Foraging Ecology of the California Gnatcatcher Deduced from Fecal Samples

Oecologia (1999) 120:304±310 Ó Springer-Verlag 1999 Jutta C. Burger á Michael A. Patten John T. Rotenberry á Richard A. Redak Foraging ecology of the California gnatcatcher deduced from fecal samples Received: 30 December 1998 / Accepted: 28 April 1999 Abstract The California gnatcatcher is a threatened Key words California gnatcatcher á Coastal sage scrub á species essentially restricted to coastal sage scrub habitat Fecal analysis á Foraging ecology á Polioptila californica in southern California. Its distribution and population dynamics have been studied intensely, but little is known about its diet. We identi®ed arthropod fragments in 33 Introduction fecal samples of the California gnatcatcher to gain in- sight into its foraging ecology and diet. Fecal samples What organisms eat is of fundamental importance to were collected from adult males, adult females, ¯edg- their ecology and behavior. Food in¯uences everything lings, and nestlings. Leaf- and planthoppers (Homopt- from predator and prey population dynamics and for- era) and spiders (Araneae) predominated numerically in aging patterns to the niche structure of populations, samples. Spider prey was most diverse, with eight fami- habitat associations, and competitive relationships. lies represented. True bugs (Hemiptera) and wasps, bees, Unfortunately, despite the obvious bene®ts that an un- and ants (Hymenoptera) were only minor components of derstanding of foraging ecology would yield in improv- the gnatcatcher diet. Gnatcatcher adults selected prey to ing management plans for threatened and endangered feed their young that was larger than expected given the birds, we know little about what most birds eat distribution of arthropod size available in their envi- (Rosenberg and Cooper 1990). Thus, our objective was ronment, and chicks were provisioned with larger prey to assess foraging ecology and dietary composition in items and signi®cantly more grasshoppers and crickets the U.S. federally threatened California gnatcatcher (Orthoptera) and spiders than adults consumed them- (Sylviidae: Polioptila c. californica) through identi®ca- selves. Both adults and young consumed more sessile tion of prey remains in fecal samples. than active prey. Further studies are needed to determine The California gnatcatcher is endemic to coastal whether arthropods sampled in coastal sage scrub that sage scrub shrubland vegetation in cismontane south- are common in fecal samples are good indicators of ern California. Its life history and distribution are well- California gnatcatcher habitat. documented (Woods 1949; Atwood 1988; Rotenberry and Scott 1998), but its dietary preferences are poorly known. Associations have been predicted and observed between arthropod prey abundance and foraging activity (MacArthur and Pianka 1966; Hespenheide 1971), habitat selection (Beaver and Baldwin 1975), J.C. Burger (&) á R.A. Redak population size (Crawford and Jennings 1989), migrant Department of Entomology, numbers (Griscom 1950), and even vagrant numbers University of California, (Patten and Burger 1998) of insectivorous birds. Riverside, CA 92521, USA However, whether or not food availability is limiting e-mail: [email protected], Fax: +1-909-7874733 the California gnatcatcher and aecting its local dis- M.A. Patten á J.T. Rotenberry tribution and/or reproduction is unknown. Arthropod Department of Biology, abundance on coastal sage scrub shrub species within University of California, Riverside, CA 92521, USA gnatcatcher territories may not be correlated with nest success (Roach 1989), but arthropod abundance and J.T. Rotenberry á R.A. Redak vegetation in concert provide a robust predictor for Center for Conservation Biology, University of California, California gnatcatcher presence at a given site (J.T. Riverside, CA 92521, USA Rotenberry and R.A. Redak, unpublished data). By 305 de®ning which arthropods in occupied habitat are ac- ignored in analysis. These particles comprised less than half of the tually known prey, we may be able to more easily de- volume of each fecal sample. Minimum numbers of arthropods per fecal sample were estimated by summing pairs of matching body tect associations between arthropods and birds, and parts and counting unique body parts for a given taxon. more accurately estimate the importance of arthropod We estimated prey lengths primarily by averaging the lengths of prey in predicting site occupation by California gnat- ®eld-collected reference specimens in species/taxonomic groups for catchers. which fragments were found. Because speci®c reference specimens We analyzed fecal samples of the California gnat- of adult Hymenoptera and Lepidoptera (moths) and larvae could not be identi®ed in feces, their lengths were assigned as the average catcher to (1) estimate the relative contribution of length of all ®eld-collected specimens of their group. Grasshoppers arthropod orders to gnatcatcher diet, (2) deduce size (Acrididae), were represented in feces by a range of sizes (from ranges of prey items, (3) compare prey size distributions nymphs to adults) that could be directly estimated using allometric and prey activity to those of arthropods sampled in equations. We measured the lengths of two heavily sclerotized body parts commonly found in samples (crescent of hind femur and coastal sage scrub to determine the degree of selectivity/ mandible) for 28 reference Orthoptera of known body length. opportunism by birds, and (4) compare diets of imma- Fragment lengths (in mm) were related to total body lengths by ture and mature birds. Once we have established which exponential functions [crescent (c): total length = 3.54e0.68c, arthropods are consumed by gnatcatchers, we will be R2 = 0.95; mandible (m): total length = 3.33e1.08m, R2 = 0.95]. better able to identify high versus low food resource sites The biomass of all orders was calculated using previously published from arthropod surveys. functions relating body length to dry mass (Rogers et al. 1977). The relative contribution of various arthropod orders to gnatcatcher diet was expressed both in terms of percent by number of individuals and by estimated biomass. Materials and methods Frequency distributions of prey lengths were compared with the distribution of arthropod lengths from vacuum samples at Mira- Fecal samples from adult and nestling California gnatcatchers were mar using a Kolmogorov-Smirnov test for goodness of ®t (Sokal collected at Marine Corps Air Station Miramar (located about and Rohlf 1981, p. 716). Only the most common arthropod orders 10 km north of San Diego, Calif., and referred to here as Miramar) found in feces (Araneae, Coleoptera, Homoptera, Orthoptera) were during the breeding season of 1995 and 1996 by federally permitted included in comparisons of lengths. Relative numbers of active researchers from San Diego State University. Samples were the (Diptera, Hymenoptera, Lepidoptera) and sessile (all others) result of ``in-hand'' defecation during banding operations, either arthropods found in gnatcatcher feces were compared with those from mist-netted adults and juveniles or from chicks in nests. Fecal collected in the ®eld using a chi-square test for goodness of ®t. samples were collected from 24 June to 25 July in 1995 and from 7 We compared diets among age classes using multivariate anal- May to 19 July in 1996. ysis of variance (MANOVA). Individual means were compared In all, we analyzed 33 samples representing birds from 26 nests. with a Tukey honest signi®cant distance (HSD) test (SAS 1996). Samples were stored in 80% ethanol at the time of collection and later provided to us for analysis. Each sample was centrifuged and treated with about 2 ml of a concentrated solution of NaOH in a hot water bath for 1±2 h to soften the feces. Samples were sub- Results sequently centrifuged, rinsed with distilled water, centrifuged again, and ®nally transferred and stored in 95% ethanol. Whenever Arthropods of ten orders were found in fecal samples. possible, we removed the mucous lining of fecal sacs before Diagnostic fragments of each could be used for identi®- softening in order to keep extraneous materials from clouding samples. Arthropod fragments were easily isolated after this cation (Table 1, Fig. 1). Homoptera were not only the treatment. most consistently found component of feces (Fig. 2), but We sampled arthropods using a modi®ed blower-vacuum also the most abundant (Table 2). However, most (Bungton and Redak 1998). Samples were collected from 30 sites Homoptera were small and therefore did not make up the in coastal sage scrub vegetation across Miramar from May to early June 1995 and in May 1996, partly overlapping in time with the bulk of biomass consumed (Table 2). Spiders (Araneae) collection of fecal samples. We considered vacuum sampling to be a were found in most samples (Fig. 2). The minimum reliable means of measuring food availability (see Cooper and number of spiders in feces could easily be quanti®ed by Whitmore 1990; Poulin and Lefebvre 1997) because it eectively counting pairs of matching chelicerae, fangs, or individ- samples the primarily sessile arthropods that would be encountered ual epigyna (reproductive structure of female). They, by birds while gleaning from shrubs. Reference collections of these arthropods were used to help identify prey items from fragments in along with Coleoptera, made up a large component of samples. In some cases, intact arthropod specimens were macerated diet both in terms of number of individuals and in terms in order to match arthropod

Load more