GROUP HUNTING BY HARRIS' HAWKS IN TEXAS

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SHORT COMMUNICATIONS
J Raptor Res. 29(4):265-267
¸ 1995 The Raptor Research Foundation, Inc.
GROUP HUNTING BY HARRIS' HAWKS IN TEXAS
JENNIFER O. COULSON
Department of Biology, Nunez Community College, 3700 La Fontaine Street, Chalmette, LA 70043 U.S.A.
THOMAS D. COULSON
Department of Biochemistry, Louisiana State University Medical Center, 1100 Florida Avenue,
New Orleans, LA 70119 U.S.A.
KEY WORDS: Harris' hawk; group hunting; Parabuteo
unicinctus; Texas.
The degree of sociality appears to vary among Harris'
hawk (Parabuteo unicinctus) populations in the United
States, with group living being most frequent in the west-
ern part of the range. Mader (1975) found that 46% of
Harris' hawk nests he studied were attended by trios and
that groups varied in size from three to six hawks in
Arizona. Dawson and Mannan (1989) reported a mean
group size of 3.8 hawks per nest in Arizona, Bednarz
(1987) found 2.8 hawks per nest in New Mexico, and
groups of more than two hawks were rare at nests in Texas
(Griffin 1976, Brannon 1980). However, the Texas pop-
ulation of Harris' hawks has not been studied in detail,
and little is known of cooperative behaviors among Harris'
hawks nesting there. Group hunting in Harris' hawks has
been reported to occur in the deserts of Arizona (Mader
1975) and New Mexico (Bednarz 1987). This paper rep-
resents the first published accounts of group hunting be-
havior in Texas.
On 23 July 1994, at 2020 H, we observed an adult
male and female Harris' hawk (we identified sex of hawks
based on body size comparisons) perched on adjacent pow-
er poles approximately 60 m from each other along a
highway in Pecos County, 1.6 km north of Ft. Stockton,
Texas. The male initiated the hunt by flying high over
the road, and was followed closely by the female. Four
other adults (three males and one female), previously un-
seen, hppeared from low perches on the east side of the
road and followed the pair across the highway. The lead
male hovered briefly and then made a shallow dive, dip-
ping just above the ground cover at a desert cottontail
(Sylvilagus audubonii). The first adult female dove steeply
behind the male. Then the hawks executed a series of five
or six steep dives apparently missing the rabbit. Five of
the birds then landed on a single yucca (Yucca sp.). The
sixth hawk perched in a nearby mesquite (Prosopis juli-
flora) about 6 m away. After 1-2 rain, all six hawks chased
the rabbit, until they were lost from view in a low spot in
the landscape. No birds flew from this low area in 15 mln
of observation.
On 1 August 1994, at 0740 H, we again observed six
adult Harris' hawks roosting on power poles at the same
location described above. Two groups of two hawks were
perched on neighboring poles, a fifth hawk was perched
on an adjacent pole, and a sixth hawk (male) was perched
eight poles away (about 480 m). The fifth hawk soon joined
one of the perched pairs on a nearby pole. Then one
Harris' hawk flew to the next pole in line. Immediately
after it perched, another hawk flew toward it. As the
approaching hawk drew near, the first hawk vacated its
perch and flew to the next pole in line. The approaching
hawk then landed on the recently vacated perch. All five
hawks engaged in seven bouts of this "leap frog" behavior
(Bednarz 1988a) as they moved from perch to perch, ap-
parently group hunting. This method of hunting on the
move, or the move-search hunting tactic is often used by
Harris' hawks that are group hunting (Bednarz 1988a,
Dawson 1988). The "leap frog" behavior was briefly in-
terrupted two times when three Harris' hawks perched
side by side. At 0746 H, all five birds began pursuing a
cottontail and engaged in a series of stoops. One hawk
captured the rabbit, and the others landed on the ground
at the kill site.
We climbed a small hill to get a view of the kill site
and inadvertently flushed four of the five hawks from the
kill. One returned immediately to the kill while the re-
maining three perched on poles. Within about 5 min, these
three chased another cottontail. After making five stoops,
they perched together on a pole. Two of these hawks then
landed at the site of the rabbit kill, and one adult male
perched on a yucca under the power poles. At 0804 H,
an adult female carried the hindquarters of the cottontaft
to the adult male perched on the yucca. The female dropped
the hindquarters, and the male went down to it, presum-
ably to feed.
TDC also observed group hunting by four Harris' hawks
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266 SHORT COMMUNICATIONS VOL. 29, No. 4
on two occasions in Webb County, Texas, in the early
1980s in April or May. North and east of Laredo, an adult
male Harris' hawk that was perched on a power pole
executed a steep dive into the tall mesquite brush. Three
other hawks (also perched on that pole) followed, diving
in succession. Further observation was obscured by the
vegetation. On a second occasion, two adult Harris' hawks
perched on adjacent fence posts, while two more adults
ran around a large prickly pear cactus (Opuntia engel-
mannii). The two perched hawks then joined the others
in attempting to flush and ambush a prey animal in the
prickly pear. The group eventually flew off without cap-
turing anything.
The observations of cooperative hunting near Fort
Stockton may not be particularly surprising--because group
hunting is common in a nearby (approximately 225 km
away) population in New Mexico (Bednarz 1988a). How-
ever, the observations in Webb County were distant (720
km) from the New Mexico populations. Nevertheless, it
is likely that gene flow occurs between the Texas, New
Mexico, and Arizona populations. Bednarz (1988b) found
that based on morphology alone, P. u. superior and P. u.
harrisi could not be easily differentiated, that skins of New
Mexico Harris' hawks were morphologically intermedi-
ate, and that clinal variations exist between populations
in the United States and Mexico. Because Harris' hawk
populations do not appear to be geographically isolated in
the United States, behavioral propensities for sociality pos-
sibly occur throughout.
Ecological factors may contribute to the incidence of
group living and cooperative hunting. Dawson (1988) sug-
gested that both the dense and thorny nature of the brush
encourage group hunting in Arizona. These Harris' hawks
hve in a habitat characterized by a complex understory
with an abundance of thorny cover available to prey an-
imals. Groups are more successful than individuals when
hunting in this terrain (Dawson 1988). Group hunting
has also been proposed as a way for Harris' hawks to
exploit large prey (e.g., jackrabbits, [Lepus spp.], Bednarz
1988a), and prey that is active only for a short period of
time, dawn and dusk (e.g. cottontails, and woodrats, [Ne-
otoma spp.], Bednarz and Ligon 1988, Faaborg and Bed-
narz 1990).
It is unusual for all Harris' hawks in a group to be
adults (Dawson and Mannan 1989, 1991a), and it is par-
ticularly unusual for six adults to hunt together in the
summer when exclusive territories are maintained (Daw-
son and Mannan 1991b). We do not know if this group
of adults aggregated because of nest failure. No nest or
juvenile birds could be seen in the vicinity. However, groups
in New Mexico appear to compete with each other when
hunting (J. Bednarz pers. comm.). Alternatively, the group
observed in Pecos County could have been composed of
nonbreeding adult floaters (wandering adults looking for
breeding territories). Similar social aggregations are formed
by resident hawks and transients in Arizona in the fall
and winter during nonbreeding (Dawson and Mannan
1991b). In New Mexico, low prey years limit breeding in
Harris' hawks; under these circumstances, some adult males
remain with the parental group for up to 3 yr (J. Bednarz
pers. comm.). it is certainly possible that the group of six
adults we observed consisted of a breeding pair and four
adult offspring.
The Texas population of Harris' hawks appears to
consist primarily of pairs with no helpers (Griffin 1976,
Brannon 1980). While cooperative breeding (more than
two hawks per nest) may be rare in Texas, our observa-
tions suggest that cooperative hunting may be common
when hawks are not actively nesting, a situation similar
to that reported for New Mexico (Bednarz 1988a).
RESUMEN.--Caza cooperativa o grupal es realizada por
poblaciones de Parabuteo unicinctus en Arizona y New
Mexico. Observamos cuatro incidentes de caza grupal pro-
tagonizaddos por P. unicinctus en Texas, dos en el Condado
de Webb (principios de los aftos ochenta) y dos en el
Condado de Pecos (1994). E1 tamafio grupal vari6 de
cuatro aves a grupos de seis adultos (cuatro machos y dos
hembras). Es posible que el agrupamiento para la caza y
como forma de vida pueda ser caracterlstico de algunos P
unicinctus, a travis del rango total de la especie.
[Traducci6n de Ivan Lazo]
ACKNOWLEDGMENTS
We are grateful to J.W. Dawson and w.J. Mader for
helpful comments on the earlier draft of this manuscript,
and to referees J. Bednarz, J.W. Dawson, and M.W.
Mannan for their critical reviews.
LITERATURE CITED
BEDNARZ, J.C. 1987. Pair and group reproductive suc-
cess, polyandry, and cooperative breeding in the Har-
ris' Hawk. Auk 104:393-404.
1988a. Cooperative hunting in Harris' hawks
( Parabuteo unicinctus ). Science 239:1525-1527.
1988b. Harris' hawk subspecies: is superior
larger or different than harrisi? Pages 294-300 in R.L.
Glinski, B. Giron Pendleton, M.B. Ross, M.N. Le-
Franc, B.A. Millsap and S.W. Hoffman [EDS.], Pro-
ceedings of the southwest raptor management sym-
posium and workshop. Natl. Wildl. Fed. Sci. Tech.
Ser. 11, Washington, DC U.S.A.
-- ^ND J.D. LIGON. 1988. A study of the ecological
bases of cooperative breeding in the Harris' hawk.
Ecology 69:1176-1187.
B•NNON, J.D. 1980. The reproductive ecology of a
Texas Harris' hawk (Parabuteo unicinctus harrisi) pop-
ulation. M.S. thesis, Univ. Texas, Austin, TX U.S.A.
DawsoN, J.W. 1988. The cooperative breeding system
of the Harris' hawk in Arizona. M.S. thesis, Univ.
Arizona, Tucson, AZ U.S.A.
-- ^ND R.W. Mt•NN^N. 1989. A comparison of two

DECEMBER 1995 SHORT COMMUNICATIONS 267
methods of estimating breeding group size in Harris'
hawks. Auk 106:480-483.
--AND . 1991a. Dominance hierarchiesand
helper contributions in Harris' hawks. Auk 108:649-
660.
-- AND -- 1991b. The role of territoriality
in the social organization of Harris' hawks. Auk 108:
661-672.
FAABORG, J. AND J.C. BEDNARZ. 1990. Galfipagos and
Harris' hawks: divergent causes of sociality in two
raptors. Pages 357-383 in P.B. Stacey and W.D. Ko-
enig [EDs.], Cooperative breeding in birds: long-term
studies of ecology and behavior. Cambridge Univ. Press,
Cambridge, U.K.
GRIFFIN, C.R. 1976. A preliminary comparison of Texas
and Arizona Harris' hawk (Parabuteo unicinctus) pop-
ulations. Raptor Res. 10:50-54.
M^DV.R, W.J. 1975. Biology of the Harris' hawk •n
southern Arizona. Living Bird 14:59-85.
S^NDV. RS, R.R. ^ND W.J. G^BRmL. 1985. Soil survey
of Webb County, Texas. Natl. Coop. Soil Surv., U S
Gov. Print. Office, Washington, DC U.S.A.
Received 7 November 1994; accepted 17 May 1995
J Raptor Res. 29(4):267-269
¸ 1995 The Raptor Research Foundation, Inc.
ACCIPITERS PREY ON NESTLING BIRDS IN JAPAN
MASAOKI TAKAGI
Laboratory of Applied Zoology, Faculty of Agriculture, Hokkaido University,
Sapporo 060, Japan
MUTSUYUKI UETA
Research Center, Wild Bird Society of Japan, 15-8 Nanpeidai, Shibuyaku, Tokyo 150, Japan •
SHO IKEDA
Group for Biological Research of Falcons, 2269- 72, Shishiuchi, Toubetu,
Ishikarigun, Hokkaido 067-37, Japan
KEY WORDS: Accipiter gentilis; A. gularis; Japanese less-
er sparrowhawk; nestling predation; Northern goshawk.
The diet of Accipiter spp. has been investigated in var-
ious localities (e.g., Brown and Amadon 1968, Opdam et
al. 1977, Kenward 1982, Goszczynski and Pilatowski 1986,
Petty 1989, Hirano and Kimiziwa 1992, Ueta 1992). In
Japan the northern goshawk (Accipiter gentilis) preys on
medium- to small-sized birds (Yamashina 1941, Ishizawa
and Chiba 1967). The Japanese lesser sparrowhawk (A.
gularis) exclusively hunts small birds such as tree sparrows
and great tits (Parus major) during the breeding season
(Hirano and Kimizawa 1992, Ueta 1992). In addition,
hawks adjust their own fledgling periods to the season that
prey bird species fledge (Newton 1986, Ueta 1993). Some
previous reports that hawks hunt nestlings have been pub-
lished (Opdam et al. 1977, Newton 1986), but these au-
thors only observed prey delivered to hawk nests. There-
fore, they did not observe the hunting technique used to
capture nestlings. In this paper, we present observations
on nestling hunting by northern goshawks and Japanese
lesser sparrowhawks.
Northern Goshawk. A northern goshawk preying on
a nestling bull-headed shrike (Lanius bucephalus) was re-
corded on videotape in western Hokkaido, northern Japan.
The nest was being videotaped as a part of a study on
shrike ecology, and was positioned among vines (Vitis co•g-
netiae) about 1.2 m above ground. The four 14-d-old nest-
lings were recorded on videotape starting at 0800 H, 21
June 1992. The adult shrikes frequently brought prey to
their nest, but during their absence the nestlings were often
exposed to predators. Nest attack and subsequent behavior
of the hawk and shrikes were as follows: At 1648 H the
male shrike brought prey to the nest and left carrying a
fecal sac. At 1649 H the goshawk approached the nest
while parents gave alarm calls nearby. At 1650 H the
hawk grasped one nestling and flew away with it. At 1652
H one nestling left the nest. The other two nestlings also
left the nest at 1655 H and 1657 H, respectively. At 1700
H the parents again gave alarm calls. A few moments later
the hawk came back and searched the vacant nest for a
few minutes. The hawk remained in the vicinity and might
have been searching for the rest of the nestlings for several
minutes and then left the observation site. Even though