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The Condor96:647653 0 The CooperOrnithological Society 1994

FORAGING BEHAVIOR AND NESTLING DIET OF -BACKED CHICKADEES IN MONTEREY

P. K. KLEINTJES*AND D. L. DAHLSTEN Laboratory of Biological Control, Universityof , Berkeley, CA 94720

Abstract. The foragingbehavior and nestlingdiet of Chestnut-backedChickadees (Parus rufescens)was studied during the breeding season(March-May) in a Monterey pine () , 199l-l 992. Adult spent 79% (? 7.1 SD) of their foraging time on Monterey pine as a result of prey availability. The majority of this time was spent perch gleaning and hang gleaning prey from the outer needles of the upper crown. Nestling diet was composedof approximately 43% Monterey pine larvae (Acantholydaburkei, Hymenoptera: Pamphiliidae) and 17% tree camel crickets (Gammarotettix bilobatus,Or- thoptera: Rhaphidophoridae). Both insects feed upon Monterey pine foliage. Spiders and individual Homoptera, Hemiptera, Coleoptera,, and Diptera speciescomprised the remaining 40% of the diet. Monterey pine servesas an important foraging resourcefor Chestnut-backedChickadees during the breeding seasonand may have contributed to the range expansion and population increaseof this speciesin the San Francisco Bay region. Key words: Chestnut-backedChickadees; Parus rufescens; Monterey pine; foraging be- havior; diet; sawfly larvae; insectivorousbird

INTRODUCTION known to breed in Monterey pine Changesin native vegetation often influence and in many areas, coastal populations of Chest- population abundance, distribution and behav- -backed Chickadees overlap the distribution ior (Dixon 1954, DeGraaf and Wentworth 198 1, of both native and planted Monterey pine (Grin- Morrison and Keane 1988, Brennan and Mor- nelll904, Brennan and Morrison 199 1, Kleintjes rison 199 1). In the San Francisco Bay region, the and Dahlsten 1992). recent range expansion and population increase The foraging behavior of Chestnut-backed of Chestnut-backed Chickadees has been attrib- Chickadees has been well documented in the uted to the conversion of native woodland coastal live oak and mixed woodlands and grasslandsto orchardsand urban landscapes of the California Coast Ranges(Dixon 1954, (Dixon 1954, Brennan and Morrison 199 1). 1964, Rowlett 1972, Hertz et al. 1976, Wagner The Chestnut-backed Chickadee is usually as- 1981) and the mixed coniferous of both sociatedwith moist coniferous forestsof the Pa- the Pacific Northwest (Sturman 1968, Lundquist cific Coast and more recently, montane forests and Manuwal1990) and Sierra Nevada (Brennan containing Douglas- (Grinnell 1904, Brennan 1989, Brennan and Morrison 1990). However, and Morrison 199 1). Therefore, we presume that little is known about their foraging behavior in planting of , particularly stands of Mon- Monterey pine stands, nor about their insectiv- terey pine (Pinus radiata), has contributed to the orous diet (Dixon 1954, Kleintjes and Dahlsten birds’ successfulinvasion of the San Francisco 1992). East Bay region. The purpose of this study was to determine Monterey pine is native to central coastal Cal- the importance of Monterey pine for breeding ifornia, but has been widely planted as an “or- Chestnut-backedChickadees, as part of long term namental” in urban landscapes,recreation areas studies on the breeding biology of Chestnut- and plantations (Barbour and backed Chickadeesnesting in artificial nestboxes Major 1988). Chestnut-backed Chickadees are in the San Francisco East Bay region and Sierra Nevada of California (Gold and Dahlsten 1989; Kleintjes and Dahlsten 1992; Dahlsten, unpubl. data). Our objectives were to evaluate the use of ’ Received24 August1993. Accepted 28 March 1994. 2 Present address:Department of Biology, Univer- Monterey pine by determining: (1) the foraging sity of Wisconsin, Eau Claire, WI 54702. behavior of adult Chestnut-backed Chickadees

WI 648 P. K. KLEINTJES AND D. L. DAHLSTEN in a Monterey pine plantation and (2) the abun- bole (inner, middle and outer one-third of crown); dance and composition of Monterey pine feeding foraging substrate (needle, small twigs, , insectsin the diet of Chestnut-backed Chickadee and cones); tree species(Monterey pine, nestlings. coast live oak, California bay laurel, elderberry, and Salk sp.); date and time of day; METHODS weather and (if color band was visible). In STUDY AREA addition, we recorded the following foraging ac- tivities: (1) perch-gleaning (picking food from the The study was conducted on the northeastern substratewhen perched);(2) hang gleaning (pick- slope of the Berkeley Hills in the East Bay Mu- ing food from the substratewhile hanging upside nicipal Utilities District, Contra Costa County, down); (3) hover gleaning (picking food from the California, during March-May, 1988-l 992. Fif- substrate while hovering); (4) pecking (driving ty-three nestboxes were established in the area bill againstsubstrate to remove food); (5) probing during 1978 (Gold and Dahlsten 1989). They (inserting bill into substrate to remove hidden were placed along a seriesof trails at intervals of food) and (6) other. We collected at least 40 ob- 25-50 m, 1.5 m above the ground. The nestboxes servations per breeding season,as recommended were constructed of and cement and by Brennan and Morrison (1990) for the Chest- contain a removable front (Schweglerand Sons, nut-backed Chickadee in the central Sierra Ne- Munich, ). vada. The study area encompassedmature stands of Frequencies of behaviors were compared be- planted Monterey Pine that were adjacent to tween years with contingency tests (Likelihood stands of mature coast live oak (Quercus agri- Ratio Chi-, P -C 0.05). Separatetests were folia) woodland. Understory vegetation consist- used for vertical location, horizontal location, ed of poison oak (Toxicodendron diversilobum), activity, substrateand tree species.Each test con- blackberry (Rubusursinus) and regeneratingcoast tained an “other” category which contained the live oak, California bay laurel (Umbellaria cal- sum of the variables whose individuals frequen- ifornica) and elderberry (Sambucus mexicana) cies were less than 5. Becausethere was no sig- (Kleintjes and Dahlsten 1992). Relative density nificant difference between years for any of the of tree specieswas determined by a plotlesspoint- variables, except for tree height, the 199 l-l 992 center-quartermethod (Cottam and Curtis 1956). foraging data were pooled acrossyears. A row of twenty-five nestboxes(25-50 m apart) within a 15 ha2 area served as sampling points. Relative density of Monterey pine was 76%, PHOTOGRAPHY AND FECAL SAC ANALYSIS Monterey pine snags(12%) California bay laurel Photography and fecal sac analysis were used to (7%) and coast live oak (5%). determine nestling diet composition. In an ear- lier study, we found photography to provide the FORAGING BEHAVIOR most detailed information on nestling diet Foraging was observed during 1 April-31 May, (Kleintjes and Dahlsten 1992). However, me- 1990, 3 April-31 May, 1991 and 10 March-15 chanical problems with cameras can cause gaps May, 1992. We standardized collection of ob- in data collection and limit sample size. There- servations to between 06:00-12:00 hrs, three to fore, we used fecal analysis to provide supple- five times per week while walking a 3.2 km trail mental information and to serve as a safety for through the plot. At the first sighting of a bird, camera failures. we waited to avoid recording conspicious be- Photographs were obtained with an 8 mm haviors. Immediately thereafter, the first forag- movie camera attached to the back of occupied ing observation was recorded. The observer then nest boxes (Dahlsten and Copper 1979). A cam- moved a minimum distance of 50 m between era unit was used in place of an original nestbox each individual bird to increase the indepen- from the time nestlings were eight days old until dency among observations made on the same fledging. day. We used foraging variables modified from A total of 561.5 hr of film from six Chestnut- Remsen and Robinson (1990) to describe each backed Chickadee nests was recorded in 1989- observation: bird height within tree (meters and 1992. In 1989, we obtained film from two nests, one-third levels of crown); foragingdistance from boxes number 34 and 36. We excluded box 36 CHICKADEE DIET AND FORAGING BEHAVIOR 649 from analysis because the nestlings were reared RESULTS by one adult and their number decreasedfrom FORAGING BEHAVIOR five to two nestlings during filming. In 1990 and 199 1 we obtained film from one nest each year Chestnut-backed Chickadees spent the greatest and in 1992, we obtained film from two nests. percentage of their foraging time on Monterey Parents of all nests were recorded bringing in pine (79.2% f 7.1 SD) (Fig. 1). Approximately prey on average 14 hr a day from 06:OOhr to 20: 16% of their time was spent on coast live oak, 00 hr. Diet information was based on nestlings although oak comprised only 5% of the relative that were typically 1O-20 days old and were raised tree density in the study area. in broods of 5-7 nestlings. The greatest proportion of substrate use was We reviewed film under 25 x magnification to spent on needles (63.6% f 4.2 SD) followed by identify prey items. Prey items were counted ac- use of leaves on evergreen and deciduous cording to the number and category brought in (18.7 f 9.9 SD) (Fig. 1). The birds used perch each trip every hour, each foraging day. Each nest and hang gleaning behaviors more than any other box was considered equivalent to one sample. activity and spent a greater frequency of time in Percent abundance of each prey category was the outer, upper crown (Fig. 1). The mean for- calculated for each nest from the total number aging height ofthe birds in Monterey pine, 199 l- of prey delivered to the nest during the entire 1992 was 13.7 m (kO.84 SD). The mean height filming period (10 days). For statistical analysis, ofpine used in 1991 was 25.3 m (rt5.8 SD) and we pooled the data from the five nests (approx- 22.86 m (k4.3 SD) in 1992. Tree height signif- imately 450 hr) to calculate the mean percentage icantly differed between years (t-test, P < 0.05). of each prey categorydelivered to the 1O-20 day old nestlings. Percentageswere modified with a NESTLING DIET COMPOSITION square root transformation becausethe range of percentagesdid not lie between 30-70% (Sokal From photographic records, 8,222 prey were and Rohlf 198 1). Means and variances for each identified from a total of 9,5 14 items brought to prey category were compared with a standard the nests. The Monterey pine sawfly (Acantho- one-way analysis ofvariance (ANOVA) for equal lyda burkei) (Hymenoptera: Pamphiliidae) com- sample sizes(Zar 1984). The Tukey’s HSD meth- prised the significantly largest mean percentage od was used for pairwise comparisons(Zar 1984). (43%) oftotalprey in the nestling diet (F= 17.38, Fresh fecal sacswere collectedfrom young dur- P < 0.05, one-way ANOVA) (Table 1). The tree ing box checks(twice per week) and banding (see camel cricket (Gammarotettix bilobatus) (Or- Kleintjes and Dahlsten 1992). Each sacwas placed thoptera: Rhaphidophoridae) comprised 17% of in a small plastic vial and frozen within 3 hr of total prey. All other identifiable prey items com- collection. Samples were collected at variable prised 25% of the diet, whereas unknown prey times and dates throughout each nesting season items comprised 14% (Table 1). When identified from a minimum of three nests and a minimum prey were grouped into one of eight orders, the of 24 hr between collections from the same nest. Hymenoptera comprised the greatestpercentage Fecal sacswere collected from as many nests as of prey items (5 1.2%) followed by Orthoptera possible when nestlings were 9-20 days old. An (19.8%) Lepidoptera (8.0%) Homoptera (6.5%), explanation of sample size estimatesand analysis Hemiptera (5.8%), Arachnids (5 .O%), Diptera of sac contents is described in Kleintjes and (3.5%), and Coleoptera (0.1%) (Fig. 2). Dahlsten (1992). Forty-five nestling fecal sacs Records from fecal analysis indicated that 86 were collected during three breeding seasons;12 prey items were present and identifiable to insect fecal sacs from four nests during 13-29 April Order or as an Arachnid. The greatestpercentage 1988, eight sacs from four nests during 13-23 of identified prey belonged to the order Hyme- May 1989 and 25 sacsfrom five nests during 8- noptera (30.8%), followed by Orthoptera (22. lo/o), 28 May and 4 June 1991. Data from all fecal Homoptera (16.10/o), Lepidoptera (14.0%), sacswere pooled to calculate the mean percent- Arachnids (9.3%), Hemiptera (3.3%) Coleoptera age of each prey category delivered to the nests. (2.6%) and Diptera (1.3%) (Fig. 2). The remain- Percentagesrepresented the proportion of prey ing contents were uncounted fragments of exo- that remained intact through the digestive pro- skeleton, leg segments,eggs, setae, spiracles, veg- cess. etable material and pebbles. Photography and 650 P. K. KLEINTJES AND D. L. DAHLSTEN

Location on Bole

Upper Middle Lower 60

40

20

0 Inner Middle Outer $ 100, ’ ’ ’ 80 Species 60 2 40 F 20 z 0 E P. r. Q. a. u. c. S. m. Salix spp. Other ; 80, I Substrate

Needles Leaves Bark Cones Twigs 60 Activity 40 __I”_

20 ) ..-T- 0 / I i-5 1 r Perch glean Hang glean Peck Probe Hover

FIGURE 1. Foraging behavior of Chestnut-backedChickadees (Purus rujiiscens)in a Monterey pine (Pinus rudiuta) plantation, as a percent of total observationscollected during the breeding season,Contra Costa Co., California, 1991 (n = 66) and 1992 (n = 51). Tree speciesare P. rudiuta (P.r.), Quercusugrifoliu (Q.u.), Sumbucus mexicanu (Xm.), Salix spp. and Umbelluluria californicu (U.C.). CHICKADEE DIET AND FORAGING BEHAVIOR 651

n=S nests n=13 nests

f$XJOrthoptera

q Hemiptera

q H omoptera

q Coleoptera

q Lepidoptera q Diptera q Hymenoptera

Spiders

Photography Fecal sacs Technique of diet analysis

FIGURE 2. Mean proportionof insectOrders and spidersin the diet of Chestnut-backedChickadee (Purer uufescens)nestlings in a Montereypine (Pinusrudiuta) plantation, Contra Costa County, California, 1989-1992.

fecal sacanalysis were not statistically compared knowledge that our provision of artificial nesting due to different levels of precision. sites in the pine stand increased the likelihood of Chestnut-backed Chickadeesusing and breed- DISCUSSION ing in the area. However, it is apparent from our Our results indicate that Monterey pine is an resultsthat availability of prey on Monterey pine adequate foraging resource for Chestnut-backed during the breeding season caused the birds to Chickadeesin the San Francisco Bay region dur- spend a greater amount of time foraging on the ing the breeding season.In a stand composed of and delivering pine-feeding insects to their approximately 75% Monterey pine trees, Chest- young. Sturman (1968) and Brennan (1989) also nut-backed Chickadees spent nearly 80% of all found that Chestnut-backed Chickadees pre- foraging observations on the pines. In addition, ferred to forage on coniferous trees during the nearly 80% of the nestling diet was composed of breeding season although species of evergreen insects found on Monterey pine foliage. and deciduous were available. Previous studies of the foraging behavior of Our results support earlier observations of the Chestnut-backed Chickadees in coast live oak Chestnut-backed Chickadees preference for for- and mixed-evergreen woodlands of the San Fran- aging on outer foliage (needles, leaves or ) cisco East Bay indicate differing preferencesfor during the breeding season (Dixon 1954, Hertz native plant speciesduring the breeding season. et al. 1976, Sturman 1968, Brennan 1989). We Dixon (1954) found Chestnut-backed Chicka- also found that the birds spent the greatestpro- dees spent the greatest percentage of their for- portion of their foraging activity hang gleaning aging time on coastlive oak whereas,Root (1964) and perch gleaning prey from Monterey pine nee- found Chestnut-backed Chickadees spent nearly dles. This was not unexpected as the majority of equal amounts of foraging time on California bay prey in the nestling diet were needle feeding in- laurel, coastlive oak and madrone (Arbutus men- sects,particularly the larvae of the Monterey pine ziesii). Even though some of these specieswere sawfly. In the Central Sierra, Brennan (1989) also available on and near our study area we found found the gleaning behavior of the Chestnut- Chestnut-backed Chickadees spent the majority backed Chickadee to be most common during of their foraging time on Monterey pine. We ac- the breeding seasonwhich he associatedwith an 652 P. K. KLEINTJES ANDD. L. DAHLSTEN

TABLE 1. Mean percentageof totalprey delivered to Acantholyda larvae have been found in the diet Chestnut-backedChickadee (Purus rufecens) nestlings of nestling Mountain Chickadees in northern in a Montereypine (Pinus rudiata) plantation,Contra CostaCounty, California. Diet wasrecorded with super California (Grundel and Dahlsten 199 1) as well 8mm movie cameras(n = 5 nests). as in the diets of various European parids (Tin- bergen 1960). Prey R % (SD) Tree camel crickets were the second most abundant prey in the nestling diet of Chestnut- Orthoptera Rhaphidophoridae backed Chickadees. These insects are generalist (Gammarotettitix bilobatus) 17.0 (11.7) herbivores of trees and in the coast live oak woodlands of California (Essig 1926). The Hemiptera crickets have not been previously recorded from Reduviidae(Zelus cervicalis) 4.9 (3.6) Monterey pine, although during random beating Homoptera and visual examination of lower crown Monterey Cercopidae(Aphrophoru sp.) 3.9 (4.2) Cicadidae 0.91 (0.68) pine foliage, we commonly observed crickets Aphididae 0.47 (0.85) throughout the nestling period (pers. observ.). Coleoptera 0.12 (0.2) They were also observedon coastlive oak foliage which may have partially accountedfor the birds’ Lepidoptera greaterproportion of foragingtime (15%) on coast Larvae 4.4 (4.3) Adults 2.8 (2.8) live oak while the density of was low (5%). The birds also brought in numbers of reduviids Diptera and cercopid nymphs. Both have been collected Tipulidae 0.83 (1.4) on Monterey pine (Ohmart 198 1). Syrphidae 0.69 (0.88) Tabanidae 1.4 (1.6) Becausewe found Monterey pine served as a Hymenoptera major food resource for breeding Chestnut- Pamphiliidae(Acuntholydu backed Chickadees, we believe the extensive burkei) 42.6 (18.3) planting of Monterey pine and other conifers has Arachnidae 4.3 (3.3) probably contributed to the successfulestablish- Pupae 1.1 (1.0) ment and continued increase of Chestnut-back Unknown 14.5 (4.7) Chickadeesin the San Francisco Bay region (Dix- on 1954, Brennan and Morrison 199 1). Although chickadeesforage in coastal live oak woodlands, it appears that when Monterey pine is planted increased abundance of foliage feeding arthro- in close proximity to or in replacement of oaks, pods. the pines provide additional foraginghabitat. This Although photography and fecal analysis of is particularly true with regardsto the availability nestling diet could not be statistically compared, of webspinning and tree camel crickets proportions of prey in the diet were similar. Both during the breeding season. Not only did these methods indicated that Hymenoptera and Or- insectsinfluence chickadeeforaging behavior but thoptera composed the greatest percentage of their abundance in the diet potentially contrib- identifiable prey items. uted to the birds’ reproductive success.To fur- The most abundant prey in the nestling diet ther understand the reasons behind the suc- were Monterey pine sawfly larvae. These larvae cessful range expansion of Chestnut-backed are solitary webspinners and feed among the out- Chickadees,it would be of great interest to study er branchesof Monterey pine within a silken web their foraging behavior and diet in a number of coveredwith frassand needle pieces(Burke 1929). habitats, particularly in Monterey pine stands Occasionally,adult chickadeesalso brought adult and adjacent coastlive oak and mixed-evergreen sawflies to the nestlings. The only previous ob- woodlands during the breeding and non-breed- servation of avian predation upon Monterey pine ing seasons. sawflieswas recorded in Monterey County, Cal- ifornia, by Burke (1929). ACKNOWLEDGMENTS It is notable that webspinning sawfliesare also We thank Michael L. Morrisonfor helpfulcomments an important food sourcefor other -glean- on the manuscript;David L. Rowneyand William A. ing insectivorous birds. Different species of Copperfor technicalsupport and assistancein the field CHICKADEE DIET AND FORAGING BEHAVIOR 653 and, Woodward Middlekauf and David Vince for in- Ecological complementarity of three sympatric sect verification. This researchwas supported,in part, Parids in a California oak woodland. Condor 78: by a Sigma Xi Grant to Paula K. Kleintjes. 307-316. KLEINTJES,P. K., ANDD. L. DAHLSTEN.1992. A com- parison of three techniquesfor analyzing the diet of Plain Titmouse (Paw inornatus)and Chestnut- LITERATURE CITED backedChickadees (P. rufescens)nestlings. J. Field BARBOUR,M. G., AND J. MAJOR. 1988. Terrestrial Omithol. 63:276-285. vegetation of California. California Native Plant LUNDQUIST,R. W., ANDD. A. M-AL. 1990. Sea- Society Special Publ. No. 9. sonaldifferences in foraginghabitat of cavity-nest- BRENNAN,L. A. 1989. Comparative use of re- ing birds in the southernWashington Cascades,p. sourcesby Chestnut-backedand Mountain Chick- 218-225. In M. L. Morrison, C. J. 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