The percentage of canopy cover and shrub density have been measured from each plot of ground nest and shrub nest. The correlation between vegetation attributes and predation rates were analysed separately for each nest type placed in two sites.
4.8.1 Canopy cover
In Palolo valley, a comparison across five habitat types showed that the canopy cover was significantly varied over ground nest plots. The highest average of percentage of canopy cover was in natural forest (72.4 %) and the lowest was along forest edge (42.6 %). It was 46.1 % in forest garden, 42.8 % in coffee plantation and 51.4 % in secondary forest, which was considered to be an intermediate level. The percentage of canopy cover was
significantly different (Kruskal-Wallis, H = 92.802 P < 0.01, N = 243 in all cases) across habitat types.
When each nest type was analysed separately, there was no significant negative correlation between canopy cover and ground nest predation rates (Spearman-rank correlation coefficient rs = - 0.30, p > 0.05 in all cases). Figure 4.19 summarises the fact that canopy cover did not affect the nest predation on ground nests in forest margin habitats as well as in natural forest. Although the natural forest was covered with more canopy, the predation rate was relatively high.
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Figure 4.19 Average of percentage of canopy cover over ground nests in Palolo valley in comparison to nest predation rates in different habitats (bars indicate standard error ± SE).
The percentage of canopy cover over shrub nests was significantly different after applying the Kruskal Wallis test (H = 125.79, p < 0.01, in all cases, N = 243). The lowest cover was measured in coffee plantation (average 33.4 %) and the highest cover was in natural forest (average 73.8 %). It seemed that the canopy cover has an effect on nest predation rates because there was significantly negative correlation between canopy cover and nest predation on shrub nests (Spearman-rank correlation coefficient rs > - 0.70, p < 0.05) (see Figure 4.20).
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Figure 4.20 Average of percentage of canopy cover over shrub nests in Palolo valley in comparison to nest predation rates in different habitats (bars indicate standard error ± SE). Note the percentage of canopy cover in natural forest.
Visual surveys in Napu valley recorded that the percentage of canopy cover above ground nests across three habitats was significantly different (Kruskal-Wallis, H = 108.165, p <
0.01 in all cases, N= 150). The highest average of percentage of canopy cover was in natural forest (76.6 %) and the lowest was at forest edge (50.4 %) (Table 4.9). The average cover in forest garden was at an intermediate level (64.4 %). There was no correlation between canopy cover and predation rates on ground nests (Spearman-rank correlation coefficient rs = - 0.20, p > 0.05).
The average of percentage of canopy closure on shrub nests was 51.8 % along forest edge, 63 % in secondary forest and 74.6 % in natural forest (Table 4.9). There was significant variation of canopy cover across habitat types (Kruskal-Wallis, H = 96.507, p < 0.01 in all cases, N= 150). In Napu valley, there was a relationship between the percentage canopy cover and predation rates on shrub nests (Spearman-rank correlation coefficient rs > - 0.60, p < 0.05).
Table 4.9 Comparison between average percentage of canopy cover over ground and shrub nests in Napu valley and depredated nests occurring in this site.
Canopy closure (%) Depredated nests (n) Habitat
Above ground nests
Above shrub nests
Ground Shrub
Forest edge 50.4 51.8 42 37
Secondary forest 64.4 63 38 24
Natural forest 76.6 74.6 35 11
4.8.2 Shrub density
The percentage of the shrub density on ground nests at each habitat type in Palolo valley indicated that the shrub layer was significantly lower in coffee plantation than in the other habitat types. This suggests a greater exposure of nests during the day, to visually oriented predators in this habitat. The lowest average of shrub cover along coffee plantation was 41.4 % followed by forest edge 50.7 %, secondary forest 51.1 %, natural forest 52.6 % and the highest was in forest garden 61.3 %. In coffee plantation, it was relatively changeable because people regularly clean the understorey layer for crop maintenance. The percentage of shrub density was significantly different across habitat types (Kruskal-Wallis, H= 50.498 p < 0.01, N = 243 in all cases). The percentage of shrub cover in each habitat was found to be closely related to number of nests attacked. There was no correlation between shrub density and nest predation rate (Spearman-rank correlation coefficient rs = - 0.30, p > 0.05) (Figure 4.21).
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Figure 4.21 Average of percentage of shrub density over ground nests in Palolo valley in comparison to nest predation rates in different habitats (bars indicate standard errors ± SE).
In Palolo valley, there was significant variation of shrub density on shrub nests across habitat types (Kruskal-Wallis, H = 24.021, p < 0.01, N = 243). Although the percentage of shrub layer in secondary forest was higher than in other habitats, the predation rate was somewhat high compared to that occurring in natural forest (note figure 4.22). In this case, there was no significant correlation between shrub density and predation rates on shrub nests (Spearman-rank correlation coefficient rs > - 0.20, p > 0.05).
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Figure 4.22 Average of percentage of shrub density over shrub nests in Palolo valley in comparison to nest predation rates in different habitats (bars indicate standard errors ± SE).
The results obtained from Napu valley revealed that the pattern of shrub density on ground nests in forest edge, secondary forest and natural forest was similar to that occurring in Palolo valley habitats. The percentage of shrub density on ground nests was significantly lower in natural forest (45.5 %) than in the other habitat types, 56 % in secondary forest and 68.5 % along forest edge (Table 4.10). These variations were significant (Kruskal-Wallis, H = 87.523, p < 0.01), suggesting a greater exposure of nests during the day to visually oriented predators in these habitat types.
Comparing data collected from shrub nests regarding shrub density, showed that there was a significant difference among habitat types (Kruskal-Wallis, H = 83.278, p < 0.01). The highest average of percentage of shrub density was in forest edge (65 %), the intermediate level was in secondary forest (60.5 %) and the lowest was in natural forest (48.5 %). The relationship between percentage shrub density and predation rates on shrub nests was significantly negative (Spearman-rank correlation coefficient rs > - 0.70, p < 0.05).
Table 4.10 Comparison between average percentage of shrub density on ground and shrub nests in Napu valley and depredated nests occurring in this site.
Shrub density (%) Depredated nests (n) Habitat
Ground nests Shrub nests Ground Shrub
Forest edge 68.5 65 42 37
Secondary forest 56 60.5 38 24
Natural forest 45.5 48.5 35 11