4 RESULTS

4.5 Utilisation of Trees and Cavities by CNBs

4.5.1 Tree species

Of the 97 nests located in plots, 68% were found in birch trees (Table 4.12). Eight out of 13 bird species utilised birch. Poplar was the second important nest tree species, accounted for 13% of the nests and was used by 6 bird species. No nests were found in rowan, Siberian pine, fir and spruce. PCNs and SCNs showed similar patterns in tree species utilisation (?2 = 5.63, df = 5, p = 0.344), and were pooled in the following analysis.

Table 4.12 Species composition (%) of nest trees used by PCNs and SCNs.

Tree species

Willow Aspen Poplar Birch Larch Scots pine

N

PCNs 2.9 5.7 20.0 65.7 2.9 2.9 35

SCNs 1.6 1.6 9.7 69.4 9.7 8.1 62

Overall 2.1 3.1 13.4 68.0 7.2 6.2 97

The species composition of nest trees differed from the relative abundance of tree species in the forests (?2 = 144.10, df = 9, p < 0.001). Poplar, which comprised only 3% of the standing stems (Table 4.5), was strongly selected by CNBs (Yates corrected

?2 = 57.80, df = 1, p < 0.001). Birch, which was frequent in the forest (37%), was also used overproportionally (Yates corrected ?2 = 34.18, df = 1, p < 0.001). Siberian pine, fir and spruce were used underproportionally (?2 test with Yates correction, p < 0.05 for Siberian pine, p < 0.001 for fir and spruce).

Comparing the species composition of cavity trees (Table 4.5) with that of nest trees, no significant difference were found (?2 = 0.00, df = 9, p =1.000). Most of the cavities were located in birch, so were the nests. Poplar and larch were the next important cavity tree species, and were thus frequently served as nest trees.

Tree species differed among habitats, but the utilisation of nest tree species followed some general rules. In each habitat, PCNs and SCNs had similar patterns in their use

of tree species (?2 test, p > 0.05 in each habitat). And this pattern differed from the tree species composition in the corresponding habitat (?2 test, p < 0.01 in each habitat). In both the mature and the young birch-larch forests, birch was used overproportionally. In the riparian forest, poplar was used overproportionally and birch was only used in proportion to its abundance. In the spruce-fir forest, both birch and larch were used overproportionally, and Siberian pine, spruce and fir were used underproportionally. Species composition of nest trees did not differ from that of cavity trees in each habitat (?2 test, p > 0.05 in each habitat).

4.5.2 Tree DBH

Trees of different DBH were used disproportional to their abundance in the forests (?2

= 232.66, df = 2, p < 0.001). Most of the nests (60%) were located in trees of DBH 25 - 50 cm, which accounted for only about 10% of the standing stems. Trees of DBH larger than 50 cm, containing 13% of the nests, were even more strongly

selected relative to their abundance (1%). Trees of DBH under 25 cm, accounting for 88% of the standing stems and 27% of the nests, were used underproportionally.

< 25 25 - 50 > 50 DBH (cm)

0 20 40 60 80 100

Percentage (%)

PCN nests SCN nests all stems cavities

Fig. 4.16 The percentage constitution of all sampled stems (green bars; n = 863), total cavities (red bars; n = 157), nests of PCNs (cyan bars; n = 35) and nests of SCNs (blue bars; n = 62) according to tree DBH.

The DBH distribution of nest trees differed between PCNs and SCNs (Fig. 4.16, cyan and blue bars; ?2 = 10.09, df = 2, p < 0.01). Both group of birds used small trees underproportionally (Fig. 4.16, green bars), but SCNs used even less small trees than

PCNs. 46% of PCN nests were located in trees of DBH < 25 cm, while only 20% of SCN nests were found in these trees (Yates corrected ?2 = 8.53, df = 1, p < 0.01). The DBH distribution of SCN nest trees was not significantly different from the DBH distribution of cavity trees (Fig. 4.16, red bars; ?2 = 3.82, df = 2, p = 0.148).

4.5.3 Tree condition

63%, 13% and 24% of CNB nests were located in living trees, intact dead trees and broken snags, respectively. Compared with the tree condition constitution in the forests (85%, 10%, 5%, respectively; Fig. 4.17, green bars), living trees were used underproportionally (Yates corrected ?2 = 28.56, df = 1, p < 0.001), and broken snags were used overproportionally (Yates corrected ?2 = 42.56, df = 1, p < 0.001). The nest holding rate of broken snags was six times higher than that of living trees, and 3 times higher than that of intact dead trees.

living dead intact dead broken Tree condition

0 20 40 60 80 100

Percentage (%)

PCN nests SCN nests all stems cavities

Fig. 4.17 The percentage constitution of all sampled stems (green bars; n = 863), total cavities (red bars; n = 157), nests of PCNs (cyan bars; n = 35) and nests of SCNs (blue bars; n = 62) according to tree condition.

PCNs and SCNs differed in their nest tree condition (Fig. 4.17, cyan and blue bars; ?2

= 11.20, df = 2, p < 0.01). Both group of birds used living trees underproportionally and used broken snags overproportionally, while PCNs avoided living trees and preferred broken snags further than SCNs. Living trees contained 71% of SCN nests but only 49% of PCN nests (Yates corrected ?2 = 3.90, df = 1, p < 0.05). Broken

snags accounted for 13% of SCN nests, while 43% of PCN nests were established in them (Yates corrected ?2 = 9.50, df = 1, p < 0.01).

SCNs did not use cavities in trees of different condition according to their availability (Fig. 4.17, red bars). Cavities in living trees were used overproportionally (Yates corrected ?2 = 6.46, df = 1, p < 0.05), and those in broken snags were used

underproportionally (Yates corrected ?2 = 13.25, df = 1, p < 0.001).

4.5.4 Presence of fire scars

Most of the nests (64%) were located in trees with fire scars, which comprised only 28% of the standing stems (Yates corrected ?2 = 51.92, df = 1, p < 0.001). Nest holding rate of fire-scarred trees was 5 times higher than that of trees without fire scars.

0 20 40 60 80 100

Percentage (%)

All stems Cavities PCN nests SCN nests Variables

with fire scar without fire scar

Fig. 4.18 The percentage of all sampled stems (n = 863), total cavities (n = 157), nests of PCNs (n = 35) and nests of SCNs (n = 62) according to the presence of fire scars.

Both PCNs and SCNs utilised fire-scarred trees overproportionally (Yates corrected ?2

= 6.30, df = 1, p < 0.05 for PCNs, Yates corrected ?2 = 53.07, df = 1, p < 0.001 for SCNs), while SCNs nested in these trees even more frequently than PCNs (Fig. 4.18;

Yates corrected ?2 = 4.60, df = 1, p < 0.05). For SCN nests, the nest holding rate of

fire-scarred trees was 7 times higher than that of trees without fire scars, while for PCN nests only about 2 times higher. The extent SCNs using fire-scarred trees (73%) was similar to the cavity availability in these trees (76%; Yates corrected ?2 = 0.53, df

= 1, p = 0.468).

4.5.5 Presence of fungi conks

38% of the nests were found in conk-bearing trees, which accounted for only 5% of the standing stems (Yates corrected ?2 = 144.76, df = 1, p < 0.001). Nest holding rate of trees with fungi conks was 11 times higher than that of trees without.

PCNs showed higher tendency to use conk-bearing trees than SCNs (Fig. 4.19; Yates corrected ?2 = 7.16, df = 1, p < 0.01). 57% and 27% of PCN and SCN nests,

respectively, were established in trees with fungi conks. Conk-bearing stems held PCN nests 23 times more frequently, and held SCN nests 7 times more frequently, than those without conks did. The proportion SCNs using conk-bearing trees was similar to cavity availability in these trees (29%; Yates corrected ?2 = 0.01, df = 1, p = 0.911).

All stems Cavities PCN nests SCN nests Variables

0 20 40 60 80 100

Percentage (%)

with fungi conk without fungi conk

Fig. 4.19 The percentage of all sampled stems (n = 863), total cavities (n = 157), nests of PCNs (n = 35) and nests of SCNs (n = 62) according to the presence of fungi conks.

4.5.6 Cavity type

PCNs and SCNs differed in their use of cavity type (Fig. 4.20, cyan and blue bars; ?2

= 47.69, df = 3, p < 0.001). PCNs mostly nested in cavities excavated by themselves or other PCNs, while in few cases (11%) P. montanus also utilised branch hole. For SCNs, the majority (71%) of their nests were established in branch hole, and 18% in woodpecker hole.

In comparison to the relative abundance of each cavity type (Fig. 4.20, red bars), branch hole was used overproportionally (Yates corrected ?2 = 4.53, df = 1, p < 0.05) and other bird-induced hole was used underproportionally by SCNs (Yates corrected

?2 = 10.86, df = 1, p < 0.01). Woodpecker hole and bark crevice were utilised in proportion to their availability (Yates corrected ?2 = 0.55, df = 1, p = 0.458 for woodpecker hole, Yates corrected ?2 = 0.00, df = 1, p = 1.000 for bark crevice).

Woodpecker hole

Other bird-induced hole

Branch hole

Bark crevice

Cavity type 0

20 40 60 80

Percentage (%)

cavities PCN nests SCN nests

Fig. 4.20 The percentage constitution of total cavities (red bars; n = 157), nests of PCNs (cyan bars; n = 35) and nests of SCNs (blue bars; n = 62) according to cavity type.

Im Dokument Tree cavity abundance and nest site selection of cavity nesting birds in a natural boreal forest of West Khentey, Mongolia (Seite 62-68)