploceus nigricollis), two omnivorous (Speckled Thinkerbird Pogoniulus scolopaceus, Little Greenbul Andropadus virens), two Nectariniidae (Collared Sunbird Hedydipna collaris, Olive-bellied Sunbird Cynniris chloropygius) and one granivorous (Western Bluebill Spermophaga haematina) showed significantly higher abundance in land use systems (CF and/or AC) as compared to natural habitats (NF and SF) (see Appendix V.1.).
V.3.4.4.4. Species with significantly higher abundance in natural habitats
Among the 25 understorey bird species with significant responses to habitat type, only insectivorous birds, eight species, (Icterine Greenbul Phyllastrephus icterinus, Xavier's Greenbul P. xavieri, Lesser Bristlebill Bleda notata, Forest Robin Stiphrornis erythrothorax, Brown-chested Alethe Alethe poliocephala, Yellow-bellied Wattle-eye Dyaphorophyia concreta, Pale-breasted Illadopsis Illadopsis rufipennis, Blackcap Illadopsis I. cleaveri) showed significantly higher abundance in natural habitats (NF and/or SF) as compared to land use systems (CF and/or AC) (see Appendix V.1.).
V.3.4.4.5. Species with significantly higher abundance in SF and CF
Among the 25 understorey bird species with significant responses to habitat type, one omnivorous (Yellow-whiskered Greenbul A. latirostris) and one Nectariniidae (Olive Sunbird Cyanomitra obscura) showed significantly higher abundance in secondary forest and agroforestry systems (see Appendix V.1.).
V.3.4.4.6. Species uniquely captured in NF
Among the eight understorey insectivorous bird species with significantly higher abundance in natural habitats, one (Yellow-bellied Wattle-eye Dyaphorophyia concreta) was found only in near-primary forest (see Appendix V.1.).
V.3.4.4.7. Species uniquely captured in AC
Among the 15 understorey bird species with significantly higher abundance in land use systems, four insectivorous (Baumann's Greenbul Phyllastrephus baumanni, Chattering Cisticola Cisticola anonymus, Green Crombec Sylvietta virens, Black-necked Weaver Ploceus nigricollis), one granivorous (Blue-spotted Wood-dove Turfur afer) and one Nectariniidae (Collared Sunbird Hedydipna collaris) were found only in annual cultures (see Appendix V.1.).
V.3.4.4.8. Species absent only in NF
Among the 25 understorey bird species with significant responses to habitat type, one insectivorous (Grey-backed Camaroptera Camaroptera brachyura) was found in all habitat types except near-primary forest, suggesting that this species tends to avoid pristine habitats (see Appendix V.1.).