The Condor 101:67-75 0 The Cooper Ornithological Society 1999 TEMPORAL VARIATION IN RESOURCE USE BY BLACK-THROATED GRAY WARBLERS JOHN J. KEANE* AND MICHAEL L. MORRISON~ Departmentof EnvironmentalScience, Policy, and Management, Universityof California, Berkeley, CA 94720 Abstract. We studiedforaging behavior and habitatuse of male and female Black-throat- ed Gray Warblers (Dendroica nigrescens)to quantify the effects of temporal variation on interpretationsof avian resourceuse. Overall, both sexes primarily foraged by gleaning in single-leaf pinyon pine (Pinus monopbyla);however, within-seasonand between-year vari- ation in behavior and habitat use were found for both sexes. Warblers increased use of shrubs from May to mid-June, and decreaseduse of shrubs and increased use of pinyon pine from mid-June through August, during each year. Use of sagebrush(Artemisia triden- tata) and Utah juniper (Juniper osteospemta)varied between years. Sex differences were found in proportional use of foraging maneuvers.Within-season shifts in plant speciesand habitat use correspondedto changesin arthropodnumbers on the plant speciesused by the warblers, but between-year shifts in behavior did not correspondas closely with changesin arthropodnumbers. Temporal variation in microhabitatuse resulted from shifts within sea- sons in the plant speciesused for foraging, which was associatedwith temporal changesin food abundance.Our results also demonstratethe importance of consideringthe effects of temporal scale in studiesof bird-resourceinteractions. Key words: Black-throated Gray Warbler, Dendroica nigrescens,foraging behavior, habitat use, Neotropical migrant, resourceuse, temporal variation. INTRODUCTION strated variation on daily (Hutto 198 lb), intra- Data from studies of avian resource use have seasonal (Sakai and Noon 1990), interseasonal been used widely to describe the structure and (Hutto 1981a), and annual (Szaro et al. 1990) composition of bird assemblages (MacArthur time scales as well as on local (Collins 1981) 1958, Holmes et al. 1979, Rotenberry 1985), and regional (Wiens and Rotenberry 1981) spa- and have been used in the development of spe- tial scales. Furthermore, variation can result cies-habitat models designed for management from intersexual differences and factors such as applications (Vemer et al. 1986). Wiens (1986), age and reproductive status (Holmes 1986, however, showed that interpretations of assem- Grubb and Woodrey 1990). Few researchers, blage structure can be influenced by inherent however, have attempted to address the poten- temporal and spatial variation. Rice et al. (1986) tially interactive effects of between-sex, within- and Rotenberry (1986) demonstratedthat the ac- season, and between-year variation in resource curacy and predictability of species-habitatmod- use patterns on bird-centered habitat measure- els can vary depending upon the temporal and ments (Kelly and Wood 1996). spatial scalesover which they are developed and The distribution and abundance of food is a primary factor affecting the resource use pat- applied. Thus, temporal and spatial variation at terns of birds (Lack 1966, Martin 1987). For in- various scales need to be described and consid- sectivorous birds, changes in arthropod distri- ered because they can influence the results of bution and abundance may correspondto chang- ecological investigations and any subsequent es in bird resource use patterns at different tem- management applications (O’Neill et al. 1986, poral and spatial scales.For example, Brush and Block and Brennan 1993). Styles (1986) found that changes in bird abun- Studies of avian resource use have demon- dance between pine- and oak-dominated vege- tation types correspondedto changes in arthro- ’ ’ Received 1 December 1997. Accepted23 July pod biomass. Similarly, seasonalvariation in the 1998. sizes and speciesof trees used by bark foraging 2 Presentaddress: Stanislaus National Forest, 19777 birds in western North American forests is as- GreenleyRd., Sonora,CA 95370. z Presentaddress: Department of BiologicalScienc- sociated with variation in prey distribution and es, California State University, Sacramento, CA abundance (Morrison et al. 1985, Lundquist and 95819. Manuwal 1990). [671 68 JOHN J. KEANE AND MICHAEL L. MORRISON Our objective was to study the resource use moved along transect lines to avoid repeatedob- patterns of Black-throated Gray Warblers (Den- servations of the same bird. When a warbler was droica nigrescens) during the breeding season. detected,the observer watched the bird for 5 set We investigated within-season, between-year, without recording data. This period was used to and between-sex variation, and their interac- minimize bias to the most conspicuousactivities. tions, in foraging behavior and the habitat char- The observer then recorded all activities and lo- acteristics of foraging sites. Additionally, we cations where the activities occurred over the measured arthropod abundances on the plant next 15 sec. Only observations where the bird species used by the warblers to determine was in full view for the 15 set were included whether variation in warbler foraging behavior for analysis. Variables recorded were: sex of the and habitat relationships correspondedto chang- bird, time of day, plant species where the activ- es in arthropod abundances. ities occurred, height of the bird, frequency of use of foraging maneuvers (glean, hover-glean METHODS [= sally-hover], flycatch [= sally-strike to aerial STUDY AREA prey], flush-chase [= flutter-chase], fly-glean [= We worked from late April to August of 1989 sally-strike at stationary substrate],probe, peck, and 1990 in the White and Inyo Mountains, Inyo as described by Remsen and Robinson 1990), County, California (37”50’N, 118”lO’W). The and substrate where each activity occurred (fo- White and Inyo Mountains range in elevation liage, twig [< 1 cm diam.], small branch [l-lo from 1,220 to 4,345 m and each forms half of a cm diam.], medium branch [ 1 l-30 cm diam.], fault block extending approximately 170 km large branch [> 30 cm diam.], trunk, air, ground, north to south (Nelson et al. 1991). Climate is and flower). All measurementswere visually es- typically one of cold, snowy winters and warm, timated. All foraging locations were flagged for dry summers with summer precipitation occur- later habitat analysis. ring from thunderstorms (Powell and Klieforth Data analysis. We split the data for each year 1991). into two periods for analysis. Early summer was All fieldwork was conducted in pinyon-juni- defined as occurring from late April to 10 June, per woodland between 1,970-2,875 m elevation. and late summer as 11 June to late August. We Woodland overstory was composedof singleleaf justified this cut-point date biologically because pinyon pine (Pinus monophyla) and Utah juniper it correspondedwith observed plant phenologi- (Juniperus osteosperma). Antelope bitterbrush cal patterns. Bitterbrush flowered primarily from (Purshia tridentata) and big sagebrush(Artemi- late April through mid-June. These periods also sia tridentata) were the dominant componentsof broadly overlapped with changes in warbler the understory. Other shrubs present with local- breeding phenology (Keane, unpubl. data). Nest ized distributions included Parry rabbitbrush building was observed in late May and nests (Chrysothamnusparryi) and cliffrose (Cowania with eggs were located from late May through mexicana). The herbaceouslayer was poorly de- mid-June. Nests with young were found from veloped and consisted of grassesand forbs in- mid-June through early July. Thus early summer cluding eriogonum (Eriogonum spp.) and pen- broadly overlapped with the territory establish- stemon (Penstemon spp.). Spira (1991) provides ment, nest building, and incubation phasesof the further descriptions of plant assemblagesin the nesting cycle, and late summer broadly over- White and Inyo Mountains. lapped with the nestling, fledgling, and post- breeding phases, although overlap undoubtedly ACTIVITY BUDGETS occurred as a result of renesting attempts. Data collection. We sampled activity behaviors The raw data for plant species use for each of Black-throated Gray Warblers within a lOO- foraging observation was transformed from a m band on either side of 10 transect lines. Tran- discrete to a continuous variable by calculating sect lines were 4.2 km long, about 0.5 km apart, the corresponding number of observation sec- and located to span the elevational distribution onds for each plant species per 15-set sample of the pinyon-juniper woodland (see Morrison et (Brennan and Morrison 1990). This resulted in al. 1993 for further details). Four observers col- a value of percent time use for each variable. lected data during the study. All observers were We used a multivariate analyses of variance trained to standardize data collection. Observers (MANOVA; Green 1978) to investigate the ef- TEMPORAL VARIATION AND RESOURCE USE 69 fects of year, period, and sex on plant species the center of a 15-m radius plot. Within each use by Black-throated Gray Warblers. Multicol- plot we counted the number of pinyon and ju- linearity between variables was reduced by elim- niper trees within three height classes(1.5-3 m, inating one of a pair of variables with a product > 3-6 m, > 6 m). We established a 30-m line moment correlation > 0.70 (Sokal and Rohlf transect bisecting the plot along a random com- 1981). The variable retained was the one with pass bearing. We measured the presence of pin- the higher F-value in