5 DISCUSSION

5.5 Nest Site selection of Individual Species

5.5.3 Niche overlap and role of competition

Among the three PCN species, D. major had larger niche breadth, in terms of its ability of excavating in diverse tree species and substrate of different condition.

However, it was confined to using substrates above certain diameter and thus needed larger trees. The two smaller PCNs, D. minor and P. montanus, were not likely to suffer from competition with D. major, since their preferred nest sites were distinct.

As dead wood specialists, the nest sites of D. minor and P. montanus were similar to some extent. However, on average P. montanus used smaller trees and excavated in thinner substrate. It also appeared to associate with more heavily decayed wood than D. minor. Thus competition for nest sites between these two species was not

observed in the study period. Snag availability was not likely to be a limiting factor for these two species. In the young birch-larch forest, where birch snags were abundant, the density of both species was not higher than that in the mature birch-larch forest and the riparian forest. Territoriality, predation or food supply were possible factors limiting their populations.

Among SCNs, C. familiaris bred earliest, and it utilised unique niche, thus it was basically free from inter-specific nest site competition. S. europaea, also as an early breeder, encountered little nest site competition from other species as well. Its nest-site, mostly in living or freshly dead large trees, placed high up with the cavity opening minimised by itself, seemed an optimum in terms of both reduced predation and favourable microclimate. From preliminary observations, S. europaea appeared to have higher breeding success than P. montanus, F. albicilla and P. auroreus.

The specialisation on branch holes of P. ater and P. major did not appear to be forced by the competition of S. europaea. There were always some usable woodpecker holes left for the later-coming P. auroreus and F. albicilla. Moreover, in fact, without the ability of minimising cavity opening, middle-sized woodpecker holes were not likely to be a good breeding place. In Bialowieza, some P. palustris bred in woodpecker holes, and such nests were depredated more often than the nests in branch holes (WESOLOWSKI 2002). It is interesting that both P. ater and P. major were never found using the old nests of P. montanus, which were a plentiful resource and located high with small opening. P. ater and P. major had to make the choice of nesting low but in living or at least more intact substrates, or nesting high in decayed wood. And they gave the priority to the former. PERRINS (1979) explained low nesting height of P.

major by preferred foraging sites in the breeding season. However, this didn’t fit to P. ater, which usually foraged high in thin twigs (GLUTZ VON BLOTZHEIM & BAUER

1993).

Competition was most likely to occur between theses two species. Though on

average P. ater used cavities of smaller opening than that of P. major, certain overlap existed. Competition for nest sites between them were observed a few times during the study. The dominant competitor, P. major, had relatively low density in most of the study area. In fragmented West European stands where nest boxes were

supplemented, its density frequently reached 35 pairs/10 ha (DHONDT &

SCHILLEMANS 1983, VAN BALEN 1984, WESOLOWSKI et al. 1987). While in the birch-larch forests of the study area, its density was lower than 2 pairs/10 ha. In

Bialowieza, a similar low density of 2 – 3.5 pairs/10 ha was also reported (TOMIALOJC

et al. 1984, WESOLOWSKI et al. 1987). The low density of this dominant competitor should not be due to cavity availability, since many usable nest sites were left for P.

ater. And for P. ater, the constraint on nest site acquisition was more likely due to intraspecific territoriality rather than interspecific competition.

F. albicilla, facing the same choice of nesting in branch holes or excavated holes, gave the priority to the latter. It was possible that, for this latest-breeding migrant, most of the favourable branch holes were already occupied by P. ater and P. major.

However, this pattern might be more likely to reflect the different trade-offs of different species. As a canopy fly-catcher, nesting high would be advantageous for

reduced exposure of adult birds and nest sites to predators. Since branch holes were rare in higher parts of trees, F. albicilla used more woodpecker holes and P. montanus holes. Despite the distinct nest site use of F. parva in Europe, other two European Ficedula species, F. albicollis and F. hypoleuca, also often nest high above ground when they use natural cavities (SACHSLEHNER 1995, CZESZCZEWIK & WALANKIEWICZ

2003; but NILSSON 1984b). In Bialowieza, where cavities were abundant, F.

hypoleuca still nested higher than Parus species (CZESZCZEWIK & WALANKIEWICZ

2003).

P. auroreus, being a nest site generalist, had certain extent of overlap with most of other SCN species, as indicated by the confounding results of discriminant function analysis. The nest-site selection of this species was most likely to be shaped by interspecific competition, as it often utilised cavities with unnecessarily large

openings. Nevertheless, its density might not be limited by the availability of cavities, since many cavities occupied by the later-coming F. albicilla could be usable for P.

auroreus as well. Its density was low in most of the study area, mainly due to its association with open area and shrubs. Experimental manipulation would be

necessary to investigate whether its nest site use was shaped by ongoing competition, or being less selective in nest site had become an adaptation of this migrant.

In general, the role of interspecific competition in shaping nest-site selection and limiting population density appeared much less important than that reported in other studies. Many of such results could arise from impoverished nest site availability in managed forests (Brawn & Balda 1988). Moreover, most of the superior competitors in other studies were edge species or introduced species, with the association of human activities. In the Netherlands, competition of S. vulgaris shaped nest-site use and population size of P. major (VAN BALEN et al. 1982). In Sweden, the dominance of Jackdaws Corvus monedula forced other users of D. martius holes to use shallower and lower cavities, and made Stock Doves Columba oenas used cavities deeper in forest (JOHNSSON et al. 1993). In United States, the nest niche of introduced S.

vulgaris and House Sparrow Passer domesticus overlapped with Bluebirds Sialia spp., and caused great decline of Salia populations (ERSKINE & MCLAREN 1976, POGUE & SCHNELL 1994, SEDGWICK 1997). The competition with S. vulgaris also limited Tree Swallows Tachycineta bicolour to smaller cavities and nest sites farther

from woodland edges (PETERSON & GAUTHIER 1985, RENDELL &ROBERTSON 1989, DOBKIN et al. 1995). The aggressiveness of S. vulgaris could exert selection on other CNBs to shift their nesting phenology and behaviour (KOENIG 2003). In Bialowieza, WESOLOWSKI (1989) also suggested that interspecific competition, though playing some role in cavity selection (sequential use of the same cavity, interspecific fight), could be less important than in managed forest.

5.6 Sequential Cavity Use

Cavity reuse was often studied by observing the sequential use of the cavities of specific woodpecker species (e.g. KÜHLKE 1985, JOHNSSON et al. 1993, BONAR 2000, MEYER & MEYER 2001, WIESNER 2001, KOTAKA & MATSUOKA 2002), while only very few have considered the reuse pattern of the whole CNB community. The overall reuse rate in the present study was 37%, very similar to the few available documented values: 36% in pine-Douglas fir forest (SAAB et al. 2004), 37% in boreal pine-Douglas fir forest (AITKEN et al. 2002), and 38% – 56% in cottonwood

bottomland (SEDGWICK 1997). The comparison was influenced slightly by whether the authors included unusable cavities in the analysis, but the proportion of unusable cavities was usually less than 5%. The reuse rates reported by all studies should be considered as the minimum of the reality, since the use by nocturnal species were generally underestimated, and the nesting attempts failed at an early stage could be difficult to detect.

Im Dokument Tree cavity abundance and nest site selection of cavity nesting birds in a natural boreal forest of West Khentey, Mongolia (Seite 126-129)