Ritualized Aggression and Unstable Dominance in Broods of Crested Ibis (Nipponia nippon)

2004·3.4 MB·English

by  Xinhai Li

Wilson Bulletin, 116(2), 2004, pp. 172-176
RITUALIZED AGGRESSION AND UNSTABLE DOMINANCE IN
BROODS OE CRESTED IBIS (NIPPONIA NIPPON)
XINHAI LI,' DIANMO LI,' ZHIJUN MA,^ TIANQING ZHAI,^ AND
HUGH DRUMMOND^
—
ABSTRACT. In broods of Crested Ibis {Nipponia nippon), aggressive dominance was unstable over time,
even within feeding sessions. All chicks took turns pecking aggressively while broodmates hung their heads
submissively, although roles were contested at the start offeeding bouts when chicks were 11-17 days old. In
all broods, at least halfofall pecks werefalsepecks, which did not strike broodmates even when within reach.
False pecks seem to be ritualized displays that function to solicit food from parents and possibly to threaten
rivals. Received 9 July 2002, accepted 24 March 2004.
We describe an extraordinary form of ago- days, 4 weeks after departure from the nest.
nistic interaction between broodmates of the Because of food competition, broods of most
Crested Ibis {Nipponia nippon), a critically ibis species are facultatively reduced to two
endangered ciconiiform (Liu 1981). Aggres- fledglings (Matheu and del Hoyo 1992), but
sion among altricial broodmates occurs in a brood reduction is relatively uncommon in
variety of avian taxa, including some ibises, Crested Ibis: 78.3% ofhatchlings fledge (Zhai
egrets, raptors, boobies, anhingas, guillemots, et al. 2001), compared to 56% ± 14.1 (SD)
and kingfishers (reviews in Mock 1984, Mock that fledge in 29 bird species with parental
and Parker 1997, Drummond 2002). Gener- feeding and a modal clutch size greater than
ally, broodmate hierarchies are formed one (reviewed by Royle et al. 1999).
through pecking and biting (review in Drum- We recorded ibis behavior at hillside nests
mond 1999), and, in species where siblicide in Shaanxi Province (33° 18' N to 33° 24' N
is facultative (Lack 1947, 1954; Ricklefs and 107° 23' E to 107° 28' E), China. Observ-
1965), the intensity of aggression varies with ers sat upslope ofthe colony at vantage points
the amount of food provided by parents 15-50 m away from nests and watched broods
(Drummond 2001a, 2001b; but see Mock et through a telescope from 07:00-19:00 UTC
al. 1987, Forbes and Mock 1994). Threatening + 08. Hatching order (a-chick, b-chick, and
postures and calls are common (Drummond so on) was evident from marked differences
2001b), but no species has been reported to in body size that persisted throughout the nest-
show ritualized attacks that do not impact the ling period (as in the Bald Ibis, Geronticus
victim. eremita; Hirsch 1979). In 1999, we recorded
The Crested Ibis feeds on loaches, eels, lo- behavior at nest 9918, where two broodmates,
custs, and freshwater invertebrates, including which hatched 2 days apart, were visible from
insects, and lays two to four eggs in a tree a vantage point 30 m away. We observed be-
nest; eggs hatch at 1- to 2-day intervals havior daily between hatching and fledging 41
(Zheng 1973, Li and Huang 1986). Both par-
days later, recording all feeding sessions on
ents feed the chicks by regurgitation until the video. In addition, we observed seven broods
chicks become independent at about age 70 of two, three, or four chicks (n = one, five,
one broods, respectively) on 16 days (2.7
' State Key Lab. of Integrated Management of Pest days/brood) in 1997, 1998, and 2000, when
Insects and Rodents, Inst, of Zoology, Chinese Acad-
emy of Sciences, Beijing 100080, China. broods were in Stage 3 (>18 days old).
^ Inst, of Biodiversity, Fudan Univ., Shanghai We recorded the absolute frequencies of
200433, China. feeds and pecks. During each parent’s period
Crested Ibis Conservation Station, Yang County of nest attendance, it typically fed the brood
723300, Shaanxi Province, China. in a single session of two to eight regurgita-
Inst, de Ecologia, Univ. Nacional Autonoma de
MexiCcoor,reAs.pPon7d0i-n2g75a,ut0ho4r5;10e-mDa.iFl.,: Mexico. ctihoincsk. aEgagrcehssrieognuragnidtaatisoinngleelifceietde.dAafbeoeudtwaosf
[email protected] recorded whenever a chick received food by
172
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Li et ai • AGGRESSION AND DOMINANCE IN CRESTED IBIS 173
—
—
Mutual pecks Pecks by a-chick Pecks by b-chick
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
Age (days) ofa-chick
EIG. 1. Rate ofpecking at a Crested Ibis (Nipponia nippon) nest, Shaanxi Province, China, 1999. The two
chicks (brood 9918) pecked at similar rates, taking turns at aggressive pecking.
inserting its bill (usually its whole head) into chick pecked for several seconds and the par-
the parent’s bill. Pecks included any rapid ent offered food to it. Sometimes, while the
downward thrust of the head, including real chicks were exchanging mutual pecks, the
pecks, where open bill tips struck the brood- parent offered its open bill to one of them,
mate (usually on the head or nape), diwdfalse which then fed. After submission by one
pecks, where no target was struck and the chick, the other chick seemed to diminish its
downward thrust ended at the nest floor. The attacks. In Stage 3 (18—41 days old), upon ar-
two categories of pecks were recorded simply rival of the parent, one chick (a or b) started
as pecks because we often could not tell pecking and the other usually responded by
whether a peck was real or false. When the hanging its head submissively. The aggressive
chicks at nest 9918 were aggressive during a chick then begged and was fed one or more
bout {mutual pecking), they pecked at similar times, all the while continuing to peck its un-
frequencies and it was difficult to count all fed nestmate unfil the latter started to peck the
pecks; thus, we counted only the mutual pecks aggressive chick and beg for food. Then the
ofthe more visible chick. Pecking frequencies fed chick promptly hung its head submissive-
of brood 9918 were recovered from video re- ly, and the unfed chick received the next feed-
cordings, and those of the seven broods in
ing.
Stage 3 were recorded using a hand counter.
Chicks of brood 9918 begged, without vo-
Video recordings were transferred from tape
to Audio Video Interleave files using a calizing, by raising their bills and repeatedly
tapping the parent’s bill. Pecks at the brood-
VideoKing^'^ video compress card (Beijing
1 mate usually were accompanied by simulta-
' wKeerfea Ealneacltyrzoendic uCsoi.ng 19A9d7o),beanPdretmhio.esree fi5l.e0s neous chirping, which occurred in no other
(Adobe Creative Team 1998). context and was interpreted as a threat call.
Development of aggression andfeeding. Aggression by both chicks of brood 9918 in-
In brood 9918, we categorized development creased more or less steadily throughout Stage
of feeding and agonistic behavior into three 2 and during the first 10 days of Stage 3, be-
stages, according to the age of the a-cfiick. fore declining steadily o\er the last 15 days
During Stage (0-10 days old), agonism was of Stage 3 (Eig. I). Parents pro\ided 5.3 ±
I
absent. During Stage 2 (11-17 days old), ar- 0.8 (SD) feeding sessions per day, with 4.1 ±
rival of the parent at the nest was followed by 2.1 regurgitations per session. Atlults did not
a bout of begging and mutual peeking until obviously interfere in broodmatc aggression
one chick (a or b) submitted by hanging its and tended to Iced whichever chick was beg-
head low, whereupon the other (aggressive) ging. Over the 41-day nestling period, the a-
174 THE WILSON BULLETIN • Vol. 116, No. 2, June 2004
chick received 483 feeds and the b-chiek re- chick pecked its broodmates. The a-chieks and
ceived 451 feeds. b-chicks (all broods) did not differ (86.8 ±
In the other broods, all of the chicks 36.2 and 78.3 ±31.4 pecks/day, respectively;
showed aggressive pecking on every day of Wilcoxon rank sum test, Z = 0.886, P — 0.38,
=
observation. This followed the pattern of two tailed n 7). The a-chiek, b-chick, and
brood 9918 in Stage 3, with chicks taking c-chick of each brood did not differ either
turns pecking and begging while their brood- (81.9 ± 25.0, 72.6 ± 21.5, and 73.1 ± 23.6
mates hung their heads submissively; there pecks/day, respectively; Kruskal-Wallis test,
was no clear consistency with respect to X" = 2.02, P = —0.36, n - 6).
which fed hrst or more frequently. Whenever False pecks. False pecks included the
a chick pecked, it pecked at all of its brood- threat call and frequently passed within cen-
mates. There was no significant difference in timeters of the broodmate, but they also oe-
the daily feeding frequencies of a-chicks and curred when the broodmate was out of range.
b-chicks (13.7 ± 4.7 and 12.3 ± 3.6 feeds, Despite the victim being immediately in front,
respectively; Wilcoxon rank sum test, Z = the aggressor directed pecks toward its own
0.93, P = 0.35, two tailed, n = 7) or of a- flanks, to one side and then the other, clearly
chicks, b-chicks, and c-chicks (12.3 ± 1.2, avoiding the easy target (which might be
12.1 ±2.8, and 1 1.4 ± 3.3 feeds, respectively; standing with head lowered in submission)
Kruskal-Wallis test, x" = 1.31, P = 0.52, n = and striking nothing. False pecks occurred in
6). — all eight focal broods, and they appeared to
Dominance. Dominance oecurred when represent roughly 60-70% of total pecks in
one chick was aggressive and the other adopt- brood 9918 and more than half of total pecks
ed a submissive posture. Although a chick of- in each ofthe other seven broods. During mu-
ten dominated its broodmate briefly, domi- tual peeking, false pecks decreased to <10%
nance between chicks was unstable over time, of total pecks. False pecks occurred in Stages
even within feeding sessions. In brood 9918, 2 and 3, usually after ordinary begging failed
over the 41-day nestling period, the a-chick to elicit parental feeding. They could occur in
pecked its broodmate 53.6 ± 39.4 times a day, the absence of genuine pecks at the brood-
versus 59.5 ± 48.5 pecks by the b-chick (ex- mate, but they were almost invariably per-
cluding mutual pecking). In Stage 2, neither formed by the chick that currently dominated
chick tended to dominate first and get the first its broodmate. After fledglings departed the
feeding. Sometimes when the b-chick was nest, real pecks were rare because victims
pecking aggressively and about to be fed, the promptly fled. False pecking continued during
a-chick rose up and, using its superior height, the next 4 weeks (when parental feeding was
intercepted the feed. In Stage 3, chicks were supplemented by attempts at self-feeding),
only fed while temporarily dominant, and sim- even when the broodmate was out of sight.
ilar feeding rates of the a-chick and the b- Like begging and real pecking, false pecking
ehiek (XL unpubl. data) reflect similar fre- never occurred in the absence of a parent, and
quencies of temporary dominance by the two when false pecks occurred, parents offered
broodmates. In most feeding sessions a single food exelusively to the aggressor.
chick maintained dominance throughout, but False pecking appears to be a ritualized
suceessive dominance was also eommon. form ofreal pecking, and both forms ofpeck-
Thus, in Stage 3, 68% of the a-ehick’s 152 ing may elicit parental feeding. Originally, se-
feeds were obtained in sessions where the a- lection may have favored parents feeding ag-
chick dominated throughout or initially, and gressors, either because dominant chicks are
32% in sessions where the b-chick dominated more worthy of investment or because ap-
initially; for the b-chick’s 171 feeds, the b- peasing aggressors is a way ofprotecting their
chick’s corresponding values were 64 and broodmates (when aggression is food depen-
36%, respectively. dent, Drummond 2001a). In either scenario,
Similar absence of stable dominanee ap- the door would be open for the evolution of
peared to be the rule in all dyads of the seven signal funetion: parental feeding could be elic-
broods observed in Stage 3. For each brood ited initially by aggressive pecking and sub-
we calculated the mean number of times each sequently by false pecking. For the aggressor.
Li et al. • AGGRESSION AND DOMINANCE IN CRESTED IBIS 175
the advantage of using false pecks over real 1979, Pegoraro and Thaler 1993, Tuckova
pecks may be that false pecks do less physical 1999, Ros et al. 2001).
harm to the (long- and sensitive-billed) ag-
ACKNOWLEDGMENTS
gressor itself or to its sibling broodmate.
Hence, false pecks could be more effective For advice on an earlier version of this paper, we
than ordinary begging for inducing regurgi- thank H. Kallander and D. W. Mock, as well as M.
tation and ensuring feeding priority, and less Cargill, P. O’Conner, and A. Smith. The current
costly than real pecks to the aggressor’s in- version was improved by A. Gonzalez Voyer, J. L.
Osorno Cepeda, C. Rodriguez Juarez, W. A. Estes and
dividual and inclusive fitness. two anonymous referees; C. Rodriguez Juarez helped
Selection on parents to discriminate false with manuscript preparation. The study was suppor-
pecks from real pecks would not necessarily ted by state key basic and developmental research
result in parents declining to respond to false G2000046805, CAS Innovation Program, and the Chi-
pecks. Discriminating parents could simply naNatural ScienceFund 39893360; HD’sparticipation
devalue the signal, responding to false pecks was supported by CONACYT grant PCCNCNA-
031528.
less than to real pecks (but more than to or-
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