440
SHORT COMMUNICATIONS
The Condor 89:440-442
0 The CooperOrnithologicalSociety1987
EFFECTS OF THE 198243 EL NIAO EVENT ON BLUE-FOOTED AND
MASKED BOOBY POPULATIONS ON ISLA DAPHNE MAJOR, GAL&‘AGOS
H. LISLE GIBBS
Department of Biology, Universityof Michigan, Ann Arbor, MI 48109-1048
STEVEN C. LATTA
Schoolof Natural Resources,Universityof Michigan, Ann Arbor, MI 48109- 1I1 5
JAMES P. GIBBS~
Division of Wildlife, Collegeof Forest Resources,Universityof Maine, Orono, ME 04469
Key words: El Niiio; Blue-footedBooby; Masked
Booby;breedingpopulations:GalrIpagos.
booby populations by counting the number of Bluefooted pairs nestingin the upper and lower cratersand
Masked Booby pairs nesting on the entire island. For
El Niiio events have negative effects on seabird pop- each nest, we determined clutch size, brood size, and
estimated chick age, basedon plumage characteristics
ulations in the equatorial Pacific, including the Gal&
pagosIslands, by causingreductions in food supplies described by Nelson (1978). Nest scrapestended by
(Murphy 1936, Boersma 1978, Nelson 1978, Schreiber one or more birds were considered to represent one
and Schreiber 1984). However, the nature and mag- reproductivelyactive pair; pairs of this type were rare,
nitude of such effects are poorly understood because making up between 0% to 10% of all nesting pairs in
detailed information on population sizes and repro- censuseswhen nesting was observed.
In 1982, we censusedeach booby population once;
ductive characteristicsof well-defined seabirdcolonies
immediately before, during, and after El Nifio events the Blue-footed Booby population was counted on 12
are rare. Here we report on (1) sizes of breeding pop- April and the Masked Booby population was counted
ulations, (2) clutch sizes and degree of breeding syn- on 12 February. We checked for breeding activity in
1983 duringisland-widesearchesonceevery three days.
chrony, and (3) availability of nestinghabitat for Bluefooted Booby (Sula nebouxii)and Masked Booby (S. Following El Nifio, we censusedBlue-footed Booby
populations monthly, from December 1983 to April
dactylatra) populations on Isia Daphne Major, caia1984 (n = 5) and counted Masked Booby nests once
nagos.from 1982 to 1984. This Deriod included the El
Ni?~oevent of 1982-83 (Cane 19‘83, Robinson and de1 on 12 December 1983. We also calculatedthe area of
Pino 1985), which has been describedas the most se- each crater floor available for nesting by Blue-footed
vere oceanographicdisturbanceof its type in the West- Boobies,prior to and followingEl Nifio when the growth
of vegetation had substantially altered the physiogem Pacific of this century (Cane 1983).
nomy of these areas.
METHODS
Isla Daphne Major is a small island (40 ha in area) RESULTS AND DISCUSSION
located in the middle of the Galapagosarchipelago.In
The El Niiio event of 1982-l 983 resultedin a dramatic
most years,large numbers of Blue-footed and Masked increasein ocean temperaturesand rainfall in the Gaboobies breed on the island (Nelson 1978). There is lspagos archipelago from December 1982 until Sepstrongspatialseparationin the nestinghabitat of these tember 1983 (Robinson and de1 Pino 1985). Ocean
specieson the island (D&y 1984). Most Blue-footed temperatures iemained above 25°C around’Daphne
Booby pairs (>95% basedon censusdata) nest on the duringthe entire eight months of our stayon the island,
floor of one of either of two extinct volcanic craters. reaching.a neak of 30°C in April 1983. Total rainfall
The lower crater is about five times greater in surface on the iilanh was 1,359 mm 0; 10 times the previously
areathan the uppercrater. In contrast,Masked Boobies recorded maximum (Gibbs et al. 1984).
nest exclusivelyon the rims of both cratersand on the
We counted620 pairsof Blue-footedBoobiesnesting
outer slope of the island.
in the upper and lower craterscombined in April 1982
We were present on Daphne from January to May
and 348 pairs of Masked Boobies on the whole island
1982, December 1982 to July 1983, and December in February 1982. The onset of El Nifio resulted in a
1983 to May 1984. During these periods we censused completecessationof the reproductiveactivity of these
normally continuously breeding populations: no individuals of either specieswere observed breeding on
I Received 22 September 1986. Final acceptance11 the island from December 1982 to July 1983. Breeding
December 1986.
population sizesshoweda rapid recovery immediately
* Present address:Division of Biological Sciences, following the 1982-1983 event, although total numUniversity of Missouri-Columbia, Columbia, MO
bers were lower than in 1982: Blue-footed Booby pop65211.
ulationsaveraged69% and Masked Booby populations
.
;
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441
TABLE 1. Numbers of Blue-footed and Masked booby pairs on Isla Daphne before, after, and during the 198283 El Nifio event.
Periodof observation
BeforeEl Niiio:
Februaryand
April 1982
Pairs of Blue-footed Boobies1
Pairs of Masked Boobies2
DuringEl Nifio:
December
1982
to Julv1983
620
348
AfterEl Nifio:
December
1983
to Mav 1984
425 f 663
135
’ Numberof pairsin theupperandlowercrater
i Numberof pairsontheentireisland.
’ Mean+ SE.
39% of pre-Niiio levels in late 1983 and early 1984
(Table 1).
Clutch sizes of both booby specieswere smaller in
post-Niiio populations. We estimated frequencies of
different clutch sizes for Blue-footed Boobies in 1984
by combiningdata from December,February,and April
censusesto avoid recounting the same nests in consecutivemonths. A significantlygreater proportion of
Blue-footedBoobyne& had large; clutch sizesin 1982
(clutch size 1%total nestsl: 1 117%1:2 175%1:3 18%1:
i = 446 total nests) than-in 1984 il [40%];?! [60%]:
n = 110;G = 22.9, df= 2, P < 0.001). Masked Boobies
showed a similar pattern. In both years, all clutches
consistedof one or two eggs.In 1982, 53% of all nests
had two eggs(n = 36 total nests)while only 27% had
clutchesof two in 1984 (n = 34: G = 5.1. df = 1. P =
0.024). Clutch sizes may have been smaller in ‘1984
becausea Nifio-induced changein the age structureof
both populations resulted in more young, inexperiencedbirds breedingin 1984 or becausefood supplies
remained low following the 1982-1983 event.
The degreeof breedingsynchronywassimilar in preand post-Nifio populations of both speciesalthough
most pairs were at different stagesof the breedingcycle
when censuseswere made in 1982 and 1984. For example, in April 1982 77% of all Blue-footed Booby
pairs had eggswhile in December 1983, 85% of all
pairs had chicks six weeks old or younger. Masked
Boobies showed a similar pattern of synchrony: 64%
had eight- to 12-week-oldyoungin February 1982while
67% had chickssix weeksold or voungerin December
1984. Therefore, becausea high-degreeof synchrony
alreadyexistsin normal populations,the uniform onset
of breeding following a Nifio event may not increase
synchronyfurther.
The increasein vegetative cover following El Nifio
had a demonstrable effect on the availability of nest
sitesto Blue-footed Boobiesin the large crater. Due to
the heavy, sustainedrainfall during 1982-1983, a large
proportion of the lower crater floor was covered in
thick vegetation from July 1983 onwards. This vegetation covered 3,595 m2or 46% ofthe crater floor used
by nesting boobies in 1982 and consisted mainly of
annual plant speciesincluding Cacabusmiersii, Heliotropium angiospermum, Merremia aegyptica, Ipomoea linearfolia, and Chamaesyce amplexicaulis.
Plants, previouslyabsent,also grew on the floor of the
upper crater; however, they were sparselydistributed
and did not decreasethe availability of nest sites. The
differencein cover between the cratersprovided a natural experiment with which to examine the effect of
the growth of vegetation on breeding populations of
Blue-footed Boobies.In the upper crater, the numbers
of nesting pairs, hence bird density, were remarkably
similar in the two years (1982: 104 pairs; 0.061 pairs
per m*; 1984: 101.5 pairs i 7.2 [mean + SE]; 0.063
pairs per m*). In the lower crater, there were fewer
breedingpairs in 1984 than in 1982 (1982: 5 16 pairs;
1984: 323.3 ? 22.9) but the density of pairs in the
nonvegetatedarea of the crater floor remained almost
constant(1982: 0.062 nairs ner mZ: 1984: 0.069 uairs
per m2).Thus, the reductionin the breedingpopulation
of Blue-footed Boobiesin 1984 was apparently due to
nest site limitation causedby changesin the cover of
terrestrial vegetation during the 1982-83 El Nifio.
Although quantitative data on changesin nest site
availability are lackingfor Masked Booby populations,
the dramatic increasesin vegetativecover on the outer
slope of the island (Gibbs and Grant, unpubl.) may
also account for the post-Niiio reduction in breeding
populationsof this species.In 1984, Masked Boobies
were not seennesting in newly-vegetatedareas,where
they had previously bred in 1982 (H. L. Gibbs, pers.
observ.).
In conclusion,the 1982-1983 event had important
short-term effects (cessationof breeding) and potentially long-term effects (smaller clutch sizes and reduced availability of nestinghabitat) on booby populations nesting on Daphne. Interruptions in breeding
activity have also been reported for other Galapagos
seabirdsduring this (Valle 1985) and other (Boersma
1978) El Nifio events. The vegetationin the lower crater has continued to limit thenesting habitat available
to Blue-footedBoobiesun to Januarv1986(H. L. Gibbs,
pers. observ.). Further studiesshould considerthe im:
portance of such long-term indirect effectsof El Nifio
on seabird populations nesting in arid regions of the
equatorial Pacific.
We thank Malcolm Coulter, Anne Wilson Goldizen,
and Peter Grant for commentson the paper.This work
wascarried out in conjunctionwith P. R. Grant’s studies of Galapagosfinches;we thank him for his assistance. Permissionto do this researchwas given by the
GalapagosNational Park Service.The CharlesDarwin
ResearchStation provided logistical support.
LITERATURE CITED
BOERSMA,P. D. 1978. Breedingpatternsof Galapagos
Penguins as an indicator of oceanographicconditions. Science200:1489-1493.
442
SHORT COMMUNICATIONS
CANE,M. 1983. OceanographiceventsduringEl Niiio.
Science222:1189-l 195.
Durrv, D. C. 1984. Nest site selection by Masked
and Blue-footed boobies on Isla Espanola, Galapagos.Condor 86:301-304.
GIBBS, H. L., P. R. GRANT, AND J. WEILAND. 1984.
Breedingof Darwin’s finchesat an unusuallyearly
age in an El Niiio year. Auk 101:872-874.
Muar&, E. C. 1936. -Oceanic birds of South America. American Museum of Natural History, New
York.
NELSON,
J. B. 1978. The Sulidae.Oxford Univ. Press,
Oxford.
ROBINSON,
G., AND E. M. DELPINO. 1985. El Niiio
The Condor 89:442-443
0 ne Cooper Ornithological
in the Galapagos:the 1982-83 event. CharlesDarwin Foundation, Quito, Ecuador.
SCHREIBER,
R. W.. AND E. A. SCHREIBER.1984. Central Pacific seabirdsand the El Niflo SouthernOscillation: 1982 to 1983 perspectives.Science225:
713-716.
VALLE,C. A. 1985. Alteracibn de las poblacionesde1
cormoran no volador, el pinguino y otras aves
marinasen Galapagospor efectode El Niiio 198283 y su subsecuenterecuperation, p. 245-257. In
G. Robinson and E. M.-de1 Pino [eds.], El Niiio
in the Galapagos:the 1982-83 event. CharlesDarwin Foundation, Quito, Ecuador.
Society 1987
CANNIBALISM IN AMERICAN COOTS INDUCED BY
SEVERE SPRING WEATHER AND AVIAN CHOLERA’
DAVID G. PAULLIN
U.S. Fish and Wildlife Service,MaNzeurNational Wildlife Refuge,P.O. Box 245,
Princeton,OR 97721
Key words: American Coot;Fulica americana;cannibalism;feeding behavior;avian cholera;Pasteurella
multocida.
The food habitsofAmerican Coots(Fulica americana)
havebeenwell documented(Jones1940;Stollberg1949;
Eley and Harris 1976, Ivey 1987). These studieshave
shown that plant food is the primary component of
American Coot diets, althoughinvertebratesare commonly taken. However, coots may occasionally attempt to eat larger vertebrates(McCurdy 1983). Bent
(1926) reported that coots have been known to pluck
and partially eat dead ducks.
Cannibalism includesboth the killing and eating or
scavengingof conspecificsand in birds is most commonly reportedamong raptors,where younger,weaker
nestlingsare sometimeskilled and eaten by siblingsor
parents to reduce fratricidal strife (Newton 1979). I
know of no recordsof cannibalismin American Coots,
although, Cawston (1983) reported it for the closely
related Common Moorhen (Gallinula chloropus).
On 21 February 1984 at 14:37 I observedtwo American Coots eating a fresh carcasson ice adjacent to a
small open water area on Malheur National Wildlife
Refuge (NWR), Hamey County, Oregon. Closer examination (from a distance of approximately 25 m)
1Received 15 October 1985. Final acceptance 19
November 1986.
revealedthe carcasswas a fresh coot. The sternum and
ribs were completely exposedand most of the pectoral
muscle tissue had been removed. The causeof death
was unknown but sincethe carcasswasbeneatha powerline it was assumedthat death had resulted from a
collisionwith the overheadwires. The two feedingcoots
were immediately joined by four others, and the six
beganvigorouslytearingand consumingthe remaining
muscletissueand viscera. I returned to the site within
1 hr to continue observations;however, strong winds
had broken the ice and the carcasshad sunk.The feeding coots had dispersed.
By 22 February the number of dead coots had increased to seven and by 23 February there were 15
dead coots, all near the powerline. I observedno cannibalistic behavior on 22 February but on 23 February
I observedtwo cootsseparatelyfeeding on two mostly
intact coot carcasses.Throughout Februaryand March
American Coots continued to die in the few areas of
open water. I observedAmerican Coots cannibalizing
coot carcasseson five occasionsduring this period.
By late February it was apparent that not all dead
coots could be attributed to powerline collisions because of the widening distribution of carcassesaway
from the powerline. On 11 March five fresh coot carcasseswere collectedand sent to the National Wildlife
Health Laboratory in Madison, Wisconsin for necropsy. All five birds were diagnosedas dvinn from avian
cholera (Pasteurella mult&ida). Before the epizootic
subsidedfollowing ice breakupin late March, 3 17 dead
cootswere picked up and disposedof. Sixty additional