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Journal of

Lepidopterists' Society

Volume 39                                    1985                                     Number 4

Journal of the Lepidopterists' Society 39(4), 1985, 239-261

BIRD PREDATION ON LEPIDOPTERA AND THE

RELIABILITY OF BEAK-MARKS IN

DETERMINING PREDATION PRESSURE

Mark K. Wourms and Fred E. Wasserman

Department of Biology, Boston University, Boston, Massachusetts 02215

ABSTRACT. Visually hunting predators such as birds are thought to have influenced the evolution of the wing markings and colorations of Lepidoptera. Although studies have been conducted to quantify and characterize predation by birds on butterfly populations, field observations of bird predation on butterflies have rarely been reported. A request for information on predation yielded 50 previously unpublished accounts of bird predation on butterflies.

The combination of laboratory interactions of Pieris rapae and blue jays and field collections of P. rapae allowed several variables to be examined which affect the reliability of using frequency of beak-marks on lepidopteran wings as an index of predation pressure. Beak marks occur four times more frequently during attacks on flying P. rapae than on ones at rest and blue jays were five times more efficient at capturing resting butterflies than capturing flying butterflies. Variation in wing strength makes the area where the ipsilateral wings overlap and the costal vein area of the forewing more resistant to beak-marks than the marginal areas of the fore- and hindwings and the distal tip of the forewings. These differences in wing strength may confound the use of beak-marks as an index of predation pressure.

Finally, predation efficiency and the frequency of occurrence of beak-marks during attacks, as determined in the laboratory, were used in conjunction with field data to estimate avian predation pressure on P. rapae populations.

Although birds have long been thought to be the major predators on adult Lepidoptera (Poulton, 1890, 1913; Fryer, 1913; Swynnerton, 1915; Dover, 1920; Carpenter, 1937), field observations of bird predation on butterflies in temperate North America have rarely been reported. The short time that it takes birds to capture and manipulate butterflies while feeding may account for the rarity of field observations (Bowers & Wiernasz, 1979; Collins & Watson, 1983). There is strong circumstantial evidence in the form of beak-marks and tears on wings of Lepidoptera to indicate that birds act as significant predators on butterflies (e.g., Wheeler, 1935; Carpenter, 1937; Kolyer, 1968).

The

and consumed the body. Most evidence of wing damage suggests birds are the major predators

Table 1. Continued.

Table 1. Continued.

Lepidoptera

Bird

Notes

Strymon spp. L. americana Lycaenids

Family—Pieridae Subfamily—Colladinae Colias eurytheme (Orange Sulfur) Colias spp. (Sulfur Butterfly)

Subfamily—Pierinae Pieris rapae (Cabbage White Butterfly)

P. rapae

P. rapae

rapae

Melospiza melodia (Song Sparrow)

Melospiza melodia (Song Sparrow)

Melospiza melodia (Song Sparrow)

Tyrannus verticalis (Western Kingbird)

Sayornis phoebe (Phoebe)

Sturnus vulgaris (Starling)

Passer domesticus (House Sparrow)

Passer domesticus (House Sparrow)

Pipilo erythrophthalmus (Rufous-sided Towhee)

Hawking

Hawking (also took unidentified mot

One attack in fligh Consumed all

Tried to catch by h ping off ground missed

Male House Sparro with butterfly in beak, fed to youn no adverse reacti

Female House Spar row pursued but fly in air, no con no capture

Table 1. Continued.

Table 1. Continued.

Lepidoptera

Bird

Notes

Virgo Tiger Moths

Various small diurnal moths

Dryocampa rubicunda

Nadata gibbosa

Lapara coniferarum

Geometridae

Arctiidae

Noctuidae

Notodonitidae

Tyrannus tyrannus (Eastern Kingbird)

Passer domesticus (House Sparrow)

Piranga rubra

(Summer Tanager)

Wings neatly clipped

off Fly-catching

Captured moths at r on side of building 2-5 seconds handling time, two oc casions

248

Journal of the Lepidopterists' Society

Predation by birds on Lepidoptera have been reported in studies that have been concerned with interactions between European Lepidoptera and birds (Carpenter, 1933, 1937, 1941; Collenette, 1935) or tropical Lepidoptera and birds (Fryer, 1913; Young, 1971; Brown & Neto, 1976; Smith, 1979; Collins & Watson, 1983). This study reports on the interactions between North American Lepidoptera and birds and investigates the reliability of butterfly wing-damage frequencies as a predictor of predation pressure in the European cabbage butterfly, Pieris rapae L.

Methods

A request for information from professional and amateur lepidop-terists and ornithologists regarding butterfly-bird interactions yielded 50 previously unpublished accounts of predation by birds on butterflies in temperate North America (Table 1). The results of a literature survey of the frequency of beak-marks reported in butterfly populations is summarized in Table 2 and a survey of defensive compounds found in adult Lepidoptera is presented in Table 3.

In order to document avian predation on P. rapae in the field P. rapae adults were collected for a 30 minute period every seven to 10 days in Boston, Suffolk Co., Massachusetts (Fenway Victory Gardens) and for a one hour period every seven to 10 days at two sites in Lexington, Middlesex Co., Massachusetts (Dunback Meadows and Carroll Field, 71° West, 42° North). The difficulty of moving through the Middlesex Co. sites, due to dense vegetation, Phragmites spp. and goldenrods, Solidago spp., necessitated the longer collection time per period. Captured butterflies were sexed, and the presence and location of bird-attributable wing damage were recorded for each specimen. Initially, nine possible locations of attack were identified. These were condensed to represent three directions of attack; from the front, side, or from behind (Fig. 1).

One factor that may influence the reliability of beak-marks as an index of predation is the strength of the wings. The strengths of (1) three areas on the fore wing, (2) one area on the hind wing, and (3) the area where the ipsilateral fore- and hindwing overlap, were measured on 25 specimens of P. rapae (Fig. 2). Strength measurements were obtained by removing the wings from the specimen, and positioning one wing at a time in the testing device (Fig. 3). The device slowly increased the force on the wing until tearing occurred. Data were analyzed with a single factor repeated measures analysis of variance and a Student Newman-Keuls multiple pairwise test (Zar, 1974).

To observe predatory behavior and to quantify the frequency and

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251

Fig. 1. Front, middle, behind locations of bird damage for field collected Pieris rapae.

type of butterfly wing damage that occurs during attacks, P. rapae adults were brought into the laboratory in a wire cage (25 cm high x 15 cm in diameter), where they were released into a 1 x 0.5 x 1 m holding cage made of mosquito netting. Sugar water and wild flowers were provided ad libitum. Four blue jays, Cyanocitta cristata, were captured in mist nets and baited traps. They were housed individually in 1 x 1 x 1 m wire screen cages under a long-day light cycle (18 h light, 6 h dark). All birds were provided water and sunflower seeds ad libitum, and were given canned dog food, fresh chopped vegetables, and 5-10 mealworms each morning. Two weeks prior to trials with live P. rapae, one bird was placed in a flight cage (3 x 4 x 3 m). The experimental procedure consisted of (1) placing a single live P. rapae in a 4 cm box, (2) introducing the box into a flight cage through a slot in the side of the cage, and (3) releasing the butterfly by pulling a string attached to the lid of the box.

The activities of the butterfly and the blue jay were monitored for 15 minutes with a video recorder. If the butterfly was not consumed during the 15 minute trial it was removed and another individual was presented after a 15 minute interval. No more than six trials were conducted per day. Video tapes were analyzed with slow motion and freeze-frame to identify attacks and contact points. A new blue jay was transferred to the flight cage and trained, after the previous bird had had 10 days of live presentations.

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Journal of the Lepidopterists' Society

a.

Forewing

B.

Hindwing

Fig. 2. The areas of Pieris rapae wings where resistance to tearing was measured. (* indicates points of anchoring during measurements.) A. The arrows indicate points of strength measurements, the forewing costal vein, wing tip, and distal margin. B. Distal margin of the hindwing. C. Ipsilateral overlap of fore and hindwings.

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3cm

Fig. 3. Wing strength (resistance to tear) measurement device. The wing was anchored by the clip of the Pesola scale, and the area measured by the lower clip. The crank was turned pulling upward. The resistance (g) was shown on the Pesola scale, and was recorded to the nearest 0.5 g the moment the wing tore apart.

Results

Of the 1179 P. rapae collected during the three field seasons, an average of 7.2 ± 0.28% (S.D.) of the specimens had beak-marks or beak tears. There were no significant differences in frequency of bird damage among sites or within sites over different seasons (Table 4, 6 x 2 Chi-square contingency table; x2 = 0.49, df = 4, P > 0.05). Only two of the 91 specimens collected showing bird damage had impressions of a bird's beak on the wings of the butterfly; the other 89 specimens

254                                                    Journal of the Lepidopterists' Society

Table 4. Butterfly sampling and beak-mark frequencies.

had beak tears. Henceforth, unless otherwise stated, "beak-marks" implies both marks and tears.

In 1981, sex was not distinguished during the collection of P. rapae. In 1982 and 1983, 838 (78.2%) males and 238 (21.8%) females were collected (Table 5). Chi-square analysis reveals that in 1982 and 1983 the frequency of bird damage on P. rapae was independent of sex (Table 5).

No specimen showed evidence of more than one attack. Symmetrical damage on both sets of wings suggests that the damage occurred while the butterfly was at rest with wings folded. Butterflies with damage on one wing or on an ipsilateral forewing and hindwing were assumed to have been attacked in flight (Bowers & Wiernasz, 1979; Sargent, 1973). Two specimens from 1982 and one from 1983 were omitted because symmetrical or single wing damage could not be determined.

Of the 76 bird-damaged P. rapae collected in 1982 and 1983, 51 (67%) were damaged in flight, and 25 (33%) were damaged at rest (Table 6). Significantly more specimens were damaged in flight than at rest (expected values are calculated as half of the total number of damaged specimens; x2 = 4.4, df = 1, P < 0.05). The distributions of attacks from the front, side and from behind are presented in Table 6. There was no significant difference between the distribution of attack positions occurring in flight from the distribution of attack positions occurring at rest (Table 6; 3 x 2 Chi-square contingency table; X2 = 2.44, df = 2, P > 0.05). Regardless of whether the butterfly was in flight or at rest, significantly more bird damage occurred from behind than from the side or from the front (expected values are calculated assuming equal numbers from each of the three directions: Table 6; flight, x2 = 25.52, df = 2, P < 0.05; rest, x2 = 18.39, df = 2, P < 0.05).

Analysis of variance indicated a significant difference in strengths of the five wing areas (F = 68.3, df = 96, P < 0.05). The costal vein area and the ipsilateral overlap were three times stronger than the distal margin of the hindwing and twice as strong as the distal margin

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Table 5. Comparison of the sex ratios of bird  damaged and undamaged specimens for each year.

Damaged                             Undamaged Total

1982

Male 27                              345 372

Female 9                              110 119

Total 36                              455 491

X2 = 0.012, df = 1,  P > 0.05

1983

Male 32                              434 466

Female 11                              108 119

Total 43                              542 585

X2 = 0.82, df = 1,  P > 0.05

of the forewing (Table 7). Using Student-Newman-Keuls multiple pair-wise test, we found no significant difference between the costal vein area and the ipsilateral overlap area, and no significant difference between the margins of the fore- and hindwings and the tip of the fore-wing, but there was a significant difference between these two groups of wing areas (P < 0.01).

The presentation of 104 P. rapae to four blue jays in a flight cage resulted in 182 attacks and 69 butterflies captured (Table 8). Sixty-nine percent (57/83) of the attacks on resting butterflies resulted in captures, while only 12% (12/99) of the attacks on flying P. rapae resulted in captures. The blue jays were significantly more efficient in capturing butterflies at rest than in flight. (Table 8; x2 = 27.73, df = 1, P < 0.05).

Few of the butterflies that were attacked showed wing damage. Of the 83 butterflies attacked at rest, only one of the 21 P. rapae which escaped had wing damage. Only four of the 87 P. rapae which escaped attacks in flight received wing damage.

Discussion

Despite the presence of mustard oils (Rothschild et al., 1970; Aplin et al., 1975) P. rapae were acceptable prey to blue jays in the field and laboratory and to house sparrows, Passer domesticus, purple martins, Progne subis subis, and various other avian species in the field (Table 1). In this study, an average of 7.2% of the P. rapae collected showed evidence of attacks by birds in the form of beak imprints and beak tears, and no specimen showed evidence of being attacked more than once. In California, Shapiro (1974) collected P. rapae and found 5.6% of the specimens bird-damaged, and between 0.33% and 0.50% of the

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Table 6. Frontal and rear attacks on P. rapae.

damaged specimens had multiple beak-marks. The percentages of bird damage reported by Shapiro (1974) and this study fall within the range of damage found in other lepidopteran species studied (Table 2).

At least four variables can affect the relationship between beak-marks and predation pressure (Benson, 1972; Shapiro, 1974; Robbins, 1980, 1981): 1) Damage may occur more readily during attacks on flying Lepidoptera than during attacks on resting Lepidoptera; 2) different avian predators may be many times more successful during attacks on resting prey than during attacks on flying prey; 3) different avian predators may vary significantly in capture efficiency on Lepidoptera; and 4) the strength of various butterfly wing areas differs, and this may influence the probability of obtaining beak-marks.

The live presentations of cabbage butterflies to blue jays indicates that bird damage may occur up to four times more readily during attacks on flying butterflies than during attacks on resting butterflies. Therefore, if equal numbers of butterflies are attacked in flight and at rest, a field sample would reveal a greater number of specimens showing evidence of being attacked in flight due to the higher frequency at which damage occurs (Table 6). This was also found by Bowers and Wiernasz (1979) in C. pegala and is expected if avian predators are less efficient during attacks in flight than at rest. The reliability of a beak-mark predation index is seriously jeopardized by unequal chances of obtaining beak-marks in flight and at rest. If most predation occurs while the butterflies are in the vegetation and few beak-marks result, the index would underestimate the amount of predation occurring. Likewise, if most attacks occur in flight the amount of damage may be overestimated if many prey are damaged and few captured. No previous study had quantified the relative occurrence of beak-marks due to attacks at rest and in flight.

Live trials support the hypothesis that avian predators may be much more efficient at capturing resting butterflies than at capturing butterflies in flight (Table 8). The attack efficiency of blue jays on flying P.

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Table 7. The strengths of P. rapae wing areas.

The strength of wing areas with identical superscripts were not significantly different from each other. Groups with the 'a' superscript were significantly different from those with V at the P < 0.01 level.

rapae was 12%. This was lower than that of an aerial insectivore, the spotted flycatcher, Muscicapa striata, which was reported in the field to have captured four flying P. rapae in 17 attempts for a success rate of 23.5% (Davies, 1977) and was still lower than the success rate of 100% on flying butterflies reported for a hunting northern shrike, La-nius borealis (Morrison, 1980; and pers. comm.). The predation index could be complicated by differences in the composition of avian communities in different habitats. For example, if shrikes were common in one habitat and relatively rare in another, many butterflies could have been consumed in the first area with little damage occurring, while the second area might have shown a high frequency of beak-marks but with little actual predation occurring. In this study, the avian communities of all three field sites were predominated by house sparrows, Passer domesticus, song sparrows, Melospiza melodia, and European starlings, Sturnus vulgaris.

Avian predators not only show variation in their probability of attacking butterflies, but may preferentially attack different areas on the butterfly in response to butterfly wing-markings. Butterflies attacked in stronger wing areas may show fewer beak-marks if they escape. Therefore, due to species differences in strengths of the areas of the wings, the frequencies of beak-marks may not be a reliable index of predation pressure for comparisons among species.

The percentage of bird-damaged specimens actually represents only the number of individuals which successfully survived attacks and escaped with bird damage. No data exist from field observations on the percentage of escaped butterflies showing no bird damage or on the percentages of attacked butterflies actually killed or eaten. If the laboratory efficiencies of blue jays preying on P. rapae are extrapolated to the field, avian predation on Lepidoptera becomes a much more significant selective force than previously suspected. Of the 1179 P. rapae collected, 76 had wing damage. Fifty-one specimens were attacked in flight, and 25 specimens were attacked at rest. In the laboratory only 4% of butterflies attacked in flight actually showed wing

258                                               Journal of the Lepidopterists' Society

Table 8. Live presentations of P. rapae to blue jays.

damage. Therefore, given the 4% probability of obtaining a beak-mark, the 51 specimens collected in the field that were attacked in flight may represent attacks in flight on approximately 1275 individuals.

Blue jays consumed 12% of the P. rapae they attacked in flight in the laboratory. If this efficiency is extrapolated to the field, 12% of approximately 1275 P. rapae attacked or 153 would have been consumed after capture in flight.

Blue jays damaged only 1% of the butterflies attacked at rest in the laboratory. The 25 field-collected specimens which had damage from attacks while at rest would represent 2500 butterflies attacked at rest in the field. However, 68% of the resting P. rapae attacked in the laboratory were consumed. Therefore, according to this extrapolation, approximately 1700 P. rapae would have been consumed in the field after capture while at rest.

The disparity in predation pressure in flight and at rest suggests that the major selective force of avian predation is directed at the butterfly wing surface that is exposed while the butterfly is at rest. This is the ventral surface of the wings for most butterflies (Piatt et al., 1971) but may be the dorsal surface of the wings of most moths (Sargent & Keiper, 1969; Endler, 1978) and for butterflies which expose the dorsal surfaces of the wings during basking, nectaring, and other activities. Rawlins and Lederhouse (1978) found that in the Battus philenor mimicry complex, the resemblance of model and mimic is closest on the ventral wing surface. They suggest that selection may be most intense on the underside of the wings, which are exposed while the butterflies are at rest, rather than on the dorsal surface of the wings which is only exposed in flight. This hypothesis previously had not been evaluated critically, and in fact, is not supported by accounts of avian predation on Lepidoptera (Table 1). Attacks on resting butterflies may be less noticeable than attacks in flight because they occur rapidly and are often obscured by vegetation.

Laboratory data obtained in the present study on predation by blue jays on P. rapae are the first to quantify differential predation pressure

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259

on the ventral and dorsal surfaces of the wings of P. rapae due to variation in success rates of attacks on flying and resting butterflies. Recently, this has been supported by work on Euphydryas chalcedona (Bowers et al., unpubl. manuscript). Euphydryas chalcedona males which had less red on the dorsal surface of their wings were under greater predation pressure when their wings were spread while resting, basking, and nectaring. Although the ventral surfaces of the wings were essentially identical in both groups, avian predation appears to favor dorsally red males. Realistic estimates of predation pressure on Lepi-dopteran populations are impossible due to the lack of field data. Yet, the extrapolation of laboratory and field data supports the concept that bird predation on butterflies may be a more significant selective force on Lepidopteran populations than previously assumed.

Many variables can influence the reliability of using the frequency of beak-marks on the wings of butterflies as an index of predation pressure. Thus, the interpretation of beak-mark frequencies is complicated and may not provide a reliable index of the amount of avian predation pressure on Lepidoptera.

Acknowledgments

Financial support for these investigations was provided by the Frank M. Chapman Memorial research grant from the American Museum of Natural History, the Louis Agassiz Fuertes research grant, and the Boston University chapter of Sigma Xi. Thanks are also extended to the following people for help of various kinds; S. Duncan, T. Kunz, D. Phillips, R. Regis, B. Schlinger, and J. Traniello.

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