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SHORT COMMUNICATIONS 203
The Condor 99:‘203-206
0 The
Cooper
Ornithological Society 1997
SINGING FOR YOUR SUPPER: ACOUSTICAL LURING OF AVIAN
PREY BY NORTHERN SHRIKES’
ERIC
C.
ATKINSON’
Raptor Research Center, Boise State University, Boise, ID 83725
Abstract:
Northern Shrikes (Lanius
excubitor) are
predatory songbirds in which both sexes sing much
of the year. I experimentally tested the hypothesis
that winter singing by Northern Shrikes serves the
purpose of attracting small passerines to be captured
as prey. I broadcast Northern Shrike song for 5 min
while recording the number of small passerines that
approached the tape player, time taken for approach
to occur, nearest approach, and mean number of call
notes given by each small passerine observed. A
blank tape and the song of American Robin
(Turdus
migratorius)
were controls. Treatment effect was sig-
nificant for number of passerines observed, time
taken for approach, and nearest approach, but not for
the number of call notes given by each passerine.
More small passerines were observed during the
Northern Shrike song and these songbirds responded
more quickly and approached more closely than dur-
ing the control and robin treatments. These results
support the hypothesis that Northern Shrikes acous-
tically lure prey.
Key words: Northern Shrike,
Lanius excubitor,
foraging, singing, acoustical luring, avian prey.
The production and function of song in birds has long
been a field of great interest. Singing has been shown
to serve multiple functions including acquisition and
maintenance of mates and territories, maintenance of
social structure, and synchronization of breeding ac-
tivities. Northern Shrikes (Lanius
excubitor)
main-
tain breeding and nonbreeding territories, and facili-
tate pair formation through winter singing (Miller
1931, Bent 1950, Atkinson 1991, 1993). Both sexes
sing (Miller 1931, Atkinson 1991). The winter song
of this species has been described as mimetic and is
quite variable. The repertoire consists of warbles,
trills, bzeeks, rattles, and whining calls similar to
begging and alarm vocalizations (Miller 1931, Bent
1950, Cade 1962, Atkinson 1991). In fact, portions
of the winter songs are quite reminiscent of the alarm
vocalizations given by chickadees
(Parus
spp.) and
nuthatches
(Sittu
spp.) (pers. observ.).
It was suggested more than 500 years ago (Bake
1
Received 3 May 1996. Accepted 5 November
1996.
’ Present address: Hawk Mountain Sanctuary As-
sociation, 1700 Hawk Mountain Rd., Kempton, PA
19529, e-mail: atkinson@hawkmountain.org
of St. Albans)
and again in the last century that North-
em Shrikes may attract small passerines within at-
tack range by imitating their calls and portions of
their songs (Witchell 1896, Armstrong 1973). Results
of such imitation have not been documented. I tested
the premise that the winter song of Northern Shrikes
could serve a purpose beyond territory advertizement
and mate solicitation; namely, that it may lure po-
tential prey (i.e., small passerines) within close prox-
imity.
METHODS
To test the effect of Northern Shrike song on attraction
of potential prey, I applied three treatments: shrike =
5 min of Northern Shrike song; robin = 5 min of
American Robin
(Turdus migrutorius)
song; and con-
trol = 5 min of blank tape to control for mechanical
noises involved in playback. Each treatment was fol-
lowed by a 2.5 min “cleansing” period during which
no treatment occurred at the site before the subsequent
treatment was applied. I performed field work during
the morning hours in March and early April 1994.
I played six unique treatment sequences (control,
shrike, robin; control, robin, shrike; shrike, robin,
control; shrike, control, robin; robin, shrike, control;
and robin, control, shrike) to control for order of pre-
sentation (Milliken and Johnson 1992). One se-
quence was selected randomly with the roll of a die
and played at one of 18 individual sites. Each
sequence was presented a total of three times. I se-
lected sites in riparian areas near Boise, Idaho, USA
that typified areas inhabited by Northern Shrikes dur-
ing the winter (Atkinson 1993). Selecting sites only
within brushy riparian areas served to reduce envi-
ronmental variation; for example, passerine assem-
blages and vegetation were similar among sites.
To attempt to control for specific voice character-
istics of particular individuals, I used composite tapes
of more than one individual (Kroodsma 1990, 1992).
Since recordings of Northern Shrike songs are rare,
the Northern Shrike tape contained a composite of
the songs of two different individuals; one from the
local area (15 km southeast of the study area, March
1990) and one from Alaska (Cornell Laboratory of
Natural Sounds). The American Robin song con-
tained portions of two songs taped locally. Tapes
were played at 80% of maximum volume on a Bell
and Howell Model 3179A portable cassette player.
This level approximated the normal volume of a sing-
ing Northern Shrike as heard from 20-30 meters.
From a hidden position, I recorded the following
response variables during the 5 min of playback of
204 SHORT COMMUNICATIONS
each tape: number of small passerines observed
within 15 m of the speaker, time (set) until a small
passerine first approached to within 15 m of the
speaker, nearest approach (m) to the speaker, and
mean number of calls given per small passerine dur-
ing each treatment. I tallied a “small passerine” when
an individual of a potential prey species [birds as
small or smaller than a European Starling
(Sturnus
vulgaris)
(Atkinson and Cade 1993)] was observed
within a 1.5 m radius of the speaker. After the treat-
ment sequence, I measured nearest approach dis-
tances to the nearest 0.5 m with a field tape mea-
sure.
STATISTICALANALYSES
I square-root transformed all variables to approxi-
mate normal distributions. Neither multicollinearity
nor singularity between response variables was
present. I applied multivariate analysis of variance
(MANOVA) to test for effects of order of song pre-
sentation and carry-over effects of one sequence to
another (PROC GLM, SAS Institute 1989, Milliken
and Johnson 1992). When both carry-over and order
of presentation effects were nonsignificant, I pro-
ceeded with analysis of variance (ANOVA) to assess
the significance of treatment effects. I subsequently
contrasted Northern Shrike with other treatments
(American Robin and control tape) using a
priori
lin-
ear contrasts within PROC GLM.
RESULTS
I observed Cedar Waxwings
(Bombycilla cedrorum),
European Starlings, Dark-eyed Juncos (Bunco
hyemalis),
Song Sparrows
(Melospiza melodia),
White-crowned Sparrows
(Zonotrichia leucophrys),
American Goldfinches
(Carduelis tristis),
Pine
Siskins (C. pinus), and House Finches
(Carpodacus
mexicanus)
during the experiment. Individuals of
these species are eaten by Northern Shrikes (Cade
1967, Atkinson and Cade 1993). During this experi-
ment, small passerines approached as individuals,
never arriving in flocks owing to the early spring tim-
ing of the field work.
Order of song presentation and carry-over effects
were not signif&t (MANOVA; Wilks’ Lambda =
0.42,
F ?nT”c; =
1.04.
P =
0.43. and Wilks’
G1,, I “.”
Lambda = 0.20,
F4,,8,.5 =
1.08,
P =
0.38, respec-
tively), so each univariate ANOVA could be inter-
preted directly. Treatment had a significant effect on
number of passerines observed
(F,,,, =
7.6,
P <
O.Ol), time taken for approach
(F,,,, =
4.9,
P =
0.03), and nearest approach
(F,,,, =
8.1,
P <
O.Ol),
but not on the mean number of call notes given by
each passerine
(F,,, = 1.2, P =
0.34). I observed
more small passe&es during the Northern Shrike
song (F,,, = 15.8,
P <
O.Ol), and these songbirds
responded more quickly
(F
- 6.0,
P =
0.03) and
approached more closely (F -
, 5 =
16.3,
P <
0.01)
than during the control and robin treatments (Fig. 1).
Each passerine, however, did not vocalize at a greater
rate during the shrike song than during the other
treatments
(F,,, =
0.7,
P =
0.43). These pairwise
comparisons refer only to the
a priori
linear contrasts
tested between shrike song treatment and “other”
(control and robin) treatment.
DISCUSSION
I demonstrated for the first time that the song of
Northern Shrikes lures avian prey, a function be-
yond serving as mate solicitation and territory adver-
tisement (Cade 1962, 1967, Atkinson 1993). More
small passerines approached the source of the shrike
song, these songbirds came more quickly, and
approached more closely than during American
Robin and control treatments. Small passerines can
make up a significant portion of the winter diet of
Northern Shrikes especially in areas with extended
snow cover (Atkinson and Cade 1993, unpubl. data);
therefore, luring such birds into proximity may in-
crease opportunities for prey capture (Cade 1962,
Denson 1979).
Shrikes are not alone in their capacity for attract-
ing prey species. Higuchi (1986, 1988a, 1988b) and
Preston et al. (1986) described bait-fishing by Green-
backed Herons
(Ardeola striata).
Through this
method, individual herons generally employ the use
of manmade articles, twigs, or live insects placed
upon the water’s surface to attract small fish to within
striking distance. Smith (1969) described a technique
by which forest falcons
(Micrastur
spp.) attracted
avian prey. Like shrikes, these predators perch hid-
den in vegetation while giving calls that seem to at-
tract passerines searching out the source of the calls.
Smith observed three attacks resulting from such be-
havior. Pollard (1930) noted that Australian Grey
Butcherbirds
(Cracticus torquatus)
appeared to
mimic vocalizations of prey species, thereby attract-
ing these birds. Finally, Great-homed Owls
(Bubo
virginianus),
Gymnogenes
(Polyboroides typus),
and
Northern Harriers
(Circus cyaneus)
exploit mobbing
as potential hunting techniques (Dens& 1979, Thu-
row and Black 1981, Bildstein 1982). However. be-
cause small passerines in my experiment did not emit
alarm calls at high rates and approached shrike songs
as rapidly as they approached control tapes, it appears
that they were more inquisitive regarding the source
of the vocalizations, rather than perceiving the shrike
song as an indication of danger and as a predator to
mob.
Both male and female Northern Shrikes sing in
winter from exposed territory-advertisement perches
as well as from perches low and hidden in brushy
vegetation (Miller 1931, Bent 1950, Olsson 1984, At-
kinson 1991, 1993). It is during the latter instances
that luring of passerine prey may be most effective.
Songbirds tend to flit about in such situations
attempting to search out the singing shrike. In four
natural instances, I observed shrikes seizing these
moments to make attacks, two of which were suc-
cessful. In each case, I observed a Northern Shrike
singing that was then surrounded by small flocks of
passe&es (Dark-eyed Juncos, American Gold-
finches, and Pine Siskins). Some individuals ao-
proached to within 1 m of the shrike during each in-
stance. After several moments, each shrike suddenly
stopped singing, causing all vocalizing by the small
passerines to cease. At this time the shrikes flew
swiftly and directly at the prey, capturing an Ameri-
can Goldfinch on one occasion and a Dark-eyed
Junco on another. Further study may be able to iden-
SHORT COMMUNICATIONS 205
b)
120 -
110 -
100 -
90 -
Z -
m 80 :
2
$j 70 -
s 60:
8
F4 50 -
3
2 40 -
w
30 -
20 -
IO -
01
o-
OF
CONTROL ROBW SHRIKE CONTROL ROBIN SHRIKE CONTROL ROBIN SHRIKE
TREATMENT
FIGURE 1. (a) Number of small passerines observed during each treatment, (b) time (set) before initial
observation of a small passerine during each treatment, and (c) nearest approach (m) to the tape player made
by a small passerine during each treatment. All responses were recorded within a 15 m radius centered on
the tape player. Means ? SE.
tify how commonly Northern Shrikes employ this
method of hunting in addition to describing which
portion of the song elicits these responses and
whether specialized vocalizations (i.e., mimicry) are
used.
I thank L. Bond, J. Belthoff and J. Munger for sta-
tistical advice. The reviews of M. L. Atkinson, T. Bal-
gooyen, M. Beecher, J. Belthoff, R. Gerhardt, C.
Haas, W. Koenig, P. Stoddard, and three anonymous
reviewers are greatly appreciated. I thank the World
Center for Birds of Prey (The Peregrine Fund) for
providing office space while this study was per-
formed.
LITERATURE CITED
ARMSTRONG, E. A. 1973. A study of bird song. Do-
ver Publications, New York.
ATKINSON, E. C. 1991. Winter ecology of Northern
Shrikes (Lanius
excuhitor)
in Idaho: foraging,
territories, and niche overlap with American
Kestrels (F&o
sparverius).
M.Sc. thesis, Boise
State Univ., Boise, ID.
ATKINSON, E. C. 1993. Winter territories and night
roosts of Northern Shrikes in Idaho. Condor
95515-527.
ATKINSON, E. C.,
AND
T. J. CADE. 1993. Winter for-
aging and diet of Northern Shrikes in Idaho.
Condor 95528-535.
BENT, A. C. 1950. Life histories of North American
wagtails, shrikes, vireos, and their allies. U.S.
Natl. Mus. Bull. 197.
BILDSTEIN, K. L. 1982. Responses of Northern Har-
riers to mobbing passerines. J. Field Omithol.
53:7-14.
CADE, T. J. 1962. Wing movements, hunting, and
displays of the Northern Shrike. Wilson Bull.
74:386-408.
CADE, T. J. 1967. Ecological and behavioral aspects
of predation by the Northern Shrike. Living Bird
6:43-86.
CATCHPOLE, C. K. 1982. The evolution of bird
sounds in relation to mating and spacing be-
havior, p. 297-319. In D. E. Kroodsma, E. H.
Miller and H. Ouellet [eds.], Acoustic commu-
nication in birds, Vol. I. Academic Press, New
York.
DENSON,
R. D. 1979. Owl predation on a mobbing
crow. Wilson Bull. 91: 133.
HIGUCHI, H. 1986. Bait-fishing by the Green-backed
Heron
(Ardeolu striatu)
in Japan. Ibis 128:285-
290.
206 SHORT COMMUNICATIONS
HIGUCHI, H. 1988a. Bait-fishing by Green-backed
Herons in south Florida. Florida Field Nat.
16:8-9.
HIGUCHI, H. 1988b. Individual differences in bait-
fishing by the Green-backed Heron
(Ardeola
striafu)
associated with territory quality. Ibis
130:39-44.
KROODSMA, D. E. 1990. Using appropriate experi-
mental designs for intended hypotheses in ‘song’
playbacks, with examples for testing effects of
repertoire sizes. Anim. Behav. 40: 1138-l 150.
KROODSMA, D. E. 1992. Much ado creates flaws.
Anim. Behav. 44:58G582.
MILLER, A. H. 193 1. Systematic revision and natu-
ral history of the American shrikes (Lanius).
Univ. Calif. Publ. Zool. 38: 1 l-242.
MILLIKEN, G. A.,
AND
D. E. JOHNSON. 1992. Analysis
of messy data, Vol. I: designed experiments.
Chapman and Hall, London.
OLSSON, V. 1984. Varfagelns
Lank excubitor
vin-
tervanor. I. Biotop. Var Fagelvarld 43: 113-124.
PRESTON,
C. R., H. MOSELEY,
AND
C. MOSELEY. 1986.
Green-backed Heron baits fish with insects. Wil-
son Bull. 98:613-614.
POLLARD, J. 1930. Whisper songs. Emu 30:62-63.
SAS INSTITUTE, INC. 1989. SAS/STAT user’s guide,
version 6, 4th ed., Vol. 2. SAS Institute, Inc.,
Cary, NC.
SMITH, N. G. 1969. Provoked release of mobbing-a
hunting technique of
Micrastur
falcons. Ibis
111:241-243.
THUROW, T. L.,
AND
H. L. BLACK. 1981. Ecology and
behavior of the Gymnogene. Ostrich 52:25-35.
WITCHELL, C. A. 1896. The evolution of bird-song
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London.
The Condor 99:206-210
0
The Cooper Ormthological Society 1997
THE FORAGING BEHAVIOR OF SEMIPALMATED SANDPIPERS IN THE UPPER BAY
OF FUNDY STEREOTYPED OR PREY-SENSITIVE?’
W. HERBERT WILSON, JR. AND ERIN R. VOGEL
Department of Biology, Colby College, Waterville, ME 04901,
e-mail: whwilson@colby.edu
Abstract:
Videotapes of migrant Semipalmated
Sandpipers foraging in the upper Bay of Fundy were
analyzed to test for foraging behaviors sensitive to
prey density. Over a range of prey densities, both the
number of steps set-’ and probes set-’ increased
with increasing prey density. However, the number
of steps between probes was constant over the range
of prey densities observed. The average angle of di-
rectional change during foraging and the number of
turns mini’ were constant despite large differences
in prey patchiness.
Key words: Semipalmated Sandpiper,
Calidris pu-
silla,
foraging behavior, stop-over area, migration,
Buy of Fundy,
Corophium volutator.
Ornithological studies have contributed much to the
development of foraging theory (e.g., Tinbergen,
1967, Davies, 1977, Krebs et al., 1977, Zach and
Falls, 1977). However, the foraging behavior of
many birds confounds testing many predictions be-
cause birds may be difficult to observe continuously
for extended periods, may take a diverse array of prey
and may thwart efforts to quantify their behaviors be-
cause of their rapid movements. In this contribution,
’
Received 11 July 1996. Accepted 25 October
1996.
we explore the relationship of prey density and prey
patchiness (measured by coefficient of variation) on
foraging behavior of Semipalmated Sandpipers
(Calidris pusilla)
which essentially prey on a single
species in the upper Bay of Fundy. The high visibil-
ity and confiding nature of these shorebirds allowed
us to videotape foraging behavior at close range, per-
mitting the acquisition of data on foraging behavior
that cannot be gathered for many avian species.
These data are used to test predictions of the rela-
tionship of foraging behaviors to prey density and
prey patchiness.
Many scolopacid sandpipers, including Semipal-
mated Sandpipers, undertake migrations between arc-
tic breeding grounds and subtropical or tropical win-
tering areas. The distances of these migrations place
extraordinary energetic demands on the birds. In addi-
tion, shorebirds have higher metabolic rates than ex-
pected based on other birds of similar mass (Kersten
and Piersma 1987). It is reasonable to expect that there
should be strong selective pressure to maximize food
intake at stop-over areas during migration.
Semipalmated Sandpipers nest in the low- to mid-
arctic (Harrington and Morrison 1979, Gratto-Trevor
1992). After nesting, the majority of central and east-
em Canadian breeding birds wend their way to the
upper Bay of Fundy. During an average stay of 15
days (Hicklin 1987), the sandpipers feed primarily on
the abundant amphipod crustacean,
Corophium volu-
tutor
(Hicklin and Smith 1979). These sandpipers
