The Condor90~875-884 0 The Cooper Ornithological Society 1988 ECOLOGICAL RELATIONS OF SYMPATRIC BLACK-CAPPED AND MOUNTAIN CHICKADEES IN SOUTHWESTERN ALBERTA’ BRAD G. HILL AND M. Ross LEIN~ Division of Ecology(Behavioral Ecology Group), Department of BiologicalSciences, University of Calgary, Calgary, Alberta T2N lN4, Canada Abstract. In an attempt to determine the factors permitting sympatry of Black-capped Chickadees (Parus atricapillus) and Mountain Chickadees (P. gambeli) in southwestern Alberta, we examined nest-site utilization and foraging behavior during the summers of 1982 to 1984. Characteristicsof both the nest tree itself, and the surroundinghabitat, differed significantlybetween nest sitesofthe two species.Patterns of reuseofnest sitesand behavioral observationssuggested that some interspecific competition for nest sites may occur, but is probably not important. Foraging behavior differed significantly between the two species, suggestingthat Black-cappedand Mountain chickadeesdo not compete for food during the breedingseason. Differences in habitat use by the two speciesapparently provide ecological segregation,and their coexistenceon our study area is due to the mosaic nature of the habitat. Key words: Parusatricapillus; Parus gambeli; interspecificcompetition; nest-site selection: foraging behavior;habitat selection. INTRODUCTION son than at other times, but neither speciesuses The ranges of speciesof North American chick- special song perches(Dixon and Stefanski 1970, adees (Paridae) are rarely sympatric with those pers. observ.). Instead, both species sing while of congeners,while in Europe up to six species moving and engagingin other activities, such as often coexist (Sturman 1968, Lack 1969). Lack foraging. (1969) suggestedthat widespreadcoexistence does Competition for roost sitesduring the breeding not occur among North American parids because season is also unlikely. Females of both Black- they are at an earlier stagein their evolution than capped(Odum I94 1b) and Mountain chickadees European species,and there has been insufficient (pers.observ.) “ roost” (incubate or brood) in their time for the evolution of ecological segregation. nest holes during the breeding season,while their This argument assumes that interspecific com- mates roost nearby in trees. Thus, roost-site petition directly influences the distribution of competition during the breeding seasonis more North American parids. However, only a few properly viewed as nest-site competition. Com- field studies(e.g., Brewer 1963, Smith 1967) have petition for roost sites may be more important directly examined whether or not competition during the remainder of the year. occursamong sympatric parids in North Amer- Numerous studies have demonstrated that ica. Our study seeksto clarify the ecological re- cavity-nesting birds may be subject to both intra- lationships of Black-capped Chickadees (Parus and interspecific competition for nest sites (e.g., atricapillus) and Mountain Chickadees(P. gam- Haartman 1957; Enemar and Sjostrand 1972; beli) during the breeding season in an area of Slagsvold 1978; Balen et al. 1982; Nilsson 1984, year-round sympatry. 1987; Minot and Perrins 1986). This appears to Small passerinespotentially compete for sev- be true for Mountain Chickadees,which increase eral resources, including food, nest sites, song in breeding density when provided with artificial perches, and roost sites. Competition for song nest cavities (Dahlsten and Copper 1979, Brawn perchesseems unlikely in chickadees.Both Black- and Balda 1988). Although Black-capped Chick- capped(Odum 194 1a) and Mountain chickadees adeesare primary cavity nesters,excavating their (pers. observ.) sing more during the breeding sea- own cavities, while Mountain Chickadees are secondarycavity nesters,using pre-existing cav- I Received 25 April 1988. Final acceptance2 August ities, there is still the potential for interspecific 1988. competition for suitable nesting trees. Although 2Author to whom reprint requestsshould be sent. Black-cappedChickadees apparently never reuse F3751 876 BRAD G. HILL AND M. ROSS LEIN nest cavities in subsequentbreeding seasons,they parsnip (Heracleum lanatum) and various species often reuse the same tree, and trees containing of grass.Anderson (1979) gives a more complete two or three nest cavities are not uncommon description of habitats of the region. (pers.observ.). Therefore, reuseof treesby Black- NEST-SITE CHARACTERISTICS capped Chickadees could deprive Mountain Chickadeesof potential nest sites, while use of a Active nest cavities were located by searching former Black-capped Chickadee nest cavity by areas where males were advertising and defend- Mountain Chickadees might deprive Black- ing territories. Nest-related activities such as ex- capped Chickadeesof the use of a potential nest cavation (Black-capped Chickadees only) and tree. gathering of nesting material were conspicuous, Black-cappedand Mountain chickadeesmight and most nestswere found prior to the beginning also compete interspecifically for food during the of laying. A few were not discovered until the breeding season. Both species feed mainly by activity of feeding nestlings again made the nest gleaning arthropods from the surfaces of vege- site conspicuous.Each year we revisited all active tation (Odum 1942, Brewer 1963, Sturman 1968, nest sites from previous years to determine the Dahlsten and Copper 1979). Territories of the rate of reuse of cavities. We noted whether each two specieson our study area showed extensive nest tree and nest cavity were still present (and overlap (Hill and Lein, unpubl.), and preliminary hence available), and whether they were being observations suggestedthat they foraged in sim- used. These checks were conducted during the ilar ways. Food competition is common in Eu- period when active nests contained nestlings. ropean species of Parus (Dixon 1961, Alatalo Therefore, cavities that had been reusedby pairs 1982, Alatalo et al. 1986). whose nest had failed early in the breeding cycle Documentation of resourcecompetition per se would be incorrectly assigned.However, rates of is difficult in field studies, as it requires dem- nest failure observed in this study were low (1 O- onstration both that two or more speciesoverlap 15%) and errors causedby this procedure would in their use of a resource, and that the resource be minor. in question is in limited supply (Connell 1983). We measuredhabitat variables in circular plots Demonstration of overlap in resource use alone centered on nest trees, using a modification of can be considered only as an indication of the the sampling technique of James and Shugart potential for resource competition. Becausewe (1970) and James (197 1). Each plot was 18 m in were unable to measure resourceavailability di- diameter, with an area of 0.025 ha. The senior rectly, we use data on resourceutilization to ex- author made all habitat measurements, elimi- amine the potential for competition for food and nating the possibility of observer bias in the data nest sites between Black-capped and Mountain (Gotfiyd and Hansel1 1985). chickadees. We chosevariables related to both the habitat surrounding the nest tree and the nest tree itself. METHODS Becausebirds are believed to selecttheir habitat using the overall configuration of vegetation STUDY AREA structure (the niche-gestalt), and not details of Our study area was in the Sheep River Wildlife microhabitat (James 1971) we measured only Sanctuary (50”38’N, 114”3O’W) in the upper major structural features. All trees (vegetation foothills of the Rocky Mountains, about 70 km with a diameter of main stem at breast height SW of Calgary, Alberta. Black-capped and [dbh] of 8.0 cm or greater) within each plot were Mountain chickadees breed sympatrically in categorized by species,size class (in 8.0-cm in- mixed forests of river valleys in this area. The crements), and condition (living or dead). Can- forests are dominated by trembling aspen (Pop- opy cover and ground cover were estimated along ulus tremuloides), with lesser amounts of white two transects of the plot which intersected at a spruce (Picea glauca), balsam poplar (_Populus 90” angle at the nest tree. At 20 points (10 per balsamijka), lodgepolepine (Pinus contorta),and transect),presence or absenceof green vegetation limber pine (Pinusflexilis). The understory, when was recorded by sighting directly up or down present, consists primarily of young trembling through a 3.0-cm diameter tube held at arm’s aspen, some willow (Salix spp.), and alder (Alnus length. Shrub density was estimated by counting spp.). The undergrowth consistsprimarily of cow the number of stems < 8.0 cm in diameter along ECOLOGICAL RELATIONS OF SYMPATRIC CHICKADEES 877 two transects, each 2 m wide, across the plot (Comrey 1973). A second DFA, using nonex- (total area of approximately 0.007 ha). Canopy eluded variables and data for 1982 and 1983, height was measured with a clinometer. Five evaluated differencesin nest trees and nest hab- characteristicsof the nest tree itself (species,con- itat between the two species. Two additional dition, dbh, and height ofthe nest tree, and height DFAs determined whether nest-habitat variables of the nest) were recorded. or nest-tree variables had the most power to dis- Most plots contained only one or two tree criminate between nest sites of the two species. species,each with individuals of relatively uni- The first of theseincluded only nest-habitat vari- form size. Therefore, white spruce, lodgepole ables with