Condor85:139-150 G The Cooper Omithologd Society 1983 FEEDING RATES OF WARBLERS IN SPRING JEAN W. GRABER AND RICHARD R. GRABER ABSTRACT.-Populations of migrant warblers were correlated with populations of lepidopterous larvae in spring in a heavily forested area of southern Illinois. The birds appeared to feed entirely on these larvae, mostly taken from oaks. Estimates of grosscalorie intake indicated a positive energy balance for warblers in an area with larval biomass of 0.70 g per m3, and a negative balance in an area with biomass of 0.02 g per m3. Warblers chose either the smaller size classes (under 15 mm) of larvae, or the smaller species (especially leafrollers) out of proportion to their availability, but the birds’ positive energy balance was de- pendent upon an irruption of geometrids. Lepidopterous larvae constituted 75- 98% of the available foliage invertebrates, and other groupswere insufficient alone to support migrant populations. Larvae were about 80% water, and warblers ingested 1.2-1.7 times their own weight in larvae per day in order to attain gross caloric intake of 14.8-19.0 kcal per bird-day. Warblers in an area with insufficient food resourcesingested 7.2 kcal/day, and their behavior was consistent with a goal of energy conservation. Among the problems faced by migrants after principal study to make the observations re- a flight is that of finding adequatefood supplies ported here. to continue the migration, particularly as the landing area may be completely unknown to STUDY AREAS the birds. In Illinois the only measurementsof Our study area with extensive forest habitat migrant populationsin forest habitat have been was in Shawnee National Forest in southern those of Twomey (1945) and Calef (1953) who Illinois (88”3O’W, 37”28’N): mature upland found four to eight times more birds in elm- oak-hickory, and upland forest edge in Pope maple forest at the peak of migration in May County (57.2% forested). Our study area with than were present in June. We wondered how more restricted forest was in Allerton Park in the forest accommodated large numbers of mi- east-central Illinois (88”38’W, 4O”OO’N): ma- grants, and how much forest is required by ture upland oak-hickory and upland forest-edge arboreal migrants. Kendeigh’s (1979) study of in Piatt County (1.8% forested). Figures for invertebratesin three east-centralIllinois study forest acreage in each county are from Essex areasshowed the seasonalpopulation patterns and Gansner (1965). Our original study in- of various groups of invertebrates. There was volved three arboreal habitats in each region, a close similarity between the timing of emer- but we restricted our observations on feeding genceof lepidopterous larvae and the seasonal warblers to forest-edge and shrub habitat so timing of bird migration in this region. that birds would be as low as possiblefor view- Stimulated by these papersand the question ing. Unless otherwise stated, the data in this of how migrant populations respondto habitat paper refer to forest-edgeand shrub. However, availability, we began, in 1979, concurrent the larger general study was important to this studiesof bird populations and their potential one becauseof the extensive experience it pro- food resourcesin (1) an area where forest tracts vided as background to the observations on were extensive, and (2) an area where forest feeding. was much more restricted. Our primary goal was to consider the availability of food sup- METHODS plies to migrant warblers. Our interest was not For censusingbirds we used the same fixed- in the influence of bird populations on insect width transectmethod we have used on breed- populations, but the reverse. In either case, it ing and winter populations (Graber and Gra- is important to know about rates of predation. ber 1963, Graber et al. 1977) i.e., counts of When the opportunity arose in May 198 1 to all birds detected within a strip of habitat 27.4 study food consumption by warblers on in- m wide and not lessthan 6.4 km long. In this vertebratesin forest-edgehabitat at two widely study the bird census transects were along divergent population levels, we interrupted our marked routes, and encompassed 18.7 ha [I391 140 JEAN W. GRABER AND RICHARD R. GRABER (Pope) and 19.9 ha (Piatt). The censusroutes r 0,115 were roughly rectangular, ending near the 0.040 SMALL starting point. We did not censuson days with LARVAE34 rain and/or sustained high wind (above 15 1 t knots) in the habitat. Bird censuseswere con- ducted in the morning; invertebrate censuses 0.030 were conducted in the afternoon usually on the 3 same day and in the same habitat as the bird g 0.025 census.With the techniquesused we could not 0 censusbirds and invertebratesat the sametime. ; 0.020 Lepidopterous larvae, which constituted 75- z 0.015 98% of the foliage invertebrate biomass, are not very mobile and are generally present in the same feeding area day and night during the 0.005 4 f35 0.05 period of their development. For severalgroups YIP t of larvae we checked their location and num- p 18 bers at different times over a period of two I I I I I I I weeks or more and found them consistently in 5 10 15 20 25 30 35 the same sites. LENGTHOmn) The invertebrate censuseswere counts of in- FIGURE 1. Relationship of length to weight in common dividuals ofall speciesdetected (excluding galls) typesof lepidopterouslarvae found in forest-edgeand shrub on or flying through the woody vegetation in habitat in southern and central Illinois study areas. Scale on left refersto curve for small (4-l 5 mm) larvae of species a cubic meter (measured with a meter stick) of leafrollers (Tortricoidea), loopers (Geometridae), and l-2 m above ground level. Samples (m’) were fruitworms (Noctuidae, excluding Amphipyru). Scale on spaced evenly at intervals of 32 m along the right refers to larger larvae (15 mm +) of geometrids and middle of the bird censustransect. The interval fruitworms. Most leafrollers were less than 15 mm. The number of specimensweighed and measuredis given be- was measured by pacing 20 (left foot of RRG) side the appropriate point on the curve. Vertical line in- paces. The point of the toe on the twentieth dicates i 1 SE for each sample. pace indicated which plant would be censused, i.e., from that point the nearest woody plant at least 2 m tall. As the starting point of the letting them. Our colleagues in the Illinois censuswas randomly selected (n paces along Natural History Survey Section of Faunistic the transect line), selection of plants was ran- Surveys and Insect Identifications provided dom, and reflected shrub and tree speciesat identifications of many species, genera, and least roughly proportional to their numbers families. The immature forms especially, of along the transect. One afternoon’s inverte- many species of invertebrates are unknown, brate censuscovered only about one-seventh and so we assigneddescriptive code names to of the entire bird censusroute and subsequent all forms that we encountered. Censuseswere invertebrate censuseswere taken on different conducted at first with the aid of a 2~ hand sectionsof the over-all route. After the first 20 lens,and later with a 2.5 X Optivisor magnifier. sampleswere censusedeach day, we modified In addition to counting the invertebrates, we the selection procedure to examine dominant measured in situ (to nearest mm) all individ- speciesthat had not been covered that day- uals found. Sizes of flying insects were esti- selecting plants close to, but not necessarily mated. We also weighed specimens(to nearest closestto, the point. This gave us more com- 0.000 1 g) of nearly all the common speciesand plete coverage of the dominant plant species, constructed weight-length curves (e.g., Fig. 1) and spreadthe sampling evenly along the tran- for the different speciesor different types where sect line. The counts and measurementsof in- the specieswas unknown to us (e.g., ants). Col- vertebrates were made by JWG, and were re- lected specimens were taken from the same corded by RRG. Some quadrats 5-7 m high habitats we censused,but not, with very few were also censusedfor invertebrates. Censuses exceptions, from the censustransects. We also for arboreal invertebrates in forest-edge and collected specimensof the common larvae for shrub, and adjacent forest interior were made dry-weight and caloric determinations (Table on 14 and 24 April, 1, 2, 13, 15, 22, 3 1 May, 1). Caloric determinations were made with a 19 June in central Illinois, and on 8, 17, 18, Parr bomb calorimeter on l-g (dry weight) 19, 28 April, 5, 6, 8, 17, 19, 25, 28 May, 14 samplesof caterpillars and sawfly larvae. To- June in southern Illinois. We censused23-34 mato hornworms are not arboreal species,but m3 per day in spring 198 1. were included in Table 1 to show calorie values We began to learn the common foliage in- in large larvae, and also to show variation in vertebrates in 1979 by photographingand col- replication of caloric determinations. Speci- WARBLER FEEDING RATES 141 TABLE 1. Caloric values of lepidopterouslarvae and sawfly larvae of various speciesand sizes. LZUVd Number of Fresh wt. <‘alones/g length larvae In of sample PWXlll cA&lrl;s(” ash-free Sample (mm) sample (9) dry matter dry wt. Leafrollers (Archips sp., Argyrotaenia sp.) 10 351 4.9845 19.22 5,930 6,225 Linden loopers (Erannis tiliaria) 20-25 47 5.1780 19.58 5,401 5,692 Various geometrids (Alsophila, Paleocrita, Erannis, and others) lo-25 136 6.6557 18.79 5,308 5,567 Mixed caterpillars-leafrollers, geometrids, fruitworms (Orthosia), webworms (Atteva) 7-40 55 5.4485 19.29 5,592 6,157 Uglynest caterpillar (Archips cerasivorana) 6-25 62 6.8391 18.63 5,460 6,007 Tomato homworm (Manduca quinquemaculata) 65 0.5 - - 4,807 5,181 Tomato hornworm 65 0.5 - - 5,344 5,795 Tomato homworm-heavily parasitized with braconid wasps 55 0.5 - - 4,779 5,073 Tomato hornworm-heavily parasitized 55 0.5 - - 4,728 5,015 Pine sawfly (Neodiprion taedae) 13-20 51 5.5665 19.78 4,892 5,304 Larch sawfly (Pristiphora erichsonii) 13-19 60 5.2578 22.80 5,334 5,808 mens were weighed live, most within one-half One of us (JWG) was primarily responsiblefor hour of being removed from the vegetation.