Reuse at guild level

The rate of reuse old nest cavities presumably depends on costs and benefits of using old cavities and on availability of suitable excavating substrates or cavities

(SEDGWICK 1997, AITKEN et al. 2002). Cavities not occupied in the previous year may have fewer parasites or debris (SHORT 1979, RENDELL& VERBEEK 1996). These cavities may also be less susceptible to predation, since some predators may learn the location of old nest sites and revisit them (SONERUD 1985, NILSSON et al. 1991).

Therefore, it might be advantageous to use cavities not occupied in the previous year.

However, most previously unoccupied cavities may indicate a lower quality of inner

dimension or surroundings, and competition for the few qualified and previously unoccupied ones may be strong. For PCNs, reusing old cavities has further benefit in saving considerable time and energy for excavation that could be allocated to

reproduction. However, cavity excavation is suggested to be an integral component of courtship, functioning in pair bond formation and maintenance (LAWRENCE 1967), thus many PCNs excavate one or even more new cavities each year. The reuse of WPCNs may be shaped by both the availability of suitable excavating substrate and the competition with SCNs.

Since the costs and benefits of reuse differ among guilds, the reuse pattern of each guilds may be different. The reuse rate of each guild, however, appeared discordant among studies (Table 5.3). In West Khentey, cavities previously used by SCNs had the highest reuse rate, previous WPCN cavities were seldom reused, and previous PCN cavities were reused to an intermediate extent. This pattern was most similar to another boreal study site (AITKEN et al. 2002). But in other two study sites, on the contrary, previous WPCN cavities had highest reuse rate (SEDGWICK 1997, SAAB et al. 2004).

Table 5.3 Guild level cavity reuse pattern in the literature.

Cavity reuse (%)

Location Original

occupant by different species by same species not reused

PCN 19 13 69

1 Only the reuse of 1985 cavities in 1986 was presented, since the data of between year reuse of 1986 and 1987 were not presented (Table 1 in SEDGWICK 1997). Black-capped Chickadee Poecile atricapillus was grouped as WPCN according to AITKEN et al. 2002.

2 Two mammal species were included in SCNs.

The reuse pattern also differed among studies, again with most disagreement in WPCNs. AITKEN et al. (2002) suggested that, WPCN cavities were generally more shallow and with entrances too small for some SCNs, thus they were not favoured by SCNs. WPCNs might be less able to compete with the larger SCNs, thus they reuse their own cavities. Therefore the reuse of WPCN cavities should be low and mainly by the same species. However, this could not be generalised to different sites. In West Khentey, reuse of WPCN cavities was low, but all were used by SCNs. This might be owing to enough suitable snags for excavation, thus almost all WPCNs constructed new cavities. And the smaller cavities of WPCNs were still usable by small SCN species. In the study of SEDGWICK (1997), reuse rate of WPCN cavities was high, and they were mostly reused by SCNs. This was resulted from that the smallest SCN species House Wren Troglodytes aeddon frequently used the smaller cavities of the only WPCN Black-capped Chickadee Poecile atricapillus. The competition for cavities was considered strong in that study site (SEDGWICK & KNOPF

1992), and strong competition often leads to small species using smaller cavities to reduce the chance of eviction by a larger competitor (PETERSON & GAUTHIER 1985, RENDELL & ROBERTSON 1989). In the study of SAAB et al. (2004), WPCN cavities had high reuse rate, and were mostly reused by the same species. This was the consequence of that the WPCN Lewis’s Woodpecker Melanerpes lewis frequently occupied a nest cavity over several years. The authors explained this with its very poor excavator morphology.

It was corresponding among the four studies, that the reuse of previous PCN cavities were mainly by other species, while the reuse of previous SCN cavities were mainly by the same species (Table 5.3). In Europe, WESOLOWSKI (1989) reported that woodpeckers hardly ever reused their own cavities, while SCNs often bred in cavities previously utilised by their own species. VAN BALEN et al. (1982) observed that woodpecker holes were always used by another species in the year after excavation, which indicated the high cavity demand in this Dutch study site with highest cavity occupancy ever documented (see Section 5.3.4). The constancy of the ten SCN species there averaged 64%, also higher than reported in other studies (Table 5.3).

The reuse of SCN cavities by different species may indicate the extent of nest niche overlap. Then the very low interspecific cavity reuse of SCNs in West Khentey

compared to other studies might imply more distinct niche partition and less

competition among SCN species. The reuse of PCN or WPCN cavities by different species shows the excavator-secondary user relationships, while under some

circumstances it may imply competition as well. The most often documented case was that, in North America, the aggressive invader S. vulgaris overtook the nest cavities of woodpeckers and forced the latter to excavate new ones or reuse the older ones (TROETSCHLER 1976, INGOLD 1989, 1996). The snatch of PCN or WPCN nest cavities by SCN species has not been observed in West Khentey.