Short Communications Wilson Bulletin 117(4):400±402, 2005 Interspeci®c Nest Sharing by Red-breasted Nuthatch and Mountain Chickadee Patrick A. Robinson,1 Andrea R. Norris,1 and Kathy Martin1,2,3 ABSTRACT.ÐWe report an observation of inter- ponent and have similar cavity preferences speci®c nest sharing between Red-breasted Nuthatches (Ghalambor and Martin 1999, McCallum et (Sitta canadensis) and Mountain Chickadees (Poecile al. 1999, Martin et al. 2004). gambeli) near Williams Lake, British Columbia, Can- ada. The nest contained two Red-breasted Nuthatch Chickadees and nuthatches, along with and three Mountain Chickadee nestlings. The nest was Downy Woodpeckers (Picoides pubescens), attended by a pair of Mountain Chickadees earlier in comprise a sub-group of small-bodied cavity the observation period and later by an adult female nesters competing for cavity resources in the Red-breasted Nuthatch; all ®ve nestlings ¯edged. nest web (Martin et al. 2004). The rate of ex- Competition for nest sites due to a decrease in cavity tra-group cavity reuse among nuthatches and availability may have contributed to this occurrence. Received 5 November 2004, accepted 18 July 2005. chickadees is low (17%) relative to the rate of reuse (70%) for primary cavity nesters (Ait- ken et al. 2002). Thus, high intra-group cavity reuse is the primary source of competition for The advantage of nesting in cavities is often nest sites among chickadees and nuthatches. high success, but cavity nesters must compete If absolute or relative availability of suitable with other individuals and species to secure cavities decreased, competition in this group this resource. Competition for cavities can would increase, promoting cavity acquisition limit population densities where cavity avail- strategies, such as usurpation or nest sharing. ability is low (Brush 1983, Peterson and Gau- Steeger and Dulisse (2002) reported increased thier 1985, Holt and Martin 1997). Red- competition and aggression among cavity breasted Nuthatches (Sitta canadensis) regu- nesters in response to changes in the relative larly excavate new cavities; however, they abundance of nest sites. Usurpation also oc- also may reuse or renovate existing cavities. curs in response to decreased nest-site abun- Mountain Chickadees (Poecile gambeli) pri- dance (McCallum et al. 1999). Although not marily reuse existing cavities, but very infre- previously reported among Red-breasted Nut- quently renovate or excavate cavities (KM un- hatches and Mountain Chickadees, nest shar- publ. data). Both species are common at our ing could result from cavity competition if study sites in the Williams Lake area of Brit- nest initiation by a subordinate pair occurs pri- ish Columbia, Canada. The area consists of or to occupation by a dominant pair, and if the interior Douglas-®r (Pseudotsuga menziesii) new occupants do not destroy the progeny of and lodgepole pine (Pinus contorta) inter- the initial pair. In this note, we report a case spersed with patches of grassland and stands of interspeci®c nest sharing, where adults of of quaking aspen (Populus tremuloides; Mar- both species attended the nest, and young of tin and Eadie 1999). Red-breasted Nuthatches both species were reared to ¯edging. and Mountain Chickadees are resident species that compete for similar nest sites, as both pre- OBSERVATION fer mixed forest with a strong conifer com- In May and June 2004, during the course of our 10-year ®eld investigation of cavity 1 Centre for Applied Conservation Research, Faculty nesters in an area approximately 40 km west of Forestry, Univ. of British Columbia, 2424 Main of Williams Lake, British Columbia, Canada, Mall, Vancouver, BC V6T 1Z4, Canada. we monitored a case of nest sharing involving 2 Canadian Wildlife Service, 5421 Robertson Rd., RR1, Delta, BC V4K 3N2, Canada. Mountain Chickadees and Red-breasted Nut- 3 Corresponding author; e-mail: hatches (Martin et al. 2004) in a quaking as- [email protected] pen. On 31 May, we observed two adult 400 SHORT COMMUNICATIONS 401 Mountain Chickadees attending the nest and was situated at the edge of the cutblock. In taking insects into the cavity. On 1 June, PAR 2004, the ®rst post-cut year, we monitored two ¯ushed an adult Mountain Chickadee from the Red-breasted Nuthatch and two Mountain cavity. This was the last detection of adult Chickadee nests in addition to the shared nest chickadees at or near the nest. At this time, cavity. This was our only observation of in- the cavity was presumed to contain Mountain terspeci®c nest sharing and brood rearing in Chickadee chicks of unknown age. On the our 10-year study of cavity nesters, during next visit (7 June) a female Red-breasted Nut- which we monitored 691 nests of small cavity hatch was tending the nest; she entered the nesters (52 Black-capped Chickadee, Poecile cavity with food twice within 5 min. PAR vi- atricapillus; 42 Downy Woodpecker; 340 sually inspected the cavity and found ®ve Mountain Chickadee; and 257 Red-breasted chicks (two nuthatch and three chickadee). On Nuthatch). 10 June, the female nuthatch made frequent (approximately once per min) foraging trips DISCUSSION from a nearby Douglas-®r tree to the nest. At Although active competitionÐin the form least two fecal sacs were removed during 6 of aggression before clutch initiation and nest min of observation. On 11 June, ARN ob- usurpation before and during incubationÐis served all ®ve chicks still in the cavity, and frequently reported (Ghalambor and Martin two nuthatch chicks (estimated at 16 days of 1999, McCallum et al. 1999), this is the ®rst age) ¯edged during the observation period. record of Mountain Chickadees and Red- The ¯edgling nuthatches were seen the next breasted Nuthatches successfully rearing their day foraging with the adult nuthatch on and young in a nest attended by both parental spe- near the nest tree while the cavity still con- cies. In our study area, nuthatch nest density tained three healthy chickadee nestlings. With more than tripled from 0.03 nests/ha during fewer chicks in the cavity, ARN could see that 1996±2000 to 0.10 nest/ha during 2001±2004; the nest was lined with fur, typical of chick- during the same period, chickadee nest density adee nests, but fresh pitch had been applied increased from 0.05 to 0.14 nests/ha (KM un- to the cavity entrance, which is typical of Red- publ. data). This may be a result of regional breasted Nuthatch nests. During this obser- changes in tree condition and an increased vation, the adult female nuthatch arrived at the abundance of forest insects (KM unpubl. cavity without food and vocalized toward the data). Nuthatches and chickadees prefer dead cavity from a nearby branch, apparently en- and decaying aspen trees, which composed couraging the remaining Mountain Chickadee ,7% of trees on our stands (Martin et al. nestlings to ¯edge. The female nuthatch then 2004). Furthermore, nest-site availability de- provisioned the chickadee nestlings twice, re- creased at a local scale, due to cutting on the moving fecal sacs following both visits. On site. Thus, both the absolute and relative avail- 16 June, the cavity was empty, and with no ability of nest sites decreased in our study evidence of predation, we presumed that the area. These factors, combined with the recent chickadees had ¯edged successfully. Because tripling of nuthatch and chickadee popula- no birds were banded, subsequent sightings of tions, may have increased encounter rates and Red-breasted Nuthatches or Mountain Chick- interspeci®c competition, facilitating the nest- adees in the area could not be associated with sharing occurrence. this nest. We were able to con®rm nest sharing be- The study plot where the observation oc- cause we visited the nest tree and inspected curred was in a 26-ha stand of mixed decid- the cavity visually on multiple occasions. Un- uous and coniferous forest consisting of 85% fortunately, we did not locate this nest until Douglas-®r, 4% lodgepole pine, 8% spruce after the eggs had hatched; thus, we could not (Picea spp.), and 3% quaking aspen. In 2002, determine the circumstances during clutch ini- we found four Red-breasted Nuthatch nests, tiation and incubation. We suspect that Moun- and in 2003, we found one nuthatch and ®ve tain Chickadees initiated the nest because the Mountain Chickadee nests. The study plot was cavity was lined with fur. In addition, Moun- selectively harvested in the fall of 2003. The tain Chickadees consistently ¯edge in 21 days nest tree (recently dead aspen, 30.2 cm dbh) (McCallum et al. 1999), whereas Red-breasted 402 THE WILSON BULLETIN x Vol. 117, No. 4, December 2005 Nuthatches remain in the nest anywhere from search Council of Canada (NSERC) grant to K. Mar- 14 to 21 days (Ghalambor and Martin 1999); tin. A. R. Norris was supported by an NSERC Indus- therefore, the nuthatches could have ¯edged trial Post-graduate Scholarship (sponsored by Tolko Ltd.), Environment Canada's Science Horizons Youth before the chickadees, even if the nuthatch Internship Program, and a Southern Interior Bluebird eggs were laid after the chickadee eggs. Last, Trail Society grant. We thank D. A. McCallum and C. we did not observe a male nuthatch at the K. Ghalambor for discussions about prevalence of this nest. Given the aggressive nature of male nut- unusual behavior. K. A. Otter and D. A. McCallum hatches and the fact that nuthatch pairs can made helpful suggestions in the revision of the man- out-compete Mountain Chickadee pairs (ARN uscript. unpubl. data), we suspect the absence of a LITERATURE CITED male nuthatch was an important contributing factor in this occurrence of interspeci®c nest AITKEN, K. E. H., K. L. WIEBE, AND K. MARTIN. 2002. Nest-site reuse patterns for a cavity-nesting bird sharing.