Biological Journal of the Linnean Society ( 1 989), 36: 29553 1 1
Discriminant analysis of Lepidopteran prey
characteristics and their effects on the outcome
of bird-feeding trials
DAVID B. MACLEAN
Department of Biological Sciences, Youngstown State Universily, Youngstown, Ohio 44555
THEODORE D. SARGENT
Department of <oology, University of Massachusetts, Amherst, Massachusetts 01003
AND
BONNIE K. MACLEAN
Department of Biology, Thiel College, Greenville, Pennsylvania 16125
Received 16 j%ne 1988, accepted f n r publicatton 26 October 1988
Discriminant analysis was used to analyse the results of 348 bird-feeding trials conducted from 1982 to
1985 in Leverctt, Massachusetts. Four size classes, seven appearancc categories, and five larval host
types, based on 163 species ofmoths and butterflies used as prey in two or more trials, were selected as
predictor variables t o discriminate between prey takrn and not taken by birds. Discriminant analysis
of individual fccding trials correctly
sified 97.5 percent of prry takrn or not-taken and ranked thc
predictor variables according to their relative importance in determining prey arreptability.
Characteristics most acceptable to birds were: ( I ) large size, (2) bark-like appearance, (3) warning
colouration, (4) woody generalist, (5) dead-leaf-like appearance, (6) woody specialist, and ( 7 )
medium size. Characteristics least acceptable to birds were: ( I ) srnall size, (2) mimetic appearance,
( 3 ) butterfly appearance, (4) herbaceous specialist food type, (5) black-and-white appearance, ( 6 )
extra large size, and ( 7 ) overall generalist feeder. A summary of thc analyscs includes a discriminant
function based on Icpidoptrran characteristics that r a n be uscd to predict the prey acccptability of
species not uscd in this study. A multiple regression analysis ofprey taken revealed that sizc alone and
larval host type combined with other prey rharacteristics were the most important variables in
determining the selection of prey regardlrss of their abundance in the trials.
KEY WORDS:
Adaptive colouration
analysis feeding trials - Lepidoptera
rolouration visual predators.
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aspect diversity bird ferding behaviour discriminant
multivariate analysis prey characteristics - protective
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CONTEN'I'S
Introduction .
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Methods . . . . . . . . . . . . . . . . . .
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Bird-feeding trials .
. . . . . .
Coding of p r y characteristics and dat a analysis
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D. B. MACLEAN E 7 i l L
Results
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Chi-square analysis of prey characteristics
Discriminant analysis of feeding trials .
Discussion.
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References.
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INTRODUCTION
In most terrestrial communities, there is a wide range of insect sizes and
appearances. T h e aspect diversity of cryptic and noncryptic appearances is
especially striking among families of Lepidoptera (Ricklefs & O’Rourke, 1975).
Appearances described as bark-like and dead leaf-like are cryptic on the
appropriate backgrounds and presumably offer protection from visually
searching predators such as birds (Sargent, 1966; 1968; 1969a, b; 1973; Erichsen,
Krebs & Houston, 1980; Pietrewicz & Kamil, 1981). Bright and sometimes
contrasting colours are known to serve as startle stimuli, e.g. Catocala spp.
(Sargent, 1976; Schlenoff, 1985), as warning or advertising stimuli (Coppinger
1969; Guilford, 1985; 1986), and as Batesian or Muellerian mimetic stimuli
(Waldbauer & Sternburg, 1976; Sternburg, Waldbauer & Jeffords, 1977;
Jeffords, Sternburg & Waldbauer, 1979).There have been few prior studies of the
relative acceptabilities of a wide array of lepidopteran prey to avian predators in
the field. Jones (1932, 1934) conducted a n extensive series of experiments
involving dead insects presented to wild birds at a feeding station on the island of
Martha’s Vineyard, Massachusetts. These studies, though similar in some respects
to those reported here, involved inconsistencies in experimental design that
precluded rigorous statistical analysis of the results. A general comparison,
however, ofJones’ results and those obtained in the present study is in preparation
(Sargent). Collins & Watson (1983) have also reported observations of bird
predation on a large number ofneotropical moth species, though here the moths
were either resting near light sources or hand-launched, rather than presented in
discrete feeding trials.
This paper presents the results of 348 bird-feeding trials carried out from 1982
through 1985 to evaluate how prey type (moth us. butterfly), shape (open us.
closed), size, appearance, larval host type, relative abundance, and position on
the feeding tray determined the selection of2 13 lepidopteran prey species used in
the trials. T o analyse the large amount of feeding trial data, we chose discriminant
analysis, a multivariate statistical method. The primary purpose of multivariate
methods is to simplify and express the underlying causes of variation in a set of
multivariate data by a smaller set of components or functions (Morrison, 1976;
Kachigan, 1982). This can be achieved by such techniques as factor analysis or
principal components analysis which attempt to extract a small number of factors
or components from a large set of variables. Discriminant analysis, on the other
hand, attempts to discriminate between two or more mutually exclusive criterion
groups based on a linear discriminant function derived from a set of predictor
variables. The association of individual variables with each group is expressed by
their classification function coefficients. T h e relative contribution of each
predictor variable can be evaluated as its standardized discriminant function
coefficient (Johnson & Wichern, 1982; Karson, 1982; Afifi & Clark, 1984).
T h e use of discriminant functions to predict group membership based on the
chosen predictor variables and to classify individual cases, e.g. species, to such
groups are important features of discriminant analysis. MacLean (1984) carried
LEPIDOPIERAN PREY CHARAC’IERIS’IICS
297
out a discriminant analysis of Catocala species hindwing groups using population
and life history traits as predictor variables. In this case, the inability to correctly
classify some cryptically similar species supported the hypothesis that anomaly
creates a degree of uncertainty for birds in associating hindwing patterns with
other characteristics of adult Catocala (Sargent, 1976).
The present paper addresses five questions. ( 1 ) Can the selection of butterflies
and moths by birds be evaluated by discriminant analysis of characteristics such
as prey size, appearance, and larval host type? (2) Which characteristics are most
often associated with prey that were selected by birds during the trials? (3) How
well did the results of discriminant analysis classify individual species into two
criterion groups, (a) prey taken and (b) prey not-taken? (4) Which combinations
ofprey characteristics were (a) least acceptable and (b) most acceptable to birds?
(5) How did the proportion of prey types affect prey acceptability?
METHODS
Bird-feeding trials
Data analysed in this study were obtained from 348 bird-feeding trials. All the
moths were taken at 150-watt incandescent spotlights (Westinghouse outdoor
projector) at the home of T. Sargent in Leverett, Massachusetts during the
collecting seasons of 1982 through 1985 (Sargent, 1987). Butterflies were collected
by net during the day at distances of up to 10 km from this site. All specimens were
immediately frozen in small jars in the freezer compartment of a household
refrigerator and were thawed just prior to their use in the bird-feeding trials.
A bird-feeding trial consisted of a 15-minute presentation ofsix different species
or distinctive morphs. T h e insects were placed on a light blue dish, 15.24 cm in
diameter, which was set out on an open feeding tray located 1 m from a large glass
door through which observations were made. The observer was approximately
2 m from the feeding tray and recorded the specimens taken, in order, the bird
species taking each insect, and any reaction of the birds after taking the prey (SW,
swallowed-whole; DWE, de-winged and eaten; DWD, de-winged and dropped;
PD, picked up and dropped in place; T D R , taken and dropped later). All feeding
trials were conducted between 06.00 and 08.00 hours EDST, and no more than
four trials were run on any one day. Two hundred and thirteen species of
butterflies and moths were used as prey. O n e hundred and sixty-three of these
species were tested in two or more trials.
Coding of p r q characterislics and data anabsis
All species were coded by Hodges numbers (Hodges et al., 1983) and the
following characteristics were recorded for the species used in 2 or more trials:
(1) size, (2) larval host type, (3) appearance, (4) closed us. open wings, and (5)
the reaction of the bird to capturing prey. Four size classes, based on a mean
wing span (45.0 mm) and standard deviation (14.0 mm), were established:
(1) small, <38.0 mm ( n = 6 4 ) ; (2) medium, 38.1-52.0 mm ( n = 9 4 ) ; (3) large,
53.0-67.0 mm (n=26); and (4) extra large, >67.0 mm ( n = 19).
Species were assigned to one of seven different appearance categories: ( I ) barklike (BK), (2) dead-leaf-like (DL), (3) green-plant-like (GP), (4) warninglycoloured (WC), (5) mimetic (MC), (6) black-and-white (BK), and (7)
298
U B MACLEAN E T A I ,
unclassified ( U N ) . Assignment into these categories was based on our own field
experience and on reports in the literature, but was often necessarily subjective.
The extent to which some of these assignments may be incorrect from the point of
view of the predators in this study will be discussed later. The unclassified
category was used primarily for butterflies that did not fall into one of the other
categories. Wings were also categorized as being open or closed when in the
normal resting attitude of the species in question. Lepidoptera with wings held
over the back or with all four wings exposed were clasified as ‘open’ while those
with hindwings covered by the forewings while at rest were classified as ‘closed’.
Larval host categories were: (1) overall generalist (OG), (2) herbaceous
generalist (HG), (3) herbaceous specialist (HS), (4) woody generalist (WG),and
(5) woody specialist (WS). These assignments were based on published foodplant
records (e.g. Forbes, 1923; 1948; Klots, 1951; Forbs, 1954; 1960; Covell, 1984).
Specialists were taken as those species for which the foodplants known to be
utilized belong to only one family. Because published foodplant information may
sometimes be erroneous or incomplete, some species may have been incorrectly
placed.
The compiled enumeration data were subjected to chi-square analyses to test
the hypotheses that the number of Lepidoptera taken us. not taken were
independent of: butterflies and moths; butterfly families; moth families; prey size,
aspect, larval host type, wing shape, cryptic apearance; and bird behaviour.
The prey characteristics listed above, and the bird species were treated as
dichotomous or binary (0, 1) predictor variables for the discriminant analysis of
feeding trials. Data analysed for each trial were: size, appearance, host, taken or
not-taken, rank-taken, and bird species. The four most abundant bird species in
the trials were the blue jay, Cyanocitta cristata (L.); black-capped chickadee, Parus
atricapillus (L.); white-breasted nuthatch, Sitta carolinensis Latham; and tufted
titmouse, Parus bicolor (L.). Since chi-square analyses showed that the size,
appearance, and larval host type of prey selected were not dependent on their
location on the dish, position was eliminated as a predictor variable.
A total of 37 584 data from 348 feeding trials was subjected to discriminant
analysis. Since criterion groups must be mutually exclusive and few species were
either always taken or never taken, it was necessary to analyse individual feeding
trials. Based on the results of each trial, prey species were assigned to two mutually
exclusive criterion categories: ( 1) taken and (2) not-taken. Discriminant analysis
was carried out by the SPSS program on the Youngstown State University
Amdahl 5868 computer. Methods and options selected were the largest increase
in the generalized distance (Rao’s V) for the selection of predictor variables and
group size (numbers taken and not-taken) for determining the probability of
group membership.
RESULTS
Chi-square analysis of prey characleristics
‘The results of contingency table analyses carried out on prey characteristics
and bird-feeding behaviour are summarized in Tables 1 and 3 . The mean ranktaken (1-6) for prey selected by birds during the trials is given in Table2 for
families of butterflies and moths and prey size, aspect, and larval host type. T h e
only variable dependent upon the position of prey on the feeding tray was rank-
LEPIDOPTERAN PREY CHARACTERISTICS
299
taken for all years except 1982. This was no doubt due to the location of shelter
nearest to positions 1, 2, and 3. Fortunately, size, appearance, larval host, ranktaken and position taken were independent of bird species. Thus, feeding
differences among birds did not affect the interpretation of the results.
The number ofprey taken was highly dependent on the taxonomic category of
the prey (Table 1 ) . Many fewer butterflies and many more moths were taken than
expected. Some families of butterflies (Hesperiidae and Pieridae) and moths
(Lasiocampidae, Sphingidae, Notodontidae, and Lymantriidae) were preferred
over others. Fewer butterflies of the Nymphalidae, Lycaenidae and Danaidae;
and fewer moths of the Limacodidae, Drepanidae, Geometridae and Arctiidae
were taken than expected.
The number of prey taken was also highly dependent on prey shape. Fewer of
the ‘open’ and more of the ‘closed’ shape were taken than expected. T h e number
of prey taken was also highly dependent on crypsis. Many more cryptic prey and
r
1
1ABLE 1. Chi-square analyses of lepidopteran prey taken and not-taken in
two or more of348 bird-feeding trials carried out from 1982 through 1985
in Leverett Massachusetts. Results are givcn for six categories of prey
characteristics
Comparison
Butterflies us. moths
Butterflies
Moths
Butterfly families
Danaidae
Hesperiidae
Lyraenidae
Nymphalidae
Papilionidae
Pieridae
Satyridae
Moth families
Arctiidae
Drepanidae
Geornetridae
Lasiocarnpidae
Limarodidae
Lymantriidae
Noctuidae
Notodontidae
Pyralidae
Saturniidae
Sphingidae
’I‘hyatridae
Wing shapr
Open
Closed
Larval host type
Generalist
Specialist
Appearance
Cryptic
Non-cryptir
* P < 0.05; * * * P < 0.001
d.f.
x2
I
204.28* * *
6
9
1
1
1
No. of Prey
__
Taken
Not-taken
192
1206
180
198
6
14
2
56
27
73
21
11
2
5
61
24
51
21
1I 6
13
186
76
5
63
795
113
14.57*
265.23* **
257.43** *
53
3
60
74
2
15
73
3
5
9
92
0
0
16
3
0
370
1366
266
176
985
708
20 1
209
1285
364
167
275
10.88***
262.81 ***
300
D. B. MACLEAN E T AL.
many fewer non-cryptic prey were taken than expected. Hostplant generalists
were significantly preferred over hostplant specialists. Subdividing the
contingency table revealed, however, that this was due to significantly fewer
herbaceous specialists being taken than expected compared to other specialist
types. Indeed more woody specialists were taken than expected, indicating that
CO ABLE 2. Mean rank (1-6) oflepidopteran prey
taken in two or more of 348 bird-freding trials
carried out from 1982 to 1985 in Ixverctt
Massachusetts. Results are given for eight
categories of prey characteristics
Catcgory
Butterfly families
Danaidae
Hesperiidae
Lycaenidar
Nymphalidae
l’apilionidac
Pirridar
Satyridae
Moth familics
Arctiidae
Drepariidac
Geometridar
Lasiocampidac
Limacodidae
Lymaritriidac
Noctuidac
Notodontidac
Pyralidae
Saturriiidac
Thyatridae
Size
Small
Medium
Large
X-large
Prey aspect
Bark-like BK
Dead-leaf DL
Green-plant GP
Warning colour W C
Mimetic M C
Black & white BW
Unclassificd U N
Larval host type
Overall generalist OG
Herbaceous generalist H G
Herbaceous specialist HS
Woody generalist WG
Woody specialist WS
Wing shape
Open
Closed
Larval host type
Generalist
Specialist
Appearance
Cryptic
Non-cryptic
Mean rank taken
3.89
4.20
3.30
3.50
3.80
4.00
4.30
4.10
3.00
3.80
4.70
4.40
3.80
3.80
4.80
3.10
3.20
3.30
2.20
5.00
3.45
3.10
2.31
3.10
2.56
3.22
3.75
3.07
3.73
3.97
4.12
3.41
3.11
3.77
2.86
2.82
3.84
3.09
3.13
3.29
2.89
3.73
30 1
LEPIDOPTERAN PREY CHARACTERISTICS
birds did not simply avoid taking specialists in general, but only those that
specialized on herbaceous plants.
Among the butterflies, birds took hesperiids, lycaenids, and nymphalids early
in the trials and pierids, danaids, and papilionids later (Table 2). Among the
moths, mean rank-taken indicated a definite preference by birds for the sphingids,
noctuids and notodontids, while lymantriids, drepanids, and geometrids were
usually taken later.
Behavioural reactions to the prey were highly dependent on size, appearance,
and larval host type (Table 3 ) . More small prey were swallowed-whole, and more
large and extra large prey were de-winged and eaten than expected. Many more
bark-like prey were swallowed-whole and fewer dropped than expected. Small
departures from expected were noted for dead-leaf-like prey that were de-winged
and eaten (fewer) and picked up and dropped (more). More green-plant prey
were dropped than expected. Fewer prey than expected were taken for the
warningly-coloured, mimetic, black-and-white, and unclassified appearances.
Among host plant specialists, less than half the number expected were
swallowed-whole and more than twice the number expected were dropped. More
woody generalists and specialists than expected were swallowed whole.
Discriminant analysis of feeding trials
The result of the discriminant analyses are summarized in Table4. The
number of trial solutions is less than the number of trials analysed for any one
year, since a solution could be found only for those trials in which one or more
prey were not taken (62.6%).Also, only those solutions which resulted in an eigen
value > 1.0 were included in the results (47.7%). T h e number of different
predictor variables selected ranged from 12 to 15. T h e percentage of cases
3. Chi-square analyses of five types of bird feeding behaviour and the size, aspect, and larval
host type oflepidopteran prey used in two or more bird-feeding trials carried out from 1982 through
1985 in Leverett Massachusetts. SW = swallowed-whole, DWE = de-winged and eaten,
PD = picked up and dropped in place, TDR = taken and dropped later
'rABLE
Comparison
Prey size
Small
Medium
Large
X-large
Prry aspect
Bark-like BK
Dead-leaf DL
Green-plant GP
Warning-colour WC
Mimics
Black & white BW
Unclassified U N
Larval host type
Overall generalist O G
Herbaceous generalist H G
Herbaceous specialist HS
Woody generalist WG
Woody specialist WS
***P < o.oni.
d.f.
x2
12
90.6***
24
16
sw
DWE
PD
TDR
93
193
22
4
11
74
29
9
52
47
5
3
7
25
4
5
1 in
127
6
48
1
9
3
33
43
7
51
7
14
8
7
17
4
15
0
9
3
2
7
39
22
15
146
83
12
11
4
24
40
20
8
3
12
10
7
120.3***
1
33
4
3
7
54.43***
9
17
61
29
D. B. MACLEAN E T AL
302
TABLE
4. Summary of discriminant analyses of bird-feeding trials
conducted from 1982-85
Number of
Year
1982
1983
1984
1985
'lotals
01
/O
Trials
Analysesd
Solutions'
Var.'
Corr.d
Correc.
class"
46
119
73
110
348
36
61
38
I7
218
29
53
31
53
166
12
15
13
13
13
0.824
0.839
0.844
0.861
0.861
97.1
95.0
97.2
97.5
97.5
"Trials in which one or more prey were not taken.
'"umber of solutions with an eigen value > 1.0.
'Number of predictor variables selected for all analyses.
"Mean values.
classified to the correct criterion group, that is ( 1 ) prey taken and (2) prey nottaken, was very high for all years (95.0-97.5%). Canonical correlation, which
expresses the association between the discriminant function scores and typical
(centroid) values for the two criterion groups ranged from 0.824 to 0.861. Using
the same set of predictor variables, it was not possible to discriminate between
prey taken early (ranks 1-3) and late (ranks 4-6) in the trials.
Relative importance of individual prgy characteristics to the outcome f bird-feeding trials.
A major objective was to identify and rank those characteristics which were
most often associated with prey that were (1) taken and (2) not-taken. The
relative importance of the predictor variables was determined by summing the
number of trials in which each variable was chosen and calculating the mean
classification function coefficients based on all years (Table 5).
The sign and magnitude of the coefficients indicate the relative association of
each variable with the two criterion groups. Characteristics associated with prey
that were taken are ranked according to their mean classification function
coefficients as follows: ( 1 ) large size, (2) bark-like appearance, (3) warning
colouration, (4) woody generalist, (5) dead-leaf-like appearance, (6) woody
specialist, and (7) medium size.
Characteristics associated with prey that were not taken are ranked according
to their mean classification function coefficients as follows: ( 1 ) small size, (2)
mimetic appearance, (3) unclassified appearance, (4) herbaceous specialist, (5)
black-and-white appearance, (6) extra large size, and ( 7 ) overall generalist host
t Y Pe.
Calculating the difference between the classification function coefficients for
each predictor variable resulted in the linear discriminant function given at the
bottom of Table 5. T h e value of the discriminant function that separates the two
criterion groups (Kachigan, 1982; Afifi & Clark, 1984) equals 0.739. Based on this
function, it is possible to predict whether or not a prey species not included in this
study would be taken by a bird. Specimens with scores less than 0.739 would be
assigned to the not-taken category and those with scores higher than 0.739 to the
taken category. For example, a small, black-and-white woody specialist with a
discriminant function score of Z = - 9.3 - 5.9 8.4 = - 6.8 would be assigned to
the not-taken group. A large, bark-like, woody generalist with a score of
Z=5.6+5.3+ 1.8= 12.7 would be assigned to the taken group.
+
LEPIDOPTERAN PREY CHARACTERISTICS
303
TABLE5. Summary of the relative importance of prey
characteristics selected as predictor variables from 166 discriminant
analyses of bird-feeding trials that resulted in an eigen value > 1.0
Criterion groupsb
Predictor variable
___
Size
X , Small SM
X, Medium MED
X , Largc LAR
X, Extra large XLAR
Appearance
Xi Bark-like BK
X, Dead-leaf DL
X, Green-plant GP
X, Warning colour W C
X, Mimetic MC
X,, Black & white BW
X I , Unclassified UN
Larval hosts
XI, Over. gen. O G
X I , Herb. gen. HG
X,, Herb. sper. HS
X I i Woody gen. WG
XI,,Woody spcc. WS
Bird sperics
X I , Blue jay
XI, Chickadee
X I , Titmouse
NO."
Rank
Taken
6.0
I .o
12.5
12.5
0. I
3.9
8.1
0.4
9.4
2.6
2.5
2.3
Not-taken
-
28
60
10
10
30
34
0
11
2
2
8
4.5
3.0
5.2
3.8
-0.1
1.6
11.0
18.0
18.0
15.0
6.9
1.4
-3.0
1.4
1.6
9.4
2.0
7.3
16
2
27
51
30
9.0
18.0
7.0
2.0
4.5
3.9
0.6
1.1
4.6
4.6
5.6
0.6
6.8
2.3
2.8
24
15
7
8.0
10.0
16.0
6.2
3.5
6.7
- 2.2
- 4.5
- 2.6
Z = -9.3 X , + 1 . 3 X,+5.6 X,-1.9 X,+5.3 X,
f 2 2 X,+5.3 X,-8.0 X,-5.0 XI,-5.9 X I ,
- 1.7 X12-5.7 X,,+2.3 X I , + 1.8 XI,
+8.4 X,,+8.0 X,,+9.3 X,,
C (cut-off score) = 0.739
"Number of disrriminant functions with eigen values > 1.0
"can classification function corficients.
T h e probability o f individual lepidopteran prey species being taken ly birds
Results of the discriminant analyses were next used to estimate the probability
of individual prey species being selected during a feeding trial. T h e probability of
classifying a species to the taken criterion group, along with percent taken and
average rank are given in Table6 for 95 species. The probability of species X
being taken (P(X))was calculated as the average of the probability of classifying
it to the correct criterion group (based on the discriminant analysis) and trials in
which all prey were taken. That is, for trials in which all prey were taken, P ( X )
was treated as 1 .O and averaged with the classification probabilities for species X.
Prey acceptability categories (defined for P ( X ) taken) were: ( 1 ) extremely high,
> 0.90; (2) high, 0.70-0.89; moderate, 0.40-0.69; and low, < 0.40. Spearman
rank correlations of correctly classifying a species to the taken group ( P ( X ) )with
percent taken (r,=0.926, n = 9 1 ) and mean rank (r,,= -0.521, n = 9 1 ) were
highly significant (P<0.001). The P ( X ) taken values, derived from the
classification phase of each discriminant analysis, were used to express the relative
acceptability of the lepidopteran species used as prey in the feeding trials.
Since species of moths and butterflies represent different sizes, appearances, and
larval host types, we attempted to access which combinations of prey
6. Prey charactcriatics, percent taken, average rank, a n d probability of correctly classifying
rach prey individual (X) as a membcr of the taken category for 95 species of Lepidoptera
' rA B L E
'Iaxon"
Species
Nb Size
Aspccl
Host" ?,, Taken Av. rank 1' ( X ) taken
__
__ .~
_
~
Hespcriidae
3870 Epargyreus clarus
Papiljonidac
4 I59 Papilio pnlyxenes
41 76 Papzlio glaucus
4181 Pupilin lroilus
Picridac
4197 Artogeiu rapae
4209 ( , M a s philodice
42 10 Colius eurytheme
1,ycarnidac
4282 Sdyrium calanus
Nymphalidac
Pobpnia comma
442 I
4434
Vunessa uirginiensi3
4436
C'anma atalnnta rubria
4440 Juonia covnia
445 1 Spvyeria aphrudite
448 1 Phyciode.r lharos
45 16 Euphydyyas phueton
4523 Hasilarchia archippus
Satyridae
458 7 Ceryonir pe&a
Danaidac
461 4 Danaus plexippu,hus
15 MED
DL
WS
73.3
3.6
0.827
14 XLAR
19 XLAR
18 XLAR
UN
IJN
UN
HS
M'G
WS
57.1
36.8
66.7
4.2
3.4
4.4
0.381
0.462
0.788
43 MED
59 MED
19 MED
UN
UN
UN
HS
HS
HS
46.5
56.5
73.7
4.3
4.2
4.5
0.528
0.654
0.738
7 SM
DL
WS
28.6
3.5
0.377
DL
DL
DL
DL
UN
DL
WC
MC
OG
HS
HS
HG
HS
HS
HS
WG
80.0
44.4
85.7
50.0
60.0
27.3
25.0
51.7
3.8
3.3
3.6
3.8
3.0
5.7
3.3
3.5
0.809
0.694
0.744
0.667
0.674
0.417
0.395
0.515
23 LAR
DI,
HS
73.9
2.9
0.768
17 X I A R
WC
HS
35.3
4.2
0.358
11 SM
BW
OG
27.3
3.3
0.306
10 SM
DL
DL
WS
WC
20.0
57.1
5.0
4.8
0.01 1
0.777
4.2
3.1
5.3
4.6
2.8
4.1
2.4
4.3
4.1
3.2
4.0
1.7
0.578
0.992
0.607
0.775
0.790
0.683
0.998
0.460
0.838
0.852
0.836
0.608
5
9
14
8
5
11
12
29
MED
MED
MED
LAR
XLAR
SM
MED
XLAR
I'yralidac
5159 DrsmiaJuneralis
Drepanidar
625 I
Drepana arcualu
6252 Drepana hilineata
Gcometridac
6583 Anacnmptode.c ephyraria
6720 lgtrosis unitaria
6739 Eurhlaena irrariu
6743 Xanthn&pe sospeta
6753 Pero honv.rtnriu
6796 Campueu perlatn
6797 Ennorno.! mngnaria
6798 Ennomos subsignaria
6864 Curipeta piniatu
6982 ProchoerodeJ transuer.snta
7290 Co?yphista meadii
729 1
Hydria undiulata
Lasiocampidae
7670
'Tolype iirlleda
7687 PlyllodeAma americana
7698 Malacosoma disJtriu
770 1 Malacosoma americanum
Saturniidac
7 7 15 Dvocampn rubicurida
7732 I-lemilvura lucina
Sphingidae
Ceratomia undulosa
7787
7824 Pa'aonius excaecntus
7825 Paonias myops
7827 1.aothoejuglnndir
7853 Hemaris thysbe
7 SM
11
18
11
17
14
41
16
14
25
37
II
6
SM
MED
MED
MED
SM
MED
MED
SM
SM
MED
SM
SM
BK
BK
D1,
DL
DL
BW
DL
BW
DL
DL
DL
D I,
WG
WG
OG
WG
WG
WG
WG
WS
OG
WS
WG
60.0
100.0
54.5
70.6
78.6
53.7
100.0
42.9
72.0
89.2
72.7
50.0
7
I1
13
46
MED
MED
SM
BK
DL
DL
111,
WG
WG
WG
WG
100.0
90.9
100.0
93.5
4.3
3.8
4.2
2.6
0.974
0.931
0.977
0.948
56 MED
11 MED
WC
wC
WG
WS
83.9
63.6
3.2
4.9
0.881
0.395
15 XLAR
BK
DL
DL
DL
R.I C
WG
100.0
100.0
100.0
100.0
37.5
1.5
1.5
2.0
2.4
3.3
0.981
0.988
0.906
0.875
0.488
SM
1 1 XLAR
8 LAR
7 LAR
8 MED
wc,
M'G
WG
WG
WG
_
‘1ABLE
Taxon“
Sprcics
Notodontidae
792 1
Peridea,ferruginea
Arctiidae
8090 Hypoprepiafucosa
8 107 Haploa clymenp
8 11 1
Haploa lecontei
8 1 14 Holomelina laeta
8129 Pyrrharctia i,sabella
8 134 Spilosoma congrua
8 137 Spilosoma uirginica
8203 Halysidota tessellaris
8230 Cjcnia tenera
8262 Ctenucha uirginica
8267 Ci.r.wpsfulvicolli
Lymantriidae
8302 Dasychira oblz~quata
83 16 Orgyia leucostigma
83 18 Igmantria dispar
Noctu idae
8587 Panapoda rujimarp
8588 Paaapoda carneicoJta
864 1 Qnedoida grandirena
8719 Euparthenos nuhilis
8727 Parallelia b i h a r t s
8739 Caenurgina erechtea
8781 Catocala judith
8788 Catocala retecta
8795 Catocala palaeogama
8848 Catocala andromedae
8857 Catocala ultronia
8858 Catocala crataegi
8878 Catocala amica
8904 Chrysanympha formosa
8924 Anagrapha falcijera
9183 t’anthea pallescens
9189 Charadra deridens
9200 Acronicta americana
9207
Acronicta irinotata
928 1 Agriopodes Jallax
9480
Papaipema ptersii
9483 Papaipema inqumsita
9546 Phlogophora iris
9556 Clytonix palliatricula
9638 Amphipyra pyramidoides
Sunira bicoloraga
9957
10067 Adita chionanthi
10300 Lacanobia grandis
10438 Pseudaletia unipuncta
10524 Nephelodes minians
10651 Agrotis uenerabilis
10663 Agrotis ipsilon
10670 Feltia jaculijera
10942 XeJtia adela
10969 Anomogyna dilucida
1 1 164 Schinia Jorida
N’
6-continued
Size
20 MED
19
7
17
3
47
34
17
63
3
7
8
SM
MED
MED
SM
LAK
MED
MED
MED
SM
MED
SM
yo Taken
Aspect‘
Host”
BK
WS
95.0
MC
WG
OG
OG
OG
OG
HG
OG
WG
HS
Av. rank P ( X ) taken
2.7
0.919
MC
HG
0.0
42.9
11.8
0.0
93.6
91.2
94.1
88.9
33.3
52.1
0.0
14 MED
7 SM
51 SM
UL
DL
DL
WG
OG
OG
92.9
85.7
84.3
2.6
3.8
3.1
0.697
0.834
0.770
15 MED
10 MED
DL
DL
7
10
42
6
DL
BK
D I,
DL
BK
BK
BK
BK
BK
BK
BK
DL
BK
BK
BK
BK
BK
GP
DL
DL
DL
BK
BK
DL
BK
BK
DL
DL
BK
BK
DL
DL
BK
G 1’
WS
WG
WS
WS
WG
HG
WS
WS
WS
WS
WS
WS
WS
WS
OG
WS
WS
WG
WG
WS
HS
93.3
90.0
57.1
100.0
78.6
83.3
100.0
3.4
3.7
2.5
1.6
3.4
3.0
2.5
2.0
3.0
3.2
2.4
2.9
2.8
3.0
3.6
2.3
2.7
1.7
3.3
5.0
3.4
1.6
3.6
2.6
2.4
3.8
1.8
2.6
3.6
2.4
3.2
I .9
2.0
2.9
3.3
4.2
0.905
0.917
0.613
0.977
0.741
0.787
0.914
0.972
0.784
0.981
0.876
0.857
0.901
0.760
0.989
0.969
0.937
0.980
0.823
0.61 1
0.989
0.993
0.799
0.599
0.890
0.987
0.971
0.997
0.997
0.993
0.995
0.998
1.0
0.997
0.987
0.797
10
14
8
13
369
9
30
6
8
150
35
45
36
5
11
5
11
9
60
8
6
7
13
12
16
14
7
14
19
10
SM
LAR
MED
SM
MED
XLAR
LAK
MED
LAK
MED
MED
SM
SM
MED
MED
LAR
MED
SM
SM
SM
MED
SM
MED
SM
SM
MED
MED
MED
MED
MED
SM
MhD
MED
SM
wc
WC
WC
wc
WC
wc
WC
WC
MC
HG
HS
HG
WG
OG
WS
WG
OG
HS
HG
HG
OG
OG
WS
HS
100.0
100.0
100.0
100.0
88.9
93.3
83.3
100.0
98.6
97.1
100.0
83.3
62.5
100.0
100.0
81.6
55.6
98.3
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
90.0
3.7
5.0
2.6
3.4
3.7
2.7
5.0
4.8
0.190
0.190
0.269
0.044
0.878
0.927
0.997
0.852
0.371
0.587
0.148
“Taxon = Taxon No. (Hodges et ul., 1983).
‘N = Number of trials.
‘Aspect = appcararice category: BK, bark-like; DL, drad-leaf-likr; GP, green-plant-like; WC, warningly
coloured; MC, mimrtir; BW, black-and-whitr; IJN, unclassified.
“Host = larval host categwy: O G , overall generalist; HG, herbareous generalist; HS, herbaceous specialist;
WG, woody generalist; WS, woody specialist.
306
1).B. MACLEAN
87 A L
characteristics were least acceptable and most acceptable to birds. Based on the
classification function coefficients listed in Table 5, combinations that should be
least acceptable to birds are in order: ( 1 ) small black-and-white species; (2) small
mimetics; ( 3 ) extra large butterflies; (4) extra large bark-like species; (5) mediumsized black-and-white species; (6) extra large mimetics; (7) medium-sized
butterflies; and (8) extra large warningly-coloured species. Overall, herbaceous
specialist species with any of these size-aspect characteristics should be even less
acceptable to birds.
Combinations that would be most acceptable to birds are: ( 1 ) large, warninglycoloured species; (2) large, bark-like species; (3) large, dead-leaf-like species; and
(4)medium-sized, warningly-coloured species. Overall, woody generalists and
specialists with any of these characteristics would be most acceptable to birds.
Based on the discriminant function coefficients, other combinations of prey size,
aspect, and larval host type would not be strongly selected for or against.
Pyrrharctia isabella (47 specimens), Halysidota tessellaris (63 specimens), Spilosoma
conqrua (34 specimens), Spilosoma uirginica ( 17 specimens) and Dryocampa rubicunda
(56 specimens) were initially placed in the warningly-coloured category for all
analyses and yet 93.6, 88.9, 91.2, 94.1 and 83.9 percent respectively of these
species were taken. These apparently palatable species, initially classified as
warningly-coloured, accounted for the large positive classification function
coefficient (6.9) of this trait.
Analysis of p r 9 acceptabilip by multiple regression
The relationship between the number of a prey species taken and its abundance
(expressed by N, the number of specimens of each species tested in the trials) and
relative acceptability to birds (as determined by the discriminant analysis) was
evaluated by stepwise multiple regression. Arcsine transformed values of the
percent taken for 90 species listed in Table6 were chosen as the dependent
variable. The square root ( X ’ = sqrt ( X + 0.5)) of the number of specimens used
for each species in the trials and the discriminant function coefficients for size,
aspect, and larval host (Table 5) were selected as independent variables. All
variables were standardized prior to the regression analysis.
The results of the regression analysis are given in Table 7 for the six most
important single variables and interaction terms. While the regression accounts
for less than 25% of the variation of percent taken (R2=0.208), analysis of
variance showed it to be significant (F=3.641, P < 0.01). The order ofselection of
the variables (given in parentheses) and their standardized partial regression
coefficients (Beta) show that size (X,) was the single most important characteristic
in determining prey selection. The other variables, in order of selection, were:
Size X Host (X,X,), Size X Aspect X Host (X,X,X,), Host (X5),Aspect (X5),
and abundance (N) X Aspect (X,X,). The inclusion of two interaction terms
early in the analysis indicated that combinations of prey characteristics were
better predictors of prey taken than single variables with the exception of size.
DISCUSSION
Results of this study demonstrate the ability of discriminant analysis to select
and rank those characteristics associated with species of Lepidoptera that were
most often, or least often, taken by birds during feeding trials. Most characteristics
LEPIDOPIERAN PREY CHARACTERISTICS
307
'I'ABLE
7. Summary of stepwise multiple regression analysis of transformed values of percent taken
for 90 species of Lepidoptera prey used in the feeding trials. Independent variables were the number
of spccimens of each species used in the trials and its discriminant function coefficients for size,
aspect, and larval host type. Results are given for only the six most important variables and
interaction terms. The order of selection of each variable is given in parentheses
Variable
X,!, Size ( 1 )
X,, Aspect (5)
Xi, Host (4)
B
-- _____
B
S.E
F
0.292
0.110
0.156
0.292
0.1 10
0.156
0.100
0.104
0.098
8.41
1.12
2.53
0.102
0.104
0.100
I .04
0.142
0.139
0.101
1.99
-0.164
0.136
2.75
X
2
&
N x Aspect (6)
X.IX,,
Size x Host ( 2 )
XIXJi,
Size x Aspect x
Host (3)
Constant
- 0.225
0.021
Multiple R = 0.456
R z = 0.208
S.E. = 0.921
Source
DF
SS
MS
F
Rrgression
Residual
6
83
18.537
70.430
3.089
0.848
3.641**
**P < 0.01.
were associated with both criterion groups which reflects the fact that few species
were either always taken or never taken. Lepidopteran characteristics based on
adult size, appearance and larval host type represent a continuum ranging from
species with low prey acceptability to ones with high prey acceptability.
Any attempt to evaluate the relationship between bird predation and prey
characteristics should also take into account the relative abundance of prey
species. T h e results of the stepwise multiple regression analysis showed that
relative abundance alone was not as important as size and combinations of prey
characteristics in determining the number of prey selected by birds. Apparently,
birds selected moths and butterflies largely on the basis of size and larval host
type regardless of their relative abundance. The stepwise selection of variables
showed that abundance was important in determining prey selection only when
combined with aspect.
Since size was selected as the single most important predictor of the number of
prey taken, it was not surprising that small and extra large species were taken
much less often than medium-sized species. Small conspicuous species (blackand-white and mimetics) were most often least acceptable to birds, e.g. Desmia
funeralis and Ennomos subsignaria. Small mimics of non-lepidoptera were often not
taken at all (e.g., Cisseps fulvicollis, a wasp mimic; and Hypoprepia fucosa, a firefly
mimic), Small black-and-white species were also highly unacceptable, though in
some cases these may actually have been warningly-coloured (e.g., Desmia
funeralis) or mimics of bird-droppings (e.g. Cernia cerintha). Small size even
reduced the acceptability of species with bark-like apperance, a trait that
attracted extremely heavy predation when associated with medium and large
sizes. Extra large size was most effectively combined with warning colouration
308
D. B. MACT,E.L\N E T A L .
and mimicry and less so with the unclassified butterfly aspect to reduce prey
acceptability. However, extra large wing size did not reduce the acceptability of
bark-like species which were heavily preyed upon.
Prey acceptability was generally highest for medium and large bark-like,
dead-leaf-like, and warningly-coloured species. Since birds swallowed more
small and medium prey whole, and de-winged more large and extra large prey
than expected, large adult wing size may help deter predation by small birds
such as chickadees. This warrants further investigation. T h e time and effort
required to de-wing a large adult lepidopteran may not be repaid in terms of
energy acquired, especially for small- or thin-bodied prey, like many butterflies.
Predators might than concentrate on smaller, large-bodied prey that yield more
energy because of their shorter handling times. This question of the costs of
handling time in prey selection is considered in many optimal foraging models
(e.g., Krebs, 1978; Erichsen et al., 1980) and the data reported here suggest that
this cost may be an important factor in assessing lepidopteran acceptability to
predators. Certainly, there are opportunities for experimental manipulation of
this variable in future studies.
The relative importance of larval host type as a predictor of prey acceptability
(three of the six variables in the regression analysis presented in Table 7 include
host) was somewhat surprising since this trait would not seem to be directly
ascertainable by predators, based on visual characteristics of adult Lepidoptera.
It may be, of course, that the predators in this study had previously learned to
associate palatability or unpalatability with many of the specific prey presented,
and that this in turn reflected differences in the larval hostplant utilization of
these prey. This may be reflected in the interaction terms in Table 7 where host
is combined with size and aspect.
General plant/herbivore theory suggests that unpalatability in insects will be
most often associated with larval specialization on “non-apparent” herbaceous
plants (e.g. Feeny, 1975; Futuyma, 1983; Huheey, 1984), and insects in this
foodplant category were the least acceptable in the present study. It is also
interesting to note that prey in this category were significantly more often picked
up and dropped than were prey in any other category, suggesting that olfactory
or gustatory cues to unpalatability may have contributed to the relatively low
acceptability of these herbaceous specialist prey.
“Warning colour” was assigned to many brightly coloured species which were
thought to be unpalatable to birds. However, direct evidence that secondary
plant chemicals sequestered from host plants provide a degree of protection
against avian predators has been established for only a few warningly coloured
species, e.g. Danaus spp. (Brower et al., 1968; Brower & Glazier, 1975) arid
Euphydryas phaeton (Bowers, 1980). Additionally, some species which do not
sequester chemicals may also be unpalatable (Blum, 1981). Recent reviews of the
immense literature on chemical defences o finsects include Duffey (1980), Brower
(1984) and Bowers ( 1988).
The high prey acceptability of some presumed warningly-colourcd species used
in the present study raises questions concerning the appropriateness of the
LL
warning colour” designation in such cases. Four arctiids, (Pyrrharctin isnbella,
Spilosoma congrua, S. virginica, and Halysidola tessellaris) , and one saturniid
(Dryocampa rubicunda) , were considered warningly-coloured due to their brightly-
LEPIDOPTERAN PREY CHARAC1'EKIS'I'ICS
309
coloured wings or spotted abdomens and, in some cases, prior reports of
unpalatability (e.g. H. lessellaris to bats (Dunning & Roeder, 1965; Dunning,
1968; Watson, 1980), and Spilosoma species to birds (Rothschild, 1983)). This
interesting, if unexpected, result suggests that warning colouration cannot be
inferred on the basis of appearance, at least with respect to the avian predators
considered here. In the future, some of these species should undoubtedly be
classified as dead-leaf cryptics (e.g., P. isahella, H. tessellaris), but others are not
clearly assignable to any o f t h e present appearance classes (e.g.,D.rubicunda, both
Sflilosoma species), and should be studied further.
The following summary attempts to answer the five questions stated in the
introduction. Discriminant analysis found satisfactory solutions for 47.7 percent of
the bird feeding trials based on prey size, appearance, and larval host type. If only
those trials in which one or more prey were left arc considered (2 18), the number
of trials in which a satisfactory solution was obtained increases to 76 percent.
Based on the sign and magnitude of the mean classification function coefficients
derived from the 166 discriminant analyses, those prey characteristics most
acceptable to birds were: ( 1 ) large size, (2) bark-like appearance, (3) warning
colouration, (4) woody generalist, (5) dead-leaf-like appearance, (6) woody
specialist, and ( 7 ) medium size. Prey characteristics least acceptable to birds were:
(1) small size, (2) mimicry, ( 3 ) unclassified (butterfly) appearance, (4)
herbaceous specialist, (5) black-and-white appearance, (6) extra largc size, and
(7) overall generalist feeder.
The 166 solutions correctly classified 97.5 percent of the prey specimens used
in these trials as either taken or not taken by birds.
Combinations or prey characteristics most acceptable to birds were: ( 1 ) large,
warningly-coloured species, (2) large, bark-like species, ( 3 ) large, dead-leaf-like
species, and (4) medium-sized, warningly-coloured species. Combinations least
acceptable to birds were: ( 1 ) small, black-and-white species, (2) small, mimetics,
( 3 ) extra large, butterflies, (4) extra large, bark-like species, (5) medium-sized,
black-and-white species, (6) extra large, mimetics, (7) medium-sized, butterflies,
and (8) extra large, warningly-coloured species. Except for several large and
medium-sized species that may have been mistakenly placed in the warninglycoloured appearance category, the probabilities derived from the discriminant
analyses accurately predicted the number of trials in which species were taken by
birds.
Based on our results, we feel that discriminant analysis offers a n excellent
means of evaluating the many complex behavioural and ecological variables
present in predator-prey interactions. While many more data will be needed to
test the validity of the regression equation presented here as an empirical model,
the results of this study will hopefully provide insight into multispecies
interactions between adult Lepidoptera and birds. The importance of birds in
the natural regulation of insect prey is poorly understood. Most models of insect
population dynamics have been based on single species and have concentrated
on the effects of insect predators, parasitoids, and abiotic factors. Therefore,
knowledge of how bird behaviour is affected by sizes, appearances, and larval
host types of many prey species could improve the reality and thus the accuracy
of predator-prey models. Studies such as this may serve best in posing and
refining questions that stimulate more rigorous experimental tests in the future.
310
D. B. MACLEAN E T AL.
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