Nest Web Analysis

4.8.1 Nest web of CNB community in West Khentey

Integrating tree and cavity characteristics across the CNB community, the nest site use of CNBs was summarised in the nest web (Fig. 4.27). The links indicated the

proportion of tree or cavity use of each bird species. Such a graphic visualised the linkage and interdependence among species.

Considering the linkage between level one (trees) and level 2 (PCNs, WPCNs and non-excavated cavities), birch played the most important role. All excavating species except D. martius utilised birch. D. leucotos, D. minor and P. montanus were highly dependent on this tree species, among them the nests of D. leucotos were only found in birch (n = 3). It also supported the majority (73%) of the non-excavated cavities

Fig. 4.27 Nest flow in the nest web. Cavity type was indicated by red (excavated cavities) and green (non-excavated cavities). The proportion of nest use was indicated by dotted lines (< 10%), thin lines (10 - 50%) and thick lines (> 50%). Bird species were abbreviated by the first three letters of the genus and species names. WPm referred to middle-sized woodpecker holes pooled.

Par mon WPm

Buc cla Sit eur Fic par Jyn tor Pho pho Pho aur Par ate Par maj Tur ruf Cer fam

Willow Birch Aspen Poplar Larch Scots Pine Branch

hole

Dry mar Pic can Den maj Den leu Den min Other

Level 1 Level 2 Level 3

which were used by SCNs and P. montanus. Aspen was important tree species for D.

martius and D. major. Spruce, fir and Siberian pine did not enter the nest web, since no nests were found in them.

Between level two and level three (SCNs), the large-sized woodpecker holes from D.

martius were only used by B. clangula, and this duck depended exclusively on this resources (n = 2). Middle-sized woodpecker holes supported more diversified users, including S. europaea, F. albicilla, J. torquilla, P. phoenicurus, P. auroreus and the WPCN P. montanus, but none of these users focused entirely on this resource. The sample size for J. torquilla and P. phoenicurus was small (n = 2 for each), and for both species one of the two nests found were in middle-sized woodpecker holes. S.

europaea showed highest dependence on this resource, in which 86% of their nests were established. The small-sized woodpecker holes from D. minor were only utilised by F. albicilla.

Cavities from P. montanus were the main resource for F. albicilla, supporting 56% of their nests. In some cases, usually after the cavity openings were enlarged by

woodpeckers or due to other causes, P. montanus cavities were also utilised by P.

phoenicurus and P. auroreus.

Except B. clangula and P. phoenicurus, all other SCNs and P. montanus utilised non-excavated cavities. T. ruficollis was not an obligate cavity nester. P. ater, P. major and C. familiaris depended exclusively on this resource, while others had alternative choices.

Among SCNs, F. albicilla and P. auroreus used more diversified cavities, behaving as generalists in cavity use. B. clangula, P. ater, P. major and C. familiaris behaved as specialists.

4.8.2 Simulation I: roles of woodpeckers

The nest web could be applied to investigate the roles of woodpeckers in the CNB community. Supposing that D. martius was removed from the system, according to the nest web (Fig. 4.27), it was most likely that B. clangula would disappear as well.

However, other parts of the CNB community were basically unaffected.

In the case that all middle-sized woodpecker species were excluded from the CNB community, the influence would be more complicated (Fig. 4.28). First, this could affect other members in the same level. D. minor and P. montanus might gain some advantages due to the release of competition for nest substrates. However, their benefit might not be substantial. The preferred nest site of D. major did not greatly overlap with that of D. minor and P. montanus (see Section 4.6). Other middle-sized woodpecker species only appeared with low density in the study area. It also affected the resource availability of SCNs. Of the five SCN species nesting in middle-sized woodpecker holes, none was totally dependent on such cavities. S. europaea, which had the highest preference for middle-sized woodpecker holes, could also well use branch holes.

With the simplified assumption that each of the users of middle-sized woodpecker cavities simply shift to its alternative resources proportionally, the users of D. minor

Fig. 4.28 The supposed nest web in the case that middle-sized woodpeckers were excluded from the system.

Par mon WPm

Buc cla Sit eur Fic par Jyn tor Pho pho Pho aur Par ate Par maj Tur ruf Cer fam

Willow Birch Aspen Poplar Larch Scots Pine Branch

hole

Dry mar Pic can Den maj Den leu Den min Other

x 1.2 7% à 8.4%

cavities and P. montanus cavities would become 1.3 times more, and that of branch holes 1.2 times more. Branch hole was originally the cavity type most heavily used by SCNs (see Section 4.5.6). The original occupancy of branch hole was

approximately 7%. This would rise to 8.4% when middle-sized woodpecker holes were no more available. These numbers indicated that there might still be enough available branch holes to buffer the lost of middle-sized woodpecker holes.

The removal of D. minor seemed to cause few stirs to the nest web. Only F. albicilla used D. minor holes. And in the study area, there were abundant P. montanus

cavities, which served as an alternative for F. albicilla.

4.8.3 Simulation II: roles of some tree species

Nest web could also be applied to observe the importance of each tree species to the CNB community. From the nest web (Fig. 4.27), it was clear that the removal of spruce, fir and Siberian pine had no direct impact on the CNBs, from the viewpoint of cavity supply.

If birch was removed from the system and replaced by, for example, spruce, far-reaching influences could arise (Fig. 4.29). Firstly, birch was the main nesting substrate for several PCNs and WPCNs, supporting 65% of their nests. D. leucotos, D. minor and P. montanus were highly dependent on birch. Though these excavators could shift their resource use, the impact was not likely to be fully adjusted. Aspen and poplar were the favourite alternatives for D. minor and P. montanus (see Section 4.6.1), but these tree species had either low abundance or limited distribution. Thus the intra- and interspecific competition among PCNs and WPCNs would rise.

Another critical function of birch was that it supplied large amount of non-excavated cavities. About 80% of the branch holes utilised by SCNs were located in birch.

When birch was excluded from the system, the abundance of branch hole would reduced to one-fifth, which indicated the occupancy would rise to five times, given the same amount of users. The original occupancy 7% would than become 35%, which might lead to strong competition among SCNs for cavities of good quality. P.

ater and P. major, which nested exclusively in branch hole, might suffer the most.

For other species, the opportunity of shifting to other resources would be limited as

Fig. 4.29 The supposed nest web in the case that birch was excluded from the system.

well, since the availability of PCN and WPCN cavities might also decrease both in number and in spatial distribution.

The removal of aspen would influence mainly D. martius and D. major. D. major was adaptive to diverse tree species. D. martius was more limited by the availability of large trees. Suitable alternatives were, however, likely to be available in its large territory. As the result of the adjustability of these PCNs, the influence on SCNs due to the removal of aspen might not be substantial.

The removal of larch would lead to about 67% decrease of special non-excavated cavities. Since T. ruficollis was not an obligate cavity nester, only C. familiars might suffer to a great extent. Other parts of the CNB community were not likely to be substantially affected.

Par mon WPm

Buc cla Sit eur Fic par Jyn tor Pho pho Pho aur Par ate Par maj Tur ruf Cer fam

Willow Birch Aspen Poplar Larch Scots Pine Branch

hole

Dry mar Pic can Den maj Den leu Den min Other

80%

7% à 35%