Nest site selection of SCN species

In the study area, S. europaea showed highest preference for woodpecker holes compared to other SCN species, and utilised this resource overproportionally (Fig.

5.8). More than 80% of S. europaea nests were established in woodpecker holes. In Bialowieza National Park, Poland, S. europaea also showed relative preference for woodpecker holes, but the reported using rate was much lower than in this study, 32.8% in ash-alder stands and 41.4 % in oak-hornbeam stands (WESOLOWSKI 1989).

This might be partially caused by the competition from the S. vulgaris. S. vulgaris was the main second user of woodpecker holes in Bialowieza, placing 56% of its nests in woodpecker holes. And the density of S. vulgaris was higher in ash-alder stands than in oak-hornbeam stands (WESOLOWSKI et al. 2002).

Since S. europaea highly preferred middle-sized woodpecker holes, which in the study area were mainly excavated by D. major, the nest site of S. europaea consequently resembled that of D. major. S. europaea used cavities in aspen overproportionally. This largely reflected the cavity availability of aspen-preferring D. major, instead of direct selection on tree species of S. europaea. It also used fresh dead trees overproportionally, placed its nest higher than all other SCNs, and bred in cavities with round opening shape with little variance in opening dimension, which were all characteristic for D. major. The only difference between S. europaea and D.

major nest sites was that on average S. europaea nested in higher cavities than D.

major did. This was probably because that S. europaea selected higher ones from available old D. major holes, with the advantage of reduced predation of higher nests.

In Europe, S. europaea was often reported as one of the SCNs which had highest nest sites (VAN BALEN et al. 1982, NILSSON 1984b, WESOLOWSKI 1989).

Fig. 5.8 Nests of S. europaea in a woodpecker hole (left) and in a branch hole (right).

F. albicilla

Similar to S. europaea, F. albicilla also nested mainly in excavated cavities. Of the two types of excavated cavities, F. albicilla used woodpecker holes and other bird-induced holes according to their abundance. But the abundance of the latter was about twice as many, thus most of F. albicilla nests were placed in other bird-induced holes, which were mainly excavated by P. montanus in the study area. As a

consequence, the tree use and nest cavity characters of F. albicilla largely resembled those of P. montanus. Thus among the six SCN species studied, F. albicilla used the smallest trees, nested in cavities higher than all others except S. europaea, and its nest

cavities located mostly in dead substrate and had mainly round openings. On average, F. albicilla bred in higher cavities than P. montanus did, indicating that F. albicilla selected higher cavities from the available ones, possibly an adaptation against predation (NILSSON 1984b). The mean nest tree diameter of F. albicilla was larger than that of P. montanus, which was mainly because of some F. albicilla utilising woodpecker holes or branch holes.

Till few years ago, F. albicilla was considered as a subspecies of Red-breasted Flycatcher F. parva, with F. p. parva in west Palaearctic and F. p. albicilla in east Palaearctic. They were then separated into two species based on that the latter has different winter quarters (India instead of Africa), much lighter weight, almost unspotted white eggs, distinctly different song, and a more complete pre-breeding moult leading to the appearance of the orange patch in males at their first spring, and the patch never reaching the breast (BOURSKI, pers. com.).

These two closely related species also use completely different nest sites. In

consistence with the present study, the studies in Central-Siberian Biosphere Reserve documented F. albicilla as an excavated cavity user. ROGACHEVA et al. (1991)

reported that 9 out of 11 F. albicilla nests were arranged in P. montanus holes, and the others were in holes of D. major or D. minor. Recent 15 – 20 observations by the authors were basically in agreement, with some few nests found in branch holes (BOURSKI, pers. com.). In contrast, F. parva used mainly chimneys and half cavities, with relatively low nest height above ground (MIERA 1978, GLUTZ VON BLOTZHEIM &

BAUER 1993, MITRUS & SOCKO 2004). Only 9 out of 49 nests near St. Petersburg were reported in “partially destroyed” woodpecker and P. montanus holes

(MALCHEVSKI & PUKINSKI 1983).

What might be the selection force leading to such distinct nest site use of these two closely related species? The availability of excavated cavities may play an important role. P. montanus is numerous in Siberia, and almost no other species competes with F. albicilla to be a secondary user (BOURSKI, pers. com.). However, though P.

montanus is less numerous in Europe, its distribution covers almost all the breeding range of F. parva, and the density is still in general higher than the latter. Several European SCN species have been observed using P. montanus cavities, but mostly as accidental cases (GLUTZ VON BLOTZHEIM & BAUER 1993). Thus it is not yet clear if

the availabilities of P. montanus cavities for F. albicilla and for F. parva differ to a great extent.

P. ater and P. major

Opposite to above two species, P. ater and P. major behaved as branch hole specialists (Fig. 5.9). They were not selective with respect to tree species or tree diameter, as they frequently use deciduous trees and large trees, following the higher availability of holes in such trees. However, they showed high preference for cavities in living trees and in living substrates. Tit species breeding almost exclusively in living trees was also reported in Europe (WESOLOWSKI 1989, 1996, GÜNTHER &

HELLMANN 1995). As described above (Section 5.4.3), the structural stability of living trees is higher, and cavities in living substrates had less predation risk and more favourable microclimate. Another factor could be that many cavities in dead

substrates might have been too heavily decomposed and become unsuitable. Thus cavities with suitable inner conditions in dead trees should have been overestimated.

As branch hole specialists, the nest cavity characters of P. ater and P. major followed the appearance of such cavities. Such cavities usually occurred lower in thicker parts

Fig. 5.9 P. ater prefers nesting in branch holes in living trees.

of trees, which had undergone a longer time enough for cavities to develop. Such cavities also mostly had slit-like opening shape, due to the nature of branch fall and wood texture, as well as the mechanism of frost crack. Such nest site characters of non-excavating tit species were frequently documented in Europe (VAN BALEN et al.

1982, WESOLOWSKI 1989, 1996, SANDSTRÖM 1992). In the study of the Marsh Tit Parus palustris in Poland, such patterns of nest-site use were suggested as anti-predator adaptations (WESOLOWSKI 2002).

C. familiaris

Distinct from all others, C. familiaris specialised on slit-like cavities, and most of its tree use could be explained by the availability of such cavities. It preferred larch in comparison with other CNB species, and used cavities in larch overproportionally.

This mainly reflected the fact that long crevices mostly occurred behind the thick bark of larch. It nested in larger trees than other SCNs except S. europaea, since bark crevices occurred mainly in old trees, and old larch trees could develop very large diameters. When slits were formed in smaller trees, as encountered in this study due to fire fissure and shear force when snags broke, it utilised small trees as well. It accepted a very wide range of cavity opening length, while the cavity opening width was generally very small.

P. auroreus

P. auroreus behaved as a generalist in many aspects. It utilised tree species, tree DBH and tree condition according to cavity availability, indicating that it was not selective to these tree attributes. It was the only SCN species having no specific preference for certain cavity type, even one nest was found in a ground hole at the river bank. P. auroreus also used cavities of most variable opening dimensions and opening shape. The Common Redstart Phoenicurus phoenicurus in Europe appeared less selective in nest sites as well (VAN BALEN et al. 1982, GLUTZ VON BLOTZHEIM &

BAUER 1993). Using diverse nest sites could be more likely to develop in such later breeding migrants.

In summary, though overlap existed, the preferences were still distinct between most of these SCN species. As demonstrated by the discriminant function analysis, nest cavity type was the most important variable for distinguishing species. The inner

dimensions of cavities, which was suggested to be an important factor of cavity occupancy and often differs among species (van Balen et al. 1982, Johnsson et al.

1993, Carlson et al. 1998), were not measured in this study. Different types of natural cavities could reflect differences in inner dimensions to certain extent. Except P.

auroreus, all SCNs had specific preference for a certain cavity type, and their nest tree use and nest site characters reflected mainly the occurrence of such cavities. This emphasised that the attributes of natural cavities are usually diverse and correlated, very different from nest boxes, which tend to be uniform.