Died of Nestling Winter Wrens in Relationship to Food Availability

The Condor9214 13-420 0 The CooperOrnithological Society 1990

DIET OF NESTLING WINTER WRENS IN RELATIONSHIP TO FOOD AVAILABILITY l

BEATRICE VAN HORNE Department of Biology, ColoradoState University,Fort Collins, CO 80523

AHMAD BADER Department of Entomology, ColoradoState University,Fort Collins, CO 80523

Abstract. We identified major arthropodtaxa in the fecesof nestlingWinter Wrens (Troglodytestroglodytes) and quantifiedtheir number,size, and biomass.These data were comparedwith samplesof arthropodsobtained from habitatsand microhabitatsthat were typical of breedingterritories. Larval Lepidopteraon shrubsin the clearcutwere more numerousand hada higherbiomass than thoseon shrubsin forestedareas. Total arthropod biomass,the biomassof Coleoptera,and numbersof Araneaeon Vaccinium shrubswere higherin the forest.On the ground,numbers and biomassof mosttaxa werehigher in the clearcutthan in the forest.Coleoptera and Araneaewere fed to nestlingsin highproportions relativeto their proportionatenumbers and biomassin the environmentalsamples. Key words: Fecal analysis;Troglodytes troglodytes; arthropod;foodavailability; nestlings; diet.

INTRODUCTION ical to understanding among-male variations in Male Winter Wrens (Troglodytes troglodytes) in habitat use, mating success,and nesting success. the spruce-hemlock (Tsuga heterophylla-Picea In this study we address the following ques- sitkensis) forests of coastal Oregon defend dis- tions: (1) Are there differences in the size and tinct breeding territories in both clearcut-logged composition of the prey base in different habitats areas containing large amounts of logging slash and microhabitats on coastal Oregon Winter (not those that have been burned), and in mature Wren territories? (2) Are certain arthropod types forests.Territories are used for both nesting and preferred as food for nestlings? If so, do these feeding. Some individuals are polygynous and preferenceschange with the age of nestlings?(3) males usually help to feed nestlings on their ter- Is there evidence that changesin prey availability ritories. Frequencies of visits to the nest are sim- influence diet composition? ilar for males and females. Adults forage on vir- Workers have taken two approaches in the tually any substrate within about 3 m of the study of foraging in free-living insectivorous pas- ground, including shrubs, logs, and the ground serines.They either observed foraging behavior, itself. They do not hawk for hying insects.Young recording microhabitats used for foraging, and/ hatch simultaneously and remain in the nest for or food being brought to nestlings, or they looked an average of 15.5 days (n = 12) on our study more directly at items consumed by examining area. In Great Britain polygyny is common gut contents, esophageal samples, or fecal ma- (Armstrong 1955; Garson 1978, 1980), and males terial and then used samples of the prey base to do not help to feed the young in the nest. Perhaps interpret their findings. The former approach because food density is lower than in the areas provides larger sample sizes but may be biased studied in Great Britain, assistanceof both par- against certain prey and microhabitat types (e.g., ents in feeding young may be required for suc- smaller prey, microhabitats in which birds are cessful nesting in coastal Oregon. If food avail- difficult to see). We took the latter approach. ability on the male territory influences nesting METHODS success,then knowledge of the relationship between nestling diets and food availability is crit- Van Home studied Winter Wrens during the breeding seasons(March-June) of 198 3-l 98 5 in the Cascade Head Experimental Forest at Otis, ’ Received18 August 1989. Final acceptance26 Jan- Oregon. Part of the study area had been clearcut- uary 1990. logged2 years previously and the remainder was

14131 414 BEATRICE VAN HORNE AND AHMAD BADER in mature, multistoried spruce-hemlock forest. significant regressionmodels (P < 0.00 1) of pre- Males were color-banded and courtship, nesting, dicted body length from the size of various body and territorial activity were monitored daily. parts for the common orders of arthropods. Territories were mapped as minimum polygons Both male and female Winter Wren parents enclosing singing perchesusing a 50-m grid over removed mucoid fecal sacs,each with one fecal the entire area in combination with topographic pellet, from the anuses of nestlings after they features. Males sang frequently from prominent were 6-7 days old. These sacswere then depos- perches on their territories during the several- ited on bare branches IO-20 m from the nest. week period prior to hatching, so there was little We did not observe any sacsbeing dropped. Fe- ambiguity regarding the location of territory cal sacswere collected from such branches daily boundaries. and storedin 70% alcohol. Not all fecal sacswere Two methods were used to sample arthropods found, nor did we attempt to analyze all possible in the environment. Pitfall traps, consisting of fecal pellets. Nestling age, date, distance of the two 8.5cm diameter plastic cups filled with 50% nest from the nearest stream, and habitat of the ethanol, were sunk to their rims at the ends of a parental territory (logged, forest, or edge) were l-m piece of lo-cm high metal flashing set into recorded. In the laboratory, each fecal pellet was the ground. The traps were coveredby small roofs teased apart and pieces of arthropods identified made of flashing. Ten pitfall traps spacedat 1O-m (usually to order and specifiedas adult or larvae; intervals were placed in transectsapproximately Calver and Wooller 1982, Ralph et al. 1985, 100 m apart perpendicular to a stream that ran Moreby 1988) and their maximum length mea- through the study area, as we suspectedthat the sured with an ocular micrometer. Within a fecal prey base might vary with distance from the pellet the minimum number of arthropods was stream. Three transects were placed in logged estimated by assuming that potentially paired and three in forested habitat. Traps were set for body parts differing by more than 1 mm in length two consecutive days at approximately 2-week were from different individuals. Measurements intervals for 6 weeks during the 1984 and 1985 of fecal arthropod parts along with the body size breeding seasons.Samples were stored in 70% regressionswere usedto predict total body length. alcohol. Where two parts could be from the same arthro- The second method of arthropod sampling pod (i.e., an Araneae fang and chelicera) we used consistedof beating shrubsover a 1-m* stretched the regressionwith the narrower 95% confidence canvas tarp. Arthropods that fell onto the tarp interval to predict body size. In addition to the were placed in 70% alcohol. Arthropods were fecal samples, two samples representing the gut collected from 10 shrubs for each sample, and contentsof five nestlingseach were collectedfrom three samples were collected from each of the nests that failed in the first l-2 days of nesting. two most common shrub species, Rubus spec- These samples were treated and analyzed in a tabilis and Vaccinium alaskensis,as we wished manner consistent with the analysis of the fecal to know whether one of the two dominant shrub samples, and produced two samples from nest- specieswas associatedwith a larger prey base, so lings too young to produce fecal sacsdeposited that the type of shrub coverage on a male terri- by parents. It is possible that there were soft- tory could provide insights into the availability bodied arthropodsthat were not detectedby these of nestling food on that territory. Samples were methods,although all the arthropod typeswe saw also collected at each of three distances(near, O- in parental bills during nest observations were 5 m; medium, 6-30 m; and far, >31 m) from also found in the fecal samples. the nearest stream in both forested and clearcut Analyses were conducted using both numbers habitats. Three collections were made about 2 of individuals and biomass of each taxon (bio- weeks apart in May and June 1984 and two col- mass predicted from size information using gen- lections were made in May 1985. eral regressionsdeveloped by Rodgerset al. 1976) Individuals in each type of sample were iden- in environmental and fecal samples. The mini- tified to order (to family for coleopterans and mum number of arthropods in each taxon and some other taxa) and their total body length was their predicted sizes were combined for fecesor measured. We used a reference collection, de- guts collected from the same nest and date and veloped from both the ground and shrub sam- expressedas numeric or biomass proportions of ples, to identify fecal fragments. We developed the total sample for the analyses of effects.Thus NESTLING WINTER WREN DIET 415

TABLE 1. Classesof effectstested and significanteffects on numbers and biomass of arthropods in shrub and ground samples. Where there were significant interactions between habitat and other effects, tests were run within habitat and other effect classes.Classes within which the effectswere significantare listed in parentheses. Biomass of “other” categorynot listed (see Methods). * = P -c 0.05, ** = P < 0.01, *** = P < 0.001.

Stream’ Date Habitat Early May (EM) Clearcut(C) Late May (LM) Forest(F) Early June(W)

ANOVA results Shrub samples Number Araneae *F > C(V) *LM > EM, EJ *V > R(C) Coleoptera **EM, EJ > LM Lepidoptera larvae **C > F *N > M,Z Diptera **EJ > EM, LM Otherb ***V > R Total number Biomass Araneae Coleoptera **F > C Lepidoptera larvae **c > F *N > M,Z Diptera Total biomass *F > C Pitfall samples Number Araneae ***C > F Coleoptera ***C > F ***EM, LM > EJ (C) Lepidoptera larvae Diptera ***c > F Other **C > F Total number ***C > F *N>Z(F) ***Z z=-N (C) Biomass Araneae ***C > F Coleoptera ***C > F (EM, LM) **EM > LM, EJ (C) Lepidoptera larvae Diptera Total biomass ***C > F (EM, LM) **C > F(EJ)

* Streamclasses used for shrubsamples only. Streamdistance was a continuousvariable in the groundsample models. For groundmodels, N > Z meansdecrease with distancefrom stream,,Z > N meansincrease with distancefrom stream. bIncludes Chilopada, Ephemeroptera, Hemlptera, Homoptera, Hymenoptera, Orthoptera, Plecoptera, Coleoptera larvae, Diptera larvae, Mecoptera, Maltophagia,Micrwxyphera, Psocoptem,Neuroptera. the original 102 feces/gut samples were consol- Individuals removed were diplopods, carabids idated into 39 samples. (Coleoptera), collembolans, and crustaceans.It Information on sizes and taxa of arthropods is possible that Collembola were consumed, but consumed by nestlings was used to remove ar- becausetheir bodies are entirely soft they did not thropods apparently not used by the birds from leave recognizablefragments in the feces.Guinan the environmental samplesprior to analysis. The and Sealy (1987) did not find Collembola in maximum prey size predicted by the regressions stomachsof House Wrens (T. aedon) despitetheir in the diets was 28 mm for Lepidoptera larvae high abundance in the environment. and 20 mm for other prey; larger individuals and Taxa in the environmental samples were thosefrom taxa not found in the diets were there- lumped into the 22 more general categoriesused fore removed from the environmental data sets. in categorizing the fecal samples (Table 1 in- 416 BEATRICE VAN HORNE ANII AHMAD BADER

SHRUB before or after any of the comparison fecal sam- !I 20 64 SAMPLES ples. Diet samplesfor 1983 were omitted as there 3 1II 3297 ARTHROPODS were no environmental samples taken that year. $0. 1111 llmm.______m-mm Only diet samples from nests in territories com- GROUND pletely within the clearcut area or completely 366 SAMPLES within the forestedarea were usedfor the analysis of preferences; edge nests were omitted. These

i= restrictions greatly reduced sample size but pro- 5 DIET duced a relatively stable result. We attempted 102 SAMPLES general summary analysesaveraging all environ- 1436 ARTHROPODS mental and diet samples using both rank and proportionate information, but these gave pref- ARTHROPOD BODY LENGTH (mm) erencesvery different than those we will present, FIGURE 1. Size distributionsof arthropodsfrom probably because there was so much change in shrubsamples, ground samples, and diet (nestling feces). diet and availability among dates and years that the averaged information was unstable with re- gard to inclusion or exclusion of environmental eluding footnote “b”). Araneae, Coleoptera samples, and irrelevant to what went on at any adults, Diptera adults, and Lepidoptera larvae given time. were sufficiently common in the diet and environment so that the effects of habitat, distance RESULTS from the nearest stream, Julian date, and shrub type (shrub samples only), and the interactions PREY SIZE betweenhabitat and eachof the other effectscould Do the size distributions of prey fed to nestlings be tested with analysis of variance procedures differ from the size distributions in the environ- (SAS). When interactions were significant (P < ment, within the sizes of prey normally fed to 0.05), tests were repeated within classesof the nestlings? The overall size-frequency distribu- effectsinvolved in the interactions. Least squares tions of arthropods in the samples differed be- means were used to interpret pairwise differ- tween the ground (pitfall) and diet samples [G- ences. statisticfor replicated goodness-of-fittests (Sokal We used an approach that employs the differ- and Rohlf 198 l), G, = 1,158 xz0.005,28= 511 and ence between rank availability and rank usageto between shrub (beating) and diet samples (G, = provide relative ranking of food preferenceamong 328, xzo.00s28= 51; Fig. 1). It appears that birds the categoriesconsidered (Johnson 1980). (The were selecting few prey less than 4 mm, more term “preference” is used in the operational sense prey in the 4-5 mm size range, and more prey to connote nonrandom use, and is not meant to in the 13-28 mm size range in comparison to imply active choice.) This approach is relatively what was available in the ground and shrub sam- insensitive to the inclusion or exclusion of sel- ples. Prey in the 4-8 mm size range were the dom-used foods. Shrub and ground sampleswere most common in the nestling diets. used to calculate numeric- and biomass-based ENVIRONMENTAL SAMPLES rank indices of availabilities of the four most common arthropod taxa, as well as the remaining How much variation is there among the envi- taxa collapsed into the category “other.” These ronmental samples, and how can this variation categorieswere used so that interpretations could be explained? Understanding what drives the remain consistent with the results of the ANO- variation will be necessaryto understanding dif- VAs, and so that the number of samples would ferencesin the availability of food for nestlings remain high relative to the number of taxon cat- among male territories, and can be used to sug- egories, producing a more robust analysis. gest further experimental work in this system. The environmental sample taken at the date The most obvious (to humans) difference in closestto the date on which the fecal sample in environment was that between the clearcut and the same habitat was taken was used to deter- logged habitat. The shrub samples indicated a mine rank availability; in no caseswere the en- higher biomass of Coleoptera and a higher total vironmental samples taken more than 10 days arthropod biomass on shrubs in the forest than NESTLING WINTER WREN DIET 417

0.5 1

FIGURE 2. Proportionof Diptera adults(numbers) FIGURE 4. Proportionof Lepidopteralarvae (num- in thediet plottedagainst the Julian date of the sample. bers)in the diet, plottedagainst days since hatching. in the clearcut (Table 1). Numbers of Araneae It would be of interest to know whether the were also higher in the forest than in the clearcut prey basechanged greatly over the 6-week period on V. alaskensisbut not on R. spectabilis.The of sampling, as this would affect availability of numbers and biomass of the Lepidoptera larvae prey for early as compared to later nests. Num- on shrubs were higher in the clearcut habitat. bers of Araneae were highest in late May and Pitfall samplesindicated higher numbers of Ara- numbers of Diptera on shrubs were highest in neae, Coleoptera, Diptera, other arthropods, and early June. Numbers of Coleoptera on shrubs total arthropods, and higher biomass of Araneae were lowest in late May, while numbers and bio- and all arthropods in the clearcut than in the mass of those sampled by pitfalls in the clearcut forest (Table I). were higher in the early and middle (May) than One might expectthat the arthropod prey base in the late (June) samples (P < 0.001). would be increased near permanent streams be- Numbers of Araneae were higher on V. alas- causeof an increase in emergent aquatics as well kensisthan on R. spectabilisin the clearcut. Oth- as larger foliage volume for foliage feeders. In- er arthropods (comprised primarily of Homop- deed, numbers and biomass of Lepidoptera lar- tera, Hymenoptera, and Plecoptera) were higher vae on shrubs were higher near streams (Table on V. alaskensisthan on R. spectabilis. 1). Total numbers of arthropods on the ground, however, increasedwith distancefrom the stream DIET SAMPLES in the clearcut (R2 = 0.06, n = 181) although What factors could be used to predict nestling they decreasedwith distance from the stream in diets? There were no significant effectsof differ- the forest (R* = 0.03, n = 181). ences in habitat, age of nestlings, or Julian date on the biomass proportions of arthropods in the diet. Proportionate numbers of Coleoptera, how- 0.7 ever, were higher for nestlingswhose parents foraged in the clearcut than for those whose parents foraged at the edge or within the forest. Propor- tionately fewer Lepidoptera larvae were fed to nestlings by adults that foraged in the clearcut compared with those that foraged at the edge or in the forest (P < 0.001 for each pairwise comparison). Proportionate numbers of Diptera decreasedwith Julian date (R* = 0.1 I, n = 38, P < 0.05; Fig 2). Proportionate numbers of Co- leoptera increased with nestling age (R* = 0.14, P c 0.05; Fig. 3) whereas proportionate num- FIGURE 3. Proportionof Coleopteralarvae (numbers of Lepidoptera larvae decreased with age bers)in the diet plottedagainst days since hatching. (R2 = 0.15, P < 0.05; Fig. 4). 418 BEATRICE VAN HORNE ANDAHMAD BADER

TABLE 2. Preferencesfor arthropodtaxa. A = Ara- neae,C = adult Coleoptera,D = adult Diptera, L = Lepidopteralarvae, 0 = othertaxa (see footnote Table 1).Taxa areordered by decreasingpreference from left to right. Thosetaxa not significantly(P < 0.05) dif- ferentfrom one anotherare underlined.n = 11 and 7 for combinedfecal samples for clearcutand forestsam- ples,respectively.

Shrub Ground

Number Clearcut ACOLD --CAOLD Forest FIGURE 5. Mean lengthof arthropodsfed to nest- ACOLD OACLD lingsplotted against days since hatching. Biomass Clearcut The mean sizes of arthropods fed to nestlings --COADL --CAOLD inereased with age of nestlings (R* = 0.61, n = Forest 11, P < 0.01; Fig. 5). The means were calculated ACOLD ACOLD from all arthropods fed to nestlings of a certain age, so that the n’s used to calculate the means averaged 120 and ranged from 24 to 240. environment is complex, possibly involving FOOD PREFERENCE maximum or minimum thresholds for certain Despite the differences in diets and arthropod prey. availability between the habitats, prey preferences were remarkably consistent among habi- DISCUSSION tats (Table 2). In the shrub samples, Araneae Understanding the relationship between nestling were ranked highestexcept when diets were com- diets and food availability may provide insights pared to ranked biomass availability in the clear- into the foraging behavior of birds and their cut. Coleoptera were consistently first- or sec- choice of territory or habitat occupancypatterns. ond-ranked. “Other” taxa (mostly Homoptera, Sampling the prey base of insectivorous birds is Hymenoptera, and Ephemeroptera)ranked in the difficult, however. It is unlikely that any single middle, while Diptera and Lepidoptera larvae method is adequate (Norment 1987). Our sam- were consistently ranked as less preferred. This pling methods provide two relative indices of low ranking of the latter taxa was even true when prey availability, and represent the microhabi- the ground samples (in which they were likely to tats in which the birds were most commonly be underrepresented)were used to calculate pref- observed foraging. erences.Rankings based on ground sampleswere Producing successfuloffspring depends in part generally consistent with those based on shrub on the parent’s ability to provide nestlings with samples.It should be emphasized, however, that adequate food. Thus, strong selection should fa- Coleoptera, Araneae, Diptera, and Lepidoptera vor efficient foraging by parents during the nest- were the most commonly consumed foods in ling phase. The optimal foraging approach taken terms of both numbers and biomass among the by Charnov (1976) is framed in terms of the ratio 22 taxa identified in the fecal samples; the pref- of energy to hunting time (E/T,,), and predicts erencesmerely establishan ordering among these that above a threshold density of large prey, an- commonly used foods. imals should stop taking small prey and concen- If preference rankings are consistent, the pro- trate on the more profitable larger prey, as long portions of the major arthropod taxa in the diet as larger prey are at least equivalent nutritionally. and the environment in that time period should Consistent with predictions of optimal foraging be positively correlated. No such correlation was theory, Winter Wrens selecteda higher frequency found, indicating that the relationship between of larger prey than was randomly available (Fig. preference and abundance of arthropods in the 1). In Great Tits (Parusmajor), adults bring in NESTLING WINTER WREN DIET 419

larger prey as the young mature, possibly as a larvae are indeed a highly preferred food) or dif- result of a greater ability of older nestlings to ferencesin the digestive physiology of the nest- handle large prey (Royama 1970). We observed lings that make it more difficult for them to con- this pattern in the Winter Wren as well. sume the more heavily scleroticColeoptera early Samples obtained by beating shrubsin the for- in the nestling period. Biermann and Sealy (1982) est contained a larger number of prey than those also found that the proportion of geometrid lar- in the clearcut area. The reverse was true for the vae in diets of very young Yellow Warblers (Den- ground samples. Because the birds forage both droica petechia) was higher than that in older on shrubsand in the slashand litter on the ground nestlings.In contrast,Meunier and B&lard (1984) it is difficult to generalize about which habitat determined that the proportion of Lepidoptera supports a larger prey base. and Diptera increased with age in nestling Sa- The pitfalls sampled more Coleoptera among vannah Sparrows (Passerculussandwichensis), the slash in the clearcut than on the mossy floor while the proportion of Homoptera decreased. of the forest, and this difference was reflected in the diets of nestlings in these two habitats. Lep- CONCLUSIONS idoptera larvae on shrubs also showed a higher The microhabitat of Winter Wrens is an impor- biomass in the clearcut than in the forest, and tant determinant of the type and quantity of ar- the larvae were more numerous near streams. thropod food available. There are differencesin Perhapshigh foliage volumes of deciduousshrubs nestling consumption patterns associated with such as R. spectabilisand V. alaskensisin light major habitat differences. Ranked preferences and moist areas can explain this result. remain fairly constant across habitats, but the Given the extent of significant differencesbe- relationship between actual consumption and tween habitats in the numbers and biomass of availability is complex. Among the four taxa most taxa in the environment and in the diet, it is commonly fed to nestlings, Araneae and adult remarkable that prey preferencevalues remained Coleoptera appear to be preferred over Lepidop- relatively constant between habitats. This indi- tera larvae and adult Diptera. Size and taxa of cates that major dietary shifts that do not show prey fed to nestlings change with nestling age. a simple correlation with changesin availability Identification of arthropod fragments in bird may lead to relatively constant patterns of ranked feces is a useful and nondestructive method for preference in the presence of considerable re- looking at relative differences in diet composi- sourcedifference and fluctuation. One of us (Van tion. In identifying preferences, it is important Home) spent many hours watching parents bring to use environmental samples temporally keyed food to nests, and it is likely that if we had used to diet samples. direct observations of food being brought to nestlings we would have identified Lepidoptera lar- ACKNOWLEDGMENTS vae as the most preferred food, as this was con- We acknowledgethe assistanceof the U.S. Forest Ser- sistently the most obvious food item carried by vice in providing housing for Van Home at the field the adults. site. Jerry Franklin single-handedly prevented the burning of our study area by the Forest Service. Boris Parentsfeeding young birds must increasetheir Kondratieff assistedwith arthropodidentifications. Rick feeding rate as the young approach the age of Redak provided helpful comments on an earlier draft. fledging. The pressureto increasethe feeding rate A Faculty Research Grant from Colorado State Uni- may have the same effectas a changein the hun- versity to Van Home supported the laboratory anal- ger state (sensu Chamov 1976) of the foragers, ysis. This researchwas conducted without direct fed- eral funding or grants. leading them to be lessselective. Royama (1970) found that spiders,a preferred food of Great Tits, LITERATURE CITED formed a lower proportion of the diet of older ARMSTRONG, E. A. 1955. The Wren. Collins New nestlings, indicating that selectivity may de- Naturalist, London. creaseas the feeding rate increases.The increase BIERMANN,G. C., AND S. G. SEALY. 1982. Parental in the proportion of Coleoptera with nestling age feeding of nestling Yellow Warblers in relation to and concurrent decrease in the proportion of brood size and prey availability. Auk 99:332-341. CALVER,M. C., AND R. D. WOOLLER. 1982. A tech- Lepidoptera could reflect either a higher selec- nique for assessingthe taxa, length, dry weight and tivity early in the nestling period when trips to energycontent of the arthropodprey of birds. Aust. the nest are much less frequent (if Lepidoptera Wildl. Res. 9:293-301. 420 BEATRICE VAN HORNE AND AHMAD BADER

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