Ecological Relations of Sympatric Black-Capped and Mountain Chickadees in Southwestern Alberta’

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 loadings of 0.20 or greater in the vari- pine, and limber pine were combined as “conif- able-reduction DFA. The other included only erous” for analysis. Trembling aspen and balsam nest-tree variables. Becausethere were only five poplar were combined as “deciduous” for size nest-tree variables, all were included in the anal- classesabove 24 cm dbh. For both deciduous ysis regardlessof their loadings in the variable- and coniferous categories,size classesof 32 cm reduction DFA. dbh or greater were combined. Descriptions of all variables used in the analyses are given in FORAGING BEHAVIOR Appendix 1. Potential dietary overlap between the two species We sampled 115 nest sites (54 Black-capped was assessedby measuring foraging behavior. In Chickadee, 61 Mountain Chickadee) over the doing so, we assume that differencesin foraging breeding seasons of 1982-1984. Nest cavities behavior reflect differencesin resourceuse. This reusedin a secondyear by Mountain Chickadees approach to assessingdietary overlap has been were included only once in the analysis, reducing commonlyused(e.g., Hertzet al. 1976, Rice 1978, the sample to 47 Mountain Chickadee nests.Be- Barlow and McGillivray 1983). cause most casesinvolved unbanded birds, we Foraging behavior is usually measured using were uncertain whether reuse was by the same either sequential observations or single point ob- individuals, and thus whether each year’s use servations (Morrison 1984). Sequential obser- representedan independent sample. Four Black- vations (following an individual bird for as long capped Chickadee nests were located in trees as possible and continuously recording data for which had previously contained another Black- a number of variables) reveal rare behaviors more capped Chickadee cavity and were excluded for completely than do single point observations similar reasons,reducing the sample to 50 Black- (Morrison 1984) but produce data that may vi- capped Chickadee nests. Nest cavities used by olate the assumption of independence of samples Black-capped Chickadees in one year, and sub- (Wiens 1983). Thus, they are unsuitable for para- sequently reusedby Mountain Chickadees,were metric statistical analysis. Single point obser- included in the analysis twice (once in the sample vations (one observation per individual, which for each species). is normally the first sighting of that individual) None of the variables differed significantly be- produce data that are suitable for parametric tween 1982 and 1983 for either species (two- analysis but which may be biased towards re- sample t-test; all P > 0.05). Therefore, all but cording visible behaviors occurring in open or one of the analyseswere performed using pooled conspicuoushabitats (Wiens 1983). data for these 2 years. Data from 1984 were used With these considerations in mind, we devel- as an independent set for testing classification oped a technique to obtain data that were suit- functions produced by discriminant analysis. able for parametric statistical analysis and reveal We used discriminant function analysis (DFA) rare behaviors, but that were not biased towards to evaluate interspecific differences, using the recording observations in conspicuouslocations. discriminant procedure of SPSS (Hull and Nie We made a series of successiveobservations on 1981). First, a “variable-reduction” DFA, in- an individual bird at predetermined intervals (an cluding all variables and data from all 3 years, observation “chain”). We used chains of up to was used to identify variables with low discrim- 10 observations made at intervals of 30 sec.Suc- inatory power. Variables with loadings (corre- cessiveobservation chains on a single individual lations with the discriminant function) of co.20 were separatedby 5 min, an interval chosen be- were excluded from subsequent analyses. This cause chickadeesrarely spend this long foraging loading is far below the level judged as “poor” in a single location. At each observation, we re- 878 BRAD G. HILL AND M. ROSS LEIN corded foraging station (tree, shrub, or ground), was tested using Box’s M. Small, but significant, tree species, tree part (trunk, branch, twig, or differences occurred in all casesand, therefore, leaf), foraging stance (erect or inverted), height the group covariance matrix of the canonical of bird, and height of tree. Foraging observations function was used in classification of cases.All began 5 min after encountering a foraging bird. habitat DFAs were performed with an equal prior This delay avoided biasing the observations to- probability of group membership. Because the wards visible behaviors occurring in conspicuous foraging observations were heavily biased to- or open habitat settings. Mean values (for con- wards Black-capped Chickadees, DFAs on these tinuous variables) or proportions (for discrete or data were performed with the prior probabilities multi-state variables, such as “foraging station”) adjusted in proportion to sample size. Discrimi- were calculated for each observation chain. Each nant functions were evaluated using an F-test of chain thus provided a single value (for each vari- the significance of the Mahalanobis distance be- able) which should not be seriously autocorre- tween groups and Cohen’s Kappa, which mea- lated with previous or subsequent observations. sures the improvement of the classification of We collected1,248 observations(938 on Black- the discriminant function over chance alone (Ti- cappedChickadees, 3 10 on Mountain Chickadees), tus et al. 1984). Statistical significancewas judged spreadover 157 chains(116 on Black-cappedChick- at the P = 0.05 level. adees,4 1 on Mountain Chickadees),in the spring RESULTS and summer of 1984. The mean number of observationsper chain was similar for Black-capped REUSE OF NEST SITES and Mountain chickadees (8.09 and 7.56, re- Black-capped Chickadeesnever reused nest cav- spectively). Observation chains were distributed ities in years subsequentto that in which the nest evenly among 10 Black-capped Chickadees and was excavated. Twelve of 14 Black-capped five Mountain Chickadees (all color-banded Chickadee nest cavities located in 1982 were still males), and were evenly distributed over the present in 1983, while two nest trees had fallen. stagesof the breeding cycle. None of these cavities were reused by Black- The various species of trees were combined capped Chickadees, although new cavities were into “deciduous” and “coniferous” categoriesfor excavated in two of the trees. Mountain Chick- analysis. Because matrix inversion during the adeesoccupied one former Black-capped Chick- calculation of the discriminant functions is not adee nest cavity in 1983. Only eight of the 1982 possible with highly correlated variables, we ex- cavities were present in 1984. None were reused, cluded one “state” of each multi-state variable. although new cavities were excavated in two ad- For two-state variables (e.g., foraging stance)the ditional trees.Of 18 Black-cappedChickadee nest exclusion of one state resulted in no loss of in- cavities discovered in 1983, 14 were still present formation. To minimize information loss in in 1984. None of these were reused by Black- three- or four-state variables (e.g., foraging sta- capped Chickadees, but one was occupied by a tion) we excludedthe staterepresenting the rarest pair of Mountain Chickadees. behavior. The full list of variables used in the In contrast, nest cavities occupied by Moun- statistical analyses can be found in Appendix 2. tain Chickadeesshowed fairly high levels of reuse Differences between the two specieswere eval- in subsequentyears. All eight nest cavities used uated using two-sample t-tests and discriminant by Mountain Chickadeesin 1982 were still pres- function analysis. Becausewe had data from only ent in 1985. Pairs of Mountain Chickadeesreused one season,we randomly selectedand removed five of these in 1983, five in 1984, and one in 20% of the original data prior to calculating the 1985. Of 20 nest cavities located in 1983, eight discriminant function and used these excluded of 17 surviving cavities were reusedin 1984, and observations to test the classification power of five of 16 surviving cavities were reused in 1985. the function. Eighteen of 19 cavities discovered in 1984 sur- vived until 1985, and six of these were reused STATISTICAL ANALYSIS by Mountain Chickadees. Variables measured as percentages or proportions were arcsine transformed for analysis. All OVERLAP IN UTILIZED NEST SITES DFAs were run using the direct option. The Eight of the 17 nest-habitat variables differed equality of group variance-covariance matrices significantly between species(Table 1). In gen- ECOLOGICAL RELATIONS OF SYMPATRIC CHICKADEES 879

TABLE 1. Characteristicsofnest habitat and nest trees of Black-cappedand Mountainchickadees. See Appendix 1 for full explanationsof variables.

Black-capped Chickadee nest sites Mountain Chickadee nest sites Variable’ R + SD (n = 50) R & SD (n = 47) Pb Nest-habitatvariables ASP1 76.52 t 56.18 44.43 + 31.63 0.001 POP1 7.80 i 18.35 7.34 + 14.35 0.891 CON1 1.72 ? 3.46 2.68 * 4.19 0.220 ASP2 25.12 +- 17.43 16.96 ? 12.19 0.009 POP2 1.72 + 4.11 2.38 * 7.31 0.580 CON2 0.40 + 0.97 2.02 & 4.09 0.008 DEC3 4.22 f 4.19 9.23 f 7.15 0.00 1 CON3 0.36 + 1.05 1.40 f 3.18 0.030 DEC45 0.78 i 1.56 3.06 * 3.60 0.001 CON45 0.20 * 0.76 1.28 t 2.70 0.008 TOTTREE 118.82 -+ 57.20 90.68 t 36.95 0.005 NLJMTRSP 2.04 f 1.01 2.36 * 0.99 0.116 CANCOV (%) 53.30 + 14.13 50.32 & 14.65 0.311 GRCOV (O/o) 65.50 f 18.52 71.89 + 17.09 0.08 1 CANHT (m) 12.89 + 2.97 13.49 f 2.81 0.076 SHRUB 44.60 i 36.31 45.70 + 33.67 0.877 PERCDEAD (%) 9.20 i 6.23 9.48 f 6.06 0.824 Nest-treevariables DBH (cm) 13.20 + 3.82 26.45 ? 9.92 0.001 HTNESTR (m) 4.57 + 2.67 10.28 ? 4.82 0.001 HTNEST (m) 3.70 f 2.07 4.82 ? 2.73 0.024

1 All units are counts unless given in parentheses. b Two-tailed, two-sample f-test. eral, Black-capped Chickadee nest sites were in from 1982 and 1983 produced a significant dis- areas dominated by small aspens(ASPl, ASP2) criminant function (P < O.OOl), with a correct while Mountain Chickadeenest siteswere in areas classification rate of an independent sample (the with larger deciduous trees (DEC3, DEC45) and 1984 data set) of 83.9%, significantly better than more conifers (CON3, CON45). All three con- tinuous nest-tree variables (DBH, HTNESTR, TABLE 2. Comparisonof correlationsbetween the HTNEST) differed significantly between the two discriminantfunctions and the discriminatingvari- species(Table 1). Mountain Chickadeenests were ablesfor analysesincluding: (a) both nest-habitatand in larger trees,and were higher above the ground, nest-treevariables; (b) nest-habitatvariables only; and than were Black-capped Chickadee nests. Of the (c) nest-treevariables only. See Appendix 1 for full explanationsof variables. two multi-state nest-tree variables, obvious interspecificdifferences were found in COND (39 Discrimmant analysis using: of 50 Black-cappedChickadee nests,but only 16 All Nest-habltat Nest-tree of 47 Mountain Chickadee nests were in dead Variable variables variables only variables only trees) but not in SPNESTR (45 of 50 Black- DBH 0.640 N/k 0.767 capped Chickadee nests, and 45 of 47 Mountain HTNESTR 0.679 N/A 0.813 Chickadee nests, were in trembling aspens). COND -0.340 N/A -0.407 0.318 0.527 N/A The variable-reduction DFA produced a high- DEC3 DEC45 0.257 0.426 N/A ly-significant function (P < 0.001) with a correct ASP1 -0.335 -0.555 N/A classification rate of 89.7%, significantly better TOTTREE -0.268 -0.444 N/A than expected by chance (Kappa = 0.792, P < CON2 0.237 0.393 N/A 0.001). Only 10 of the 22 variables had loadings CON45 0.193 0.319 N/A ASP2 -0.228 -0.377 N/A >0.20. The remaining variables had poor dis- HTNESTb - N/A 0.303 criminatory power and their removal in a trial SPNESTRb N/A -0.016 DFA produced no change in the overall correct a Not apphcable. classification rate. b Excluded in the combined nest-habitat and nest-tree analysis because of low correlation with the discriminant function m the variable-reduc- The DFA usingnonexcluded variables and data tion DFA (see Methods). 880 BRAD G. HILL AND M. ROSS LEIN chance (Kappa = 0.621, P < 0.002). The vari- often on branches (BRANCH), and less often on ables contributing most to the separation were trunks (TRUNK) and twigs (TWIG), than did dbh of nest tree (DBH), height of nest tree Mountain Chickadees.Black-capped Chickadees (HTNESTR), and condition of the nest tree foraged lower (HTBIRD) and in shorter trees (COND) (Table 2). Mountain Chickadeestended (HTTREE) than did Mountain Chickadees. to nest in trees that were taller and of larger di- The discriminant function was highly signifi- ameter than those in which Black-capped Chick- cant (P < 0.001). The correct classification rate adeesnested (Table I), and used cavities located of an independent sample (the remaining 20% of in live trees to a greater extent than did Black- foraging chains) was 94.4%, a highly significant capped Chickadees. improvement over chance (Kappa = 0.875, P < The DFA using only nest-habitat variables also 0.00 1). Three variables contributed most strong- produced a significant function (P = 0.002), with ly to discrimination (Table 4). Black-capped a correct classification rate of the 1984 data set Chickadees used deciduous trees (DEC) to a of 80.7%, significantly greater than expected by greater extent, foraged lower (HTBIRD), and in chance (Kappa = 0.460, P < 0.03). All variables shorter trees (HTTREE) than did Mountain contributed to the separation, as indicated by Chickadees. their similar loadings (Table 2). Black-capped DISCUSSION Chickadeesused nest siteswhich were surrounded by more small deciduous trees, while Moun- Interspecific differenceswere found in both nest tain Chickadees used nest sites surrounded by sites and foraging behavior of Black-capped and more coniferousand large deciduoustrees (Table Mountain chickadees in southwestern Alberta. 1). The tendency of Mountain Chickadees to nest The final DFA, including only nest-tree vari- in taller trees with larger diameters is probably ables, also produced a significant discriminant related to their requirement for pre-existing cav- function (P < 0.001). The correct classification ities. Mountain Chickadeesoften nested in aban- rate of the 1984 data set (83.9%) was significantly doned nest cavities of Yellow-bellied Sapsuckers better than that expected by chance (Kappa = (Sphyrapicusvarius), which were invariably lo- 0.62 1, P < 0.002). The variables which contrib- cated in large trembling aspens. The difference uted most heavily to the function were the same in nest habitatsbetween the two chickadeespecies as in the analysis with nest-habitat and nest-tree is probably related to this use by Mountain variables combined (DBH, HTNESTR, and Chickadees of large nest trees. Thus, in using a COND) (Table 2). Not surprisingly, SPNESTR nest tree with a large diameter, an individual contributed only very weakly to the separation necessarilyuses a nest habitat which is also char- (both speciesnested almost exclusively in trem- acterized by large trees (most aspen stands in the bling aspen). Thus, nest sites differ between the area were of relatively uniform age and size). speciesprimarily in the size of trees used, with The rates of reuse of nest cavities also suggest Mountain Chickadee nestsbeing associatedwith that competition for nest sites between the two treeswhich were larger than thoseassociated with speciesis not severeon our study area. Although Black-capped Chickadee nests (Table 1). a few nest cavities excavated by Black-capped Chickadeesare used by Mountain Chickadeesin OVERLAP IN FORAGING BEHAVIOR subsequentbreeding seasons,many are not, and Mean values of foraging variables differed sig- presumably would be available for such use. In- nificantly between Black-capped and Mountain traspecific competition for nest cavities among chickadees,with the exception of the proportion Mountain Chickadees seems more likely, as in- of observations in which the bird was erect dicated by the relatively high rate of cavity reuse (ERECT) (Table 3). The largestdifference was in in this species. the proportion of observations in deciduoustrees However, despite differencesin nest sites and (DEC). Black-capped Chickadees foraged in de- the low rate of nest-site reuse, there are several ciduous trees to a far greater extent than did indications that Black-capped and Mountain Mountain Chickadees.Black-capped Chickadees chickadeesmay sometimescompete for nest sites, foraged less in trees (TREE) and more often in at least locally. First, we examined nest-site use, shrubs (SHRUB) than did Mountain Chicka- not choice. The observed use might be affected dees.Black-capped Chickadees also foragedmore by behavioral interactions, with one speciesbeing ECOLOGICAL RELATIONS OF SYMPATRIC CHICKADEES 881

TABLE 3. Characteristicsof the foraging behavior of Black-cappedand Mountain chickadees.See Appendix 2 for full explanationsof variables.

Black-capped Chickadees Mountain Chickadees Variable’ n?SD(n=116) R ? SD (n = 47) Pb

TREE 76.01 + 20.20 88.90 ? 7.03

a All variables are percentages unless otherwise indicated. b Two-tailed, two-sample t-test. restricted to less-preferred nest sites. Minock which used the cavity throughout the remainder (1972) found that Black-cappedChickadees were ofthe breeding cycle. While we have no idea what normally able to dominate Mountain Chicka- happened to the original occupants, interspecific dees at feeding stations in winter. If a similar aggressioncan not be ruled out. dominance relationship occursduring the breed- Foraging behavior differed greatly between ing season, Black-capped Chickadees could po- Black-capped and Mountain chickadees. The tentially restrict Mountain Chickadees to less- largest single difference was in the type of tree preferred nest sites. used, with Black-capped Chickadees foraging in Second, although statistical differences were deciduous trees far more than Mountain Chick- found between the two speciesin nest-site use, adees.In addition, Black-cappedChickadees for- there was some overlap. Two cavities which were aged lower in the canopy and in smaller trees used by Black-capped Chickadees in one year than did Mountain Chickadees. The overlap in were reusedby Mountain Chickadeesduring the foraging behavior, and thus resource use, was following year. very low, as indicated by the very low rate of Third, patterns of reuse of nest cavities or nest misclassificationof casesin the foraging behavior trees indicate that there may be resource limi- DFA (5.6%). These differences in foraging be- tation, especiallyfor Mountain Chickadees.While havior are due to interspecific differencesin pref- such reuse of cavities or trees may be explained erence, rather than to differences in resource as nest-site philopatry, the observations are also availability. Territories of the two speciesoften consistent with the idea that suitable nest sites overlapped extensively on our study area, and are limited. an analysis of vegetation on territories showed Fourth, some interspecific aggression,which no significant interspecific differences (Hill and could be interpreted as a behavioral manifesta- Lein, unpubl.). tion of competition, was observed at or near nest sites. During song playback experiments per- TABLE 4. Correlations between the discriminant formed at nest sites(Hill and Lein, unpubl.), both function and the discriminating variables for the for- speciesoften responded to the playback of het- aging DFA. See Appendix 2 for full explanations of erospecificsong, although responseswere weaker variables. than those given to conspecificsong. In addition, most instances of naturally occurring interspe- Variable Correlation cific aggressionoccurred at or near nest sites(Hill DEC 0.769 and Lein, unpubl.). HTBIRD -0.436 One observation in the spring of 1983 may HTTREE -0.410 indicate direct interspecific competition for nest SHRUB 0.191 TREE -0.189 sites. A nest cavity that had been excavated by TRUNK 0.148 Black-capped Chickadees,and used by them for TWIG 0.144 a period of severalweeks, was subsequentlyfound BRANCH 0.096 to be occupiedby a pair ofMountain Chickadees, ERECT 0.070 882 BRAD G. HILL AND M. ROSS LEIN

This low overlap in foraging behavior proba- ica. There is broad overlap between the ranges bly reducesfood competition between the species. of Black-capped Chickadees and Boreal Chick- As with nest sites,it is possiblethat the observed adees(Parus hudsonicus),but the two speciesare foraging behavior is a direct result of aggressive largely segregatedby habitat within the region of interactions (i.e., that one of the two speciesis overlap (Dixon 196 1). Similarly, within the area being restricted to less-preferred foraging sta- of overlap between the rangesof the Black-capped tions and/or behaviors by the other species). Chickadee and Chestnut-backed Chickadee However, our field observations do not support (Pam rufescens)in western North America, the this. Usually, there was only one species(usually two speciesare generally found in different hab- one pair of birds) in the vicinity and thus there itats, especially during the breeding season,and was no reason for an individual to restrict its they show marked differencesin foraging behav- foraging behavior. In 32 instances when we ob- ior (Smith 1967, Sturman 1968). The exception served individuals of both speciesforaging in the to this pattern of ecological segregationby hab- same tree (in 10 caseswithin 2 m of one another), itat among North American chickadees is the no interspecific aggressionwas seen. narrow overlap between the ranges of Black- Low dietary overlap (and hence lack of com- capped Chickadeesand Carolina Chickadees(P. petition) is also suggestedby the fact that the carolinensis) in eastern North America. These bills of Mountain Chickadees on our study site two speciesuse similar habitats and nest sites, are significantly longer and thinner than those of and show similar foraging behavior (Brewer Black-capped Chickadees (two-sample t-test, P 1963). However, their extreme morphologicaland < 0.00 1; Hill, unpubl. data). While the relation- genetic similarity and their hybridization in the ship between bill dimensions and food resource region of sympatry (Rising 1968, Braun and use is controversial (e.g., Hespenheide 1975), it Robbins 1986, Robbins et al. 1986) indicate that is generally acceptedthat interspecificdifferences thesetwo forms are very closely related, and thus in bill size often reflect differencesin prey choice are qualitatively different from other sympatric (Schoener 1965, Karr and James 1975). We be- pairs of chickadeesin North America. lieve that the differencesin foragingbehavior and ACKNOWLEDGMENTS bill morphology reflect adaptation to different habitats. The differencesin foragingbehavior that We thank D. A. Boag for permitting us to use the we documented, with Black-capped Chickadees facilities of the University of Alberta’s R. B. Miller BiologicalStation. Dave &tchinson and Kelly Nordin primarily using deciduous trees and Mountain assisted in the fieldwork. Brian O’Mallev and John Chickadeesprimarily using coniferous trees,par- Wiens made valuable contributions to the-experimen- allel those found in allopatry (e.g., Smith 1967 tal designand to the statisticalanalysis. Robert Barclay, and Sturman 1968 for Black-cappedChickadees; Kevin Cash, Glen Chilton, Val Haines, L. Scott John- Manolis 1977 and Franzreb 1983 for Mountain son, Charles M. Weise, and an anonymous reviewer improved the manuscript with their comments. The Chickadees). Partridge (1976) found similar re- study was supportedby a postgraduatescholarship to lationships between foraging behavior and bill BGH and an operating grant (A9690) to MRL, both morphology in two European parids, with the from the Natural Sciencesand Engineering Research bills of conifer-foraging Coal Tits (Parus ater) Council of Canada. being longer and thinner than those of decidu- LITERATURE CITED ous-foraging Blue Tits (P. caeruleus). Partridge ALATALO,R. V. 1982. Evidence for interspecificcom- attributed these differencesto adaptation to dif- petition among Europeantits Parus spp.: a review. ferent habitats. Ann. Zool. Fenn. 19:309-3 17. Our findings indicate that, where they exist in ALATALO,R. V., L. GUSTAFSSON,AND A. LUNDBERG. sympatry in southwesternAlberta, Black-capped 1986. Interspecificcompetition and niche changes and Mountain chickadees are ecologically seg- in tits (Part0 spp.):evaluation of nonexperimental data. Am. Nat. 127:819-834. regated by their preferences for, and adaptations ANDERSON,H. G. 1979. Ecologicalland classification to, different habitats. Local sympatry is possible and evaluation, Highwood-Sheep. Energy and only where a mosaic of the preferred habitats of Natural ResourcesReport No. 93. Government both species exists, and each species uses this of Alberta. BALEN,J. H. VAN, C.J.H. BOOY,J. A. FRANEKER,AND mosaic in rather different ways. This explanation E. R. OSIECK. 1982. Studieson hole-nestingbirds is similar to those described or proposed for oth- in naturalnest sites. 1. Availability and occupation er sympatric pairs of chickadees in North Amer- of natural nest sites. Ardea 70: l-24. ECOLOGICAL RELATIONS OF SYMPATRIC CHICKADEES 883

BARLOW,J. C., ANDW. B. MCGILLIVRAY. 1983. For- KARR, J. R., AND F. C. JAMES. 1975. Eco-morpho- agingand habitatrelationships of the siblingspecies logicalconfigurations and convergentevolution in Willow Flycatcher (Empidonux traillii) and Alder speciesand communities, p. 258-29 1. In M. L. Flycatcher(E. alnorum) in southernOntario. Can. Cody and J. M. Diamond [eds.], Ecologyand evo- J. 2001. 61:1510-1516: lution of communities. Harvard Univ. Press, BRAUN.M. J.. AND M. B. ROBBINS.1986. Extensive Cambridge, MA. protein ~similarity of the hybridizing chickadees LACK,D. 1969. Tit niches in two worlds; or homage Paw atricapillus and P. carolinensis.Auk 103: to Evelyn Hutchinson. Am. Nat. 103:43-49. 667-675. MANOLIS,T: 1977. Foraging relationshipsof Moun- BRAWN,J. D., AND R. P. BALDA. 1988. Population tain Chickadeesand Pygmy Nuthatches. Western biology of cavity nestersin northern Arizona: do Birds 8: 13-20. nest siteslimit breeding densities?Condor 90:6 l- MINOCK, M. E. 1972. Interspecific aggressionbe- 71. tween Black-cappedand Mountain chickadeesat BREWER,R. 1963. Ecologicaland reproductive rela- winter feeding stations. Condor 741454-461. tionships of Black-capped and Carolina chicka- MINOT,E. O., ANDC. M. PERRINS.1986. Interspecific dees. Auk 80:9-47. interference competition-nest sites for Blue and COMREY,A. L. 1973. A first coursein factor analysis. Great tits. J. Anim. Ecol. 55:331-350. Academic Press,New York. MORRISON,M. L. 1984. Influence of sample size and CONNELL,J. H. 1983. On the prevalenceand relative sampling design on analysisof avian foraging be- importance of interspecificcompetition: evidence havior. Condor 86: 146-150. from field experiments. Am. Nat. 122:661-696. NILSSON,S. G. 1984. The evolution of nest-site se- DAHLSTEN,D. L., ANDW. A. COPPER.1979. The use lection among hole-nestingbirds: the importance of nestingboxes to study the biology of the Moun- of nest predation and competition. Ornis Stand. tain Chickadee (Parus gambeli) and its impact on 15:167-175. selectedforest insects,p. 2 17-260. In J. G. Dick- NILSSON,S. G. 1987. Limitation and regulation of son, R. N. Conner, R. R. Fleet, J. C. Kroll, and J. population density in the Nuthatch Sitta europaea A. Jackson[eds.], The role of insectivorousbirds (Aves) breeding in natural cavities. J. Anim. Ecol. in forest ecosystems.Academic Press,New York. 56:921-937. DIXON, K. L. 1961. Habitat distribution and niche ODUM,E. P. 1941 a. Annual cycleof the Black-capped relationshipsin North American speciesof Parus, Chickadee-l. Auk 58:314-333. p. 179-219. In W. F. Blair [ed.], Vertebrate spe- ODUM,E. P. 1941 b. Annual cycleof the Black-capped ciation. Univ. Texas Press, Austin, TX. Chickadee-2. Auk 58:519-535. DIXON, K. L., AND R. A. STEFANSKI.1970. An ap- ODUM, E. P. 1942. Annual cycle of the Black-capped praisal of the songof the Black-cappedChickadee. Chickadee-3. Auk 59:499-531. Wilson Bull. 82:53-62. PARTRIDGE,L. 1976. Some aspectsofthe morphology ENEMAR,A., AND B. SJOSTRAND.1972. Effects of the of Blue Tits (Paruscaeruleus) and Coal Tits (Purus introduction of the Pied Flycatcher Ficedula hy- ater) in relation to their behaviour. J. Zool. (Lond.) poleuca on the composition of a passerine bird 179:121-133. community. Ornis Stand. 3:79-89. RICE, J. C. 1978. Ecologicalrelationships of two in- FRANZREB,K. E. 1983. A comparison of avian for- terspecificallyterritorial vireos. Ecology 59:526- aging behavior in unloggedand loggedmixed-co- 538. niferous forest. Wilson Bull. 95:60-76. RISING,J. D. 1968. A multivariate assessmentof in- GOTFRYD, A. F., AND R.I.C. HANSELL. 1985. The terbreeding between the chickadees, Parus atri- impact of observer bias on multivariate analyses canillus and P. carolinensis.Svst. Zool. 17:160- of vegetation structure. Oikos 45:223-234. 189. HAARTMAN,L. VON. 1957. Adaptation in hole-nest- ROBBINS,M. B., M. J. BRAUN,AND E. A. TOBEY. 1986. inn birds. Evolution 11:339-347. Morphological and vocal variation acrossa con- HERTZ~P.E., J. V. REMSEN,JR., AND S. I. ZONES. 1976. tact zone between the chickadeesParus atricapil- Ecological complementarity of three sympatric lus and P. carolinensis.Auk 1031655-666. parids in a California oak woodland. Condor 78: SCHOENER,T. W. 1965. The evolution of bill size 307-316. differencesamong sympatric congenericspecies of HESPENHEIDE,H. A. 1975. Prey characteristicsand birds. Evolution 19:189-2 13. predator niche width, p. 158-l 80. In M. L. Cody SLAGSVOLD,T. 1978. Competition between the Great and J. M. Diamond [eds.], Ecologyand evolution Tit Parus major and the Pied Flycatcher Ficedula of communities. Harvard Univ. Press, Cam- hypoleuca:an experiment. Ornis Stand. 9:46-50. bridge, MA. SMITH, S. M. 1967. An ecological study of winter HULL, C. H., ANDN. H. NIE. 1981. SPSSupdate 7-9. flocks of Black-capped and Chestnut-backed McGraw-Hill, New York. chickadees.Wilson Bull. 79:201-207. JAMES,F. 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APPENDIX 1. Description of variables used in the analysesof nest sites.

Code Description of variable

Nest-habitat variables ASP1 Number of trembling aspensper plot in size class 1 (dbh = 8.1-16.0 cm). POP1 Number of balsam poplars per plot in size class 1. CON1 Number of coniferous trees per plot in size class 1. ASP2 Number of trembling aspensper plot in size class2 (dbh = 16.1-24.0 cm). POP2 Number of balsam poplars per plot in size class2. CON2 Number of coniferous trees per plot in size class2. DEC3 Number of deciduoustrees per plot in size class 3 (dbh = 24.1-32.0 cm). CON3 Number of coniferous trees per plot in size class3. DEC45 Number of deciduoustrees per plot in size classes4 and 5 (dbh = 32.1 cm or greater). CON45 Number of coniferous trees per plot in size classes4 and 5. TOTTREE Total number of trees of all speciesand size classeswithin each plot. NUMTRSP Total number of speciesof trees per plot. CANHT Canopy height, measuredto the nearest 0.5 m. CANCOV Percent canopy cover per plot. GRCOV Percent ground cover per plot. SHRUB Shrub density (number of stems with dbh < 8.0 cm intersectedon two 2-m wide tran- sectsper plot). PERCDEAD Percent of trees that were dead per plot. Nest-tree variables DBH Diameter at breast height of nest tree, to the nearest cm. HTNESTR Height of nest tree, to the nearest 0.5 m. COND Condition of nest tree (living or dead). SPNESTR Speciesof the nest tree. HTNEST Height of nest, to the nearest 0.5 m.

APPENDIX 2. Variables used in the analysisof foraging behavior.

Code Description of variable

TREE Proportion of observationsper chain when focal individual was foraging in a tree. SHRUB Proportion of observationsper chain when focal individual was foraging in a shrub. DEC Proportion of observationsper chain when focal individual was foraging in a deciduoustree. TRUNK Proportion of observationsper chain when focal individual was foraging on the trunk. BRANCH Proportion of observationsper chain when focal individual was foraging on a branch (limb > 2.0 cm in diameter). TWIG Proportion of observationsper chain when focal individual was foraging on a twig (limb < 2.0 cm in diameter). ERECT Proportion of observationsper chain when focal individual was foraging in erect position (as opposedto inverted position). HTBIRD Mean height of the foraging bird, to the nearest meter. HTTREE Mean height of the tree being utilized, to the nearest meter.