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Home , Jabiru, Maguari stork, Mycteria, White stork, Wood stork

MAGUARI NESTING: JUVENILE GROWTH AND BEHAVIOR

BETSY TRENT THOMAS Apartado80844, Caracas1080-A,

ABSTRACT.--Althoughit has a wide range in tropical , the (Ciconiamaguari) is poorly known. In an 11-yr study of its breedingbiology in the llanos of Venezuela I found the mean clutch size for 3 yr to be 3.2 eggs.The young hatchedasynchro- nously at 29-32 days and usually made their first flight at 60-72 days. Data on egg sizes, weights,and a probabledump egg are given. The weight and growth of two siblings,plum- age succession,and nestling and fledgling behaviorsare described.The fledging successof eggsfrom 55 nests,over 9 yr, was61%. Loss of eggs(presumably from snakepredation) was higher than nestling loss.Although the asynchronouslyhatched young in a clutch varied greatly in weight and continuedthis differencethroughout nestling life, no caseof brood reduction was found in 123 nests during the study. Nestlings were fed mainly and tadpoles,, eels, and aquaticrats; food classesand food sizesbrought by adults varied with the age of their young. Maguari Stork nestlingsdo not graspwith their hallux and so are confined to their nestsuntil they can fly. I suggestthat the two black plumagesof nestlings,between the white hatchingdown and the white firstbasic , are for crypsis of the nest-boundyoung. Received29 August1983, accepted I March 1984.

THE Maguari Stork ( Ciconiamaguari = Euxen- day of hatching to the postfledgingperiod. uragaleata) is found only in South America and Perhapsbecause of abundantfood on the study is lesswidespread than the other two storksof site, no incident of brood reduction was ob- the Western Hemisphere, the served in 123 nests. The manner and size of ( americana)and the (Jabirumyc- food presentedto the young and sibling growth teria), both of which range farther to the north data, however, suggest that brood reduction (Blake 1977). Although the three speciesare may not occurin this speciesor may be rare. largely sympatric in South America, the wide Lossof eggs,presumably to snakes,was higher ranges indicated on distributional maps are than nestling lossesfrom all causescombined. somewhat misleading, becausethese storksare One puzzling aspect of Maguari chick de- ecologicallyrestricted to tropical, low-eleva- velopment is the successionof two different tion wetlands and adjacentnonforested areas. black ,between the white hatching In Venezuela the three storksare found togeth- down and the white first basicplumage (Hum- er in the interior of the country, a vast phrey and Parkes 1959) acquired at fledging. I called the llanos. suggestthat the black plumagesare for crypsis Mayr and Amadon (1951) and Kahl (1971a) becauseof their timing and the fact that Ma- have suggestedthat this stork is closelyrelated guari Stork nestlings,like adults,do not grasp to the (Ciconiaciconia), and this pa- with their hallux and thus cannot leave their per gives a broader basisfor a comparisonof nests to escapepredators. the young with thoseof the better known White Stork (Schtiz 1942, 1943, 1944; Haverschmidt STUDY SITE AND METHODS 1949). The principal published information about the Maguari Stork is from 2 months of The studysite (08ø31'N,67ø35'W) was a cattle ranch observationsof nestingbirds in (Kahl locatedroughly in the center of the Venezuelan lla- nos, about 45 km south of Calabozo in the state of 1971b).For 11 yr I followed the seasonalbreed- Guarico. Troth (1979) provided an account of the ing of all Maguari on a ranch in the vegetation of this ranch and Thomas (1979a) an an- llanos of Venezuela.I give data on the eggsand notatedlist of its species.The llanos are grass- on the asynchronouslyhatched young from the land savannawith scatteredpalms and clumps of other trees. During the rainy season(generally May • Present address:I Wetsel Road, Troy, New York to November)Maguari Storks nest, usually from June 12182 USA. to October (Thomasin press),when much of the sa-

812 The Auk 101: 812-823. October 1984 October1984] MaguariStork Nesting 813

TABLE1. Maguari Stork hatching and fledging success.

Eggs Young Total Number Percentage Percentage success Year of nests Laid Hatched hatched Fledged fledged (%) 1973 2 5 5 100.0 4 80.0 80.0 1974 16 43 24 55.8 18 75.0 41.8 1975 15 a 47 24 51.0 19 79.1 40.4 1976 10 36 28 77.7 24 85.7 66.7 1977 2 7 4 57.1 4 100.0 57.1 1978 5 14 9 64.2 5 55.5 35.7 1979 1 4 3 75.0 3 100.0 75.0 1980 1 4 3 75.0 3 100.0 75.0 1981 3 11 10 90.9 9 90.0 81.8 Total 55 171 110 71.8 89 85.0 61.5 ' In 1975 all the young were taken from five nestsby poachers.Four of these nestsare included here, becausetheir nine young storkswere nearly old enough to fledge. vanna is flooded to a depth of 5-100 cm. Maguari Eggs.--MaguariStorks laid eggsfrom May to Storkson the study site usedan area of approximate- November;in any one year, however, the nest- ly 690 ha. I studiedthe from 1972through 1982, ing seasonlasts no more than about 4 months. and from 1973 to 1976 I banded all nestlings (n = The timing difference is causedby the varia- 128). The detailsof handling techniquesare reported tion in the beginning of the nestingperiod and elsewhere (Thomas 1977). Some observationsof nest- ling behavior were made during banding; other data amount of rainfall (Thomasin press).The only were from dawn-to-dusk observations from blinds pair that laid eggs in late May did not relay and from field censusesduring all nesting stages. after their young fledged in late August.Nest- I used8 x 32 binoculars,a 15-60x telescope,spring ing in October was by late-returning birds, balancePesola scales of 100, 300, 1,000, and 2,500 g, usually young inexperiencedindividuals. The a grocer's scale that registered to 10 kg, a vernier eggs were small in relation to the size of the scalecalibrated to 0.1 mm, and a stop watch. I took adult female(œ egg is ca.3.3% of femaleweight), notesby hand or by using a portable tape recorder. and the clutch size was usually 3-4 eggs.The Color notes were made by comparisonwith color mean was 2.8 eggs (n = 20 clutches) in 1974, swatchesin hand (Smithe 1975). Only nestswhere 3.4 (n = 17) in 1975, and 3.6 (n = 12) in 1976, the number of eggs laid and number hatched were known are listed in Table 1. Standard deviations and for an overall mean of 3.2 eggsper clutch.Late rangesare given for egg sizesand weights.The length rains and low precipitation during May-June of food items was either measured from collected in 1974 may have been associatedwith the samplesor estimated to the nearest centimeter, and smaller clutches. the weight was calculatedfrom preservedsamples of Eggs are oval to subelliptical (Palmer 1962). food species. They are nearly smoothand dull white but may become stained during incubation. The mean RESULTS length of 54 eggs from 5 yr was 75.19 mm (+ 3.29, range=69.1-85.0) and their mean Nests.--Bothmembers of Maguari Storkpairs width was 52.56 mm (+ 2.66, range = 46.0- sharein all phasesof nestbuilding, incubation, 57.0). Mean weight of 42 fresh eggs,within a brooding, and feeding the young. In Venezue- week of laying and only slightly incubated,was la their nests were built on bushes and in small 116.4g (+ 9.42,range = 100-140).One freshegg trees 1.4-6.3 m high (ThomasMS) but not on was separated into its component parts and the ground, as Kahl (1971b) found in Argen- weighed:the shell was 14 g (11%),the yolk 25 tina. Nests were platforms of sticksabout ! m g (21%), and the albumen 83 g (68%). The yolk in diametercovered with grasssloping slightly was Flame Scarlet(Smithe 1975), perhaps be- down toward the center. Most nests were in cause of the , which are the com- small colonies (77%), but some (23%) were sol- ponentsof somefish and crustacea(Roudybush itary. Details of nestsand nest-buildingwill be et al. 1979);White Stork eggsare alsored-yolked reported elsewhere. (Goldsmith 1965). 814 BETSYTI•ENT THOMAS [Auk,Vol. 101

At three closelywatched nests, the storkslaid white down on the capital,spinal, humerat, alar, their eggs on alternate days, and full-time in- and ventral tracts. By day 4, emerging black cubation began with the secondor third egg. semiplumes(Lucas and Stettenheim 1972: Fig. Thus, in a clutch of four, there was sometimes 45) cover the frontal, coronal,and occipitalareas a differenceof a week in the age of the asyn- to the dorsalcervical line. Subsequently,black chronouslyhatched young. semiplumesalso cover the auricular, malar, and I found one presumed instance of a dump submalar areas, as well as the ventral, lateral, egg. In a nestwith three eggswhere incubation and dorsal cervical areas. At 1 week, a second had progressedfor about 21 days,a fourth egg generationof down, which is BlackishNeutral appeared.Three days later a broken stork egg Gray, starts to appear on the body. The first shell was underneath the nest. No eggs were white down is protoptile and the secondblack missing from nearby nests,and the three eggs down mesoptile (Van Tyne and Berger 1971). of this nest hatched 2-5 days apart. Therefore, The seconddowny featherspush out the first the shell below the nest was probably that of white downy feathers, many of which remain the dump egg. This nest might have been par- attached to the tips, and the effect is that of ticularly vulnerable to dumping, becauseit was regularly spacedwhite stars (Thomas 1979b). the highest and most exposedof a cluster of The seconddown is extremely dense, and it nests.Unlike the Jabiru,which usuallynests in bleachesto Olive-brown by 3 weeks. On the isolatedsites where an interloperis easilyseen 18th day, the first glossyblack remigesbegin and attacked (Thomas 1981), Maguari Storks to emergefrom their sheaths.At 5 weeks,young sometimesnest less than 0.5 m apart, so there storks are well covered with black juvenal are close conspecificneighbors, and a strange plumage. At this age the distinctively different female might not be immediately conspicuous. head and neck semiplumes,which are black, Yom-Tov (1980) suggested that intraspecific lanceolate,and plumulaceous,are 2-3 cm long. parasitismis more common in "good" years: These feathers are often erected when nest- this year had the largest number of nests(29) lings are excited and occasionallyfor thermo- during the study. regulation. Thechick at hatching.--Theincubation period A third generation of down, which is white, of Maguari Stork eggswas 29-32 days,counted begins to appear at about the age of 7 weeks, from the last egg laid until the last chick and soon afterward a few white semiplumes hatched in two nests. At one nest the first chick edge the bill on the malar tract, quickly fol- of three eggsbegan to break through its shell lowed by white semiplumesin the postauric- at 1130. At 1316 the attending adult, the male, ular area.Just after 2 monthsof age,white con- stoodup, cleaned the nest, picked up one half tour feathers become clearly visible on the of the egg shell with the tip of his bill, and interscapular tract. Around the time of the ap- tossed it over one side of the nest. The other pearanceof the first white semiplumes(ca. 60- half he threw in the opposite direction. Then 65 days), most young storks make their first he resumed incubation. At 1430 the chick's flights. By the age of 3 months, the juvenile white down was dry, and it had a dull gray- stork'sfirst basicplumage looks like that of the colored bill with a small gray egg tooth. The adults. The sequenceof these plumages and young bird's gape was pink, the gular area was molts is illustrated in Fig. 5. whitish yellow, the legs and feet pinkish gray, Other morphologicalchanges.--The egg tooth and the body skin light gray. The skin of some appears to be absorbed,as it is only slightly chicksat hatching is darker, nearly black. The visible by day 9. By the 5th day the nestling's nestling weighed 90 g, and it opened its eyes legs,toes, orbital area, and bill are black. When briefly and made soft peepswhen handled.The the chick reachesits 16th day, the hallux, which secondchick of this nest,which hatched4 days previously lay folded forward with the other later, weighed 83 g approximately 2 h post- toes,becomes loose and flexible. Four days lat- hatching. Three other "day 0" nestlings er, the hallux flexes backward, but nestlings weighed 76, 78, and 88 g. Daily measurements often do not stand on their feet until they are of these two siblings are given in Figs. !-4. The a month or more old. Neither the nestling nor third egg was infertile. the adult Maguari Stork usesits hallux to grasp Plumagesequence.--On its first full day, the (Thomas MS). Maguari nestling is well coveredby expanding By the nestling's 5th day, the gular area is October1984] MaguariStork Nesting 8 ! 5

4000

3500

$ooo

2500

• 2000

1500

I000 - 500o.O"'..j'ø 0- 0 4 8 12 16 20 24 28 52 56 40 Days after hatching Fig. 1. Weight gain oœnestling Maguari Stork siblings.Solid line representsthe older, dotted line the 4-day younger bird.

Pale Sulphur Yellow, and it becomesa pro- along the edge of the bill, but the amount and gressivelystronger color: Orange Yellow at 3 pattern vary among nestlings.When the young weeks and SpectrumOrange 2 weeks later. At first fly, the gular is pale Scarlet.The orbital this time, some black dots speckle the gular area, which is red in adults, staysblack for per-

125

I00

25

I I I I I I I I I I • 0 4 8 12 16 20 24 28 :52 36 40 Days after hatching

Fig. 2. Exposedculmen growth of siblingsdifferent in ageby 4 days.Symbols as in Fig. 1. 816 BETSYTRENT THOMAS [Auk, Vol. 101

225

200

175

• 1,50- E • 125 I00 .? ....

?õ o.' ...

..•_ . . . . ' ./...'

I I I i i I i I I I I 0 4 8 12 16 20 24 28 32 36 40 Days afler hatching

Fig. 3. Tarsaland largest (middle) toe growth of siblingsdifferent in age by 4 days. Symbolsas in Fig. 1.

haps a year, however. At 3 months, the bill and eels were still alive. Representative sam- beginsto becomebicolored, similar to the adult ples of food were collected for identification. bill, and the legsand feet of someyoung start The classes of food, in the order of their nu- to turn pinkish. Both sexesof adult Maguari merical importance,were: frogs (Pseudispara- Storkshave lighter than Cream Color irides, doxus,Leptodactylus fuscus, L. ocellatus);fish (Pe- but the nestlingiris is Raw Umber. This color tenia kraussi,Cichlasoma spp., Gymnotuscarapo, does not change significantly before young Pterolebiasspp., Hoplias malabaricus, Rhamdia spp., storks leave their natal area and probably not Aequidensspp., Hoplosternum littorale, Trachycor- in their first calendaryear (ThomasMS). Dark ystesgaleatus); freshwater eels (Symbranchus iris color is the best field mark for identifying marmoratus);rodents (Holochilusbrasilensis, Zyg- young Maguari Storks. odontomysbrevicauda); crabs (Dilocarcinus denta- Nestlingfood and feeding.--Nestling Maguari tus); [Lothoceriasgrandis (Belastomati- .Storks were fed almost entirely upon aquatic dae), Dytiscid larva, dragonfly nymph, organisms.Parents carry food to the nestas a grasshopper];earthworms (Lumbricusspp.); large bolus in the throat or crop. They regur- snake(Lygophis lineatus); and bird [oneunfeath- gitate it onto the nest,often bit by bit when ered nestling (not identified)]. the chicks are young and as a large massto The relative importance of food items and older nestlings.There it is pickedup and eaten classesin 54 boll is illustrated in Fig. 6. Al- by the nestlings.Feedings are copiousbut in- though I identified some food items from a frequent (ThomasMS). blind rather than collecting them, it appeared Food brought to the nest was alwaysfresh, that different food classesdominated by weight whole, and undigested;occasionally small fish in different years (see Methods):Pseudis para- October1984] MaguariStork Nesting 817

lings are very young, many small items, like eggs, earthworm-sizedeels, and fish less 550 than 5 cm, are brought. After an adult drops food on the nest, it looks at the bolus and, sort- 525 ing the food with its bill, quickly picks up and

500 swallowslarge items, including all rodents if the chicks are small. Sometimes a relieving 5?5 adult depositedfood before the other left the nest,and its matepicked through the boll and 250 ate some of the largest items. Most boll were made up of a variety of food classes.For ex- ample,two nestlings2.5-3 weeksold received • 200 3 babyrats and 2 eachof frogs,small fish, small œ crabs,and insectsin a single meal. Three nest- • 175 lings 6-7 weeksold were fed in a meal I0 rats (5-12 cm body-size,350 g combined),31 fish (2-10 cm, 145 g), a tiny eel (1.5 g), 4 crabs(2- 5 cm, 45 g), and 2 large insects(10 cm, 46 g), for a total of 588.5 g. Adult Maguari Storks do not disgorge food until all their nestlingsare resting on their tar- si facing the center of the nest, where they beg by flapping their wings. During the transition 5O periodbetween fledging and independence,the

25 - young are still largely dependent on adult feeding. Fledglings are fed both on the nest and in the marsh for 5-6 weeks. As a result of 0 4 8 12 16 20 24 8 2 •6 40 Days after hatching this kind of food transfer,young storksreceive a fairly equal shareof food;the largestnestling Fig. 4. Wing growth of siblingsdifferent in age usually grabsthe largestitems, while smaller by 4 days. Measuredfrom the bend of the wing to sibseat smaller ones.This could lead to dispro- the tip of the longestprimary. Symbolsas in Fig. 1. portionategrowth, but, by the time the largest chick swallows a large fish or eel, the smaller ones have eaten many items. Sibling rivalry wasrare and mild. During the doxustadpoles in 1973, eels in 1974, and ro- study, there was no instance of brood reduc- dents in 1975. Pseudisparadoxus tadpoles were tion through smallersiblings starving, perhaps up to 12 cm long and weighed as much as 18.5 becauseof food abundance in the study area. g; sometimes5-6 were in a singlebolus. A large The weights of asynchronouslyhatched Ma- eel (60 x 2.5 cm, 326 g) was fed to an 18-day- guari Storksin the samenest varied greatly. It old nestling, which took severalhours to swal- was commonto find 500-1,400-g weight differ- low it. By the age of 4 weeksyoung storkswere encesbetween the largestand smallestsiblings able to eat rodents with bodies 16 cm long. The in 3-4 chick clutches, yet the smallest birds most closely observed nestlings regurgitated fledgedsuccessfully. At hatching,the younger their first pellets at the agesof 21 and 25 days of the two chicks in the most closely observed (42.3 x 78.3 mm, 75 g, and 42.2 x 53.7 mm, 30.5 nest weighed 46% of the weight of its 4-day- g; weighed when fresh and wet). At this age older sibling. A difference in weight between the young are first fed rodents, and pellets re- the two continued with age, but the percentage flectedthe larger amount of indigestiblefur, in decreased(calculated at 5-day intervals, the addition to and crab exoskeletons. smaller chick's weight being 47%, 54%, 60%, The quantity and size of food brought by 88%,74%, and 87%of the larger). Nevertheless, parent storks depended greatly on the age of their growth rates as seen in Figs. 1-4 were their nestlings,a variation that Kahl (1964) re- very similar. ported in Wood Storks. When Maguari nest- The growth rate (Fig. 3) of the bill (exposed 8 18 BETSYTRENT T•OM^S [Auk, Vol. 101

Age(days) 0 - white down

3 -- PLUMAGE blackdow/•n bla•ck(head semipand neck)lumes NATA L 9- (body)

15-- I black feathers prejuvenal molt 21-- (remiges) black contour feathers I 27-- JUVENAL

33-- PLUMAGE

39--

45-- white body down 51--

57-- first prebosic molt white semiplumes 63-- (head and neck) white contour feathers 69--

75--

81- J

87 - FIRSTPLUMAGEBASIC 93 - Sequence of plumages and molts Fig. 5. Sequenceof plumagesand molts in young Maguari Storks.The age indicated is the first time that the plumagegeneration is clearly visible, or (later) the time when feathersfirst begin to break out of their sheaths.

culmen) was closeto that of Wood Storks (Kahl Behavioraldevelopment.--On its first full day, 1962); the tarsus (Fig. 3) grew like that of a the Maguari nestling sits and moveswith agil- Wood Stork for the first 3 weeks, and then it ity on its tarsi and begswith its neck arched grew faster.The longestMaguari toe, however, and bill open. Its eyes are wide open, and it was slower growing and remained shorterthan grabsaccurately at the nest surface.By day 9, the Wood Stork toe. Wing measurements(Fig. the elder of the two closely observed chicks 4) are not comparable,because Kahl measured began to defend its nest against my approach the longestprimary and I measuredfrom the by gobbling fiercely and biting my hand, while bend of the wing to the end of the longestpri- the 4-day-younger sibling lay flat on the nest mary. Not unexpectedly,Maguari Stork nest- in akinesia, a form of deliberate immobility. ling weight (Fig. 1) was always higher than The following day both chicks used this form that of Wood Stork nestlings, and it increased of defense.Gradually, thesetwo nestlingsgrew at a more acceleratedrate; young Wood Storks increasingly aggressive,and after 5 weeks I weigh about 2,500 g at fledging, whereasMa- stopped handling them lest they injure them- guari Storksweigh about 4,000 g at that time. selves. By contrast,the parents, who attacked October1984] MaguariStork Nesting 819

Frogs ( *adpoles IlO

100 Fish

90 • 1973(22)

80 • m74 (93)

• 7o i 1975(109) I;• 1976(102) • Eels •--• 1977(18)

Rodents

o•

2o Insects Ei3rthworms

IO -- •-c• • • Othe•

Fig. 6. Food items fed to Maguari Stork nestlingsfrom 54 boli (n = 344), all sizescombined. Total items per classshown in the wide unshadedcolumns; number of items per year in the narrow bottom columns, with the percentageof eachfood classby year above.Total numberof itemsidentified each year is shown to the right of the year symbol.

me during my early nestvisits, gradually grew down onto a neighboring nest to escapeme. In tranquil as they becamehabituated to my daily eachcase only the residentyoung defendedthe nest inspections. nestby attackingme; the intrudersalways stood The younger nestling stood on its feet at 22 behind them and hissed. days,whereas its sibling did not do so until 38 VocaIizationsand othersounds.--Although the days old. There was great variation among Maguari Stork doesnot have a wide repertoire nestlings in regard to this behavior. Young of vocalizations, it does make some sounds, one nestlingsattempt to defecateover the edge of of which is mechanical.The ageswhen I first the nest from the tarsal stance,but by 4 weeks heard soundsfrom nestlings, their mnemonic of age, when most stand on their feet, they al- notations, and my interpretations of their ways defecatedover the nest edge. During the meanings are: last weeks of nest life young Maguari Storks begin to whiten their legs with excreta.This (1) First soundat 2.5 h: a thin peeping(beg- "urohidrosis" is a thermoregulatory behavior ging). of Wood Storks (Kahl 1963). By the age of 6 (2) Day 4: eheheehehe ehehe (rapid begging). weeks, when they are left unattended on the (3) Day 9: gobbling like a [defense nest, the young sometimesgape, pant, gular against nest intrusion by humans and flutter, and use ptilorerection to dissipateheat Carib Grackles(QuiscaIus Iugubris)]. during midday. (4) Days 10-14: onk onk onk onk,4-5 notes in At 1 month, most nestlings begin a strong descendingtones given by nestlingsboth defenseof their nestsby flapping their wings, beforeand after eating;sometimes I could moving toward the intruder, snapping,biting, induce this vocalization by stroking a and making various defensive sounds(see be- nestlingon the head/nape (subduedbeg- low). Maguari Stork nests are often less than ging, contentment?). 0.5 m apart, but these close nests are always (5) Week2: bill clappingand snapping; when constructedat different heights, with no ver- the chicks are very young, hardly any tical overlap. The defended is the nest sound results because of the softness of properand the airspacedirectly aboveit. Seven the tomia (defense). times when I was banding nestlingsat closely (6) Week 3: grunts, occasional (protests at spacednests, the young of one nest stepped handling). 820 BETSYTRENT THOMAS [Auk,Vol. 101

(7) Weeks 4-5: throaty hiss, similar to the begin to hunt visually in water that is seldom adult sound that accompaniesthe adult more than 20 cm deep at this time of year. Up-Down display (Kahl 1972) (defense, Gradually, the young of a colony coalesceinto agonistic, social). a loose flock, and younger chicks from a single colony often fledge earlier than normal to join Fledgingand roosting behavior.--Around the age these flocks. Fledglings never wander or fly of 6 weeks, nestling Maguari Storks begin to away from the group alone except to return to exercise their wings with increasing vigor. their nests.Apparently, young storksand their Gradually, they add small vertical jumps, with parents are able to recognize each other indi- their wings widespread,while they facestrong vidually. The young birds constantlyscan the morning breezes;then they land again on the sky, and, when one sees a parent circling to nest. As the young grow, the nest becomes land in or near the marsh, it and its sibs hur- crowded, and, if an empty neighboring nest riedly walk toward the parent. When they reach exists,often a nestling will exercisefrom it by it, they drop down on their tarsi and beg before jumping a meter or more above its surface. they are fed. Such feedings occur 100-900 m The siblings that I measureddaily were first from the next. Frequently, several adults stay left unattendedduring the day when they were in the marsh with the fledglings, and, occa- 41 and 45 days old. As the time for fledging sionally, a hungry juvenile attemptsto beg food approached, they were fed less and less, al- from an unrelated adult. In such cases,it is ig- though the parents flew low overhead and nored, rebuffed, or mildly attacked. perched nearby watching over them. Each time I was able to watch the activity of three 14- a parent returned to the area, the large chicks 15-week-old siblings for 3.5 h, becausethey flung themselvesdown on their tarsi, facedthe stayedin a dry field with low vegetation. The centerof the nest,and beggedwith wildly flap- behaviorof eachbird was recordedevery 5 min ping wings and loud vocalizations.If the adult for 1.5 h and then every 2.5 min for 2 h (for a flew away, the young promptly stopped beg- total of 177 point samples).Individual differ- ging and stood up. When a parent did bring a ences in their behaviors were small. Collec- few small bits of food, they were consumed tively, they spent 10%of the time foraging, 34% ravenouslyby the young birds. On its 67th day, , 32% standing still, 10% resting with the elder of the two made its first flight. These the head retracted,9% walking (not foraging), nestlingshad not been fed at all for a minimum and 5% flying. Their banded father flew to the of 18.5 h preceedingthe flight. By 1116,while field once and fed them. Their mother came 2.5 exercising its wings, the nestling flew off in a h later, perhaps in responseto a Jabiru harass- gust of wind, circled the nest tree, and landed ing them, and all flew away with her. Three- in the marsh nearby. An adult Maguari Stork, month-old Maguari Storks occasionally sup- which I believe was its male parent, flew down planted smaller waterbirds at marsh feeding besideit in the marsh, but after 8 min the fledg- sites (Thomas in press). The young storks re- ling flew directly back up to its nest. During mained in the study area, gradually moving to this time, the bird's sibling and two younger wetter sites as marshesdried, but by 3.5-4.5 storks in an adjacentnest ceasedall activities months postfledgingall young of the year were and watched it intently. The younger sib made gone from their natal area. its first flight 2 days later at 65 days old. The Lossesof eggsand nestlings.--The 55 neststhat ability to fly back to the nest is important for a I observedover 9 yr had a hatching successof fledgling, becausethe nest is still used by par- 71.8%and a fledging successof 61.5%(Table 1). ent birds as a feeding site, and it is a place of I found no eggs broken in a nest, a common safety. Young storks often return to stand on indicator of egg shell thinning from pesticide their nests when they are disturbed in the contamination,nor were any deformed chicks marsh during early postfledgling days, and seen, although adult storksprobably fed in an young usually roost at night on their nestsfor area of intensive rice cultivation that was less up to 6 weeks after fledging. than 3 km from the study site and was subject- Postfiedgingbehavior.--By day, siblings stay ed to a broad spectrum of chemical applica- close together in the marsh surrounding their tions.Five unhatchedeggs from five nestswere nests.They stalk through high vegetation and infertile, 3 eggsfell out of very small nests,and October1984] MaguariStork Nesting 821

3 eggsin one nest were desertedand promptly semitorquatus)or a Great Horned Owl (Bubovir- eaten by a CrestedCaracara (Polyborus plancus). ginianus),both of which frequent the study area. During the 2 yr of highest stork-nestconcen- Ten nearly fledged and banded nestlingswere tration, a minimum of 32 eggs disappeared taken as food by human poachers. gradually; they were probably taken by boa The greatestnestling losseswere large young constrictors (Constrictor constrictor). Lower (2,200-3,000g) that fell out of undersizednests numbersof eggswere lost in the samepattern that contained 3-4 chicks. In each case that I in other years at other sites. observed(n = 6), the lost bird was the largest Maguari Storkslost theseeggs one at a time, or next-to-largest.Early in the study I returned the other eggs being left unharmed and the such birds, which were banded, to their nests, nest undamaged.Yet, in all but one case,the but I found that the same overcrowded nest entire clutch gradually disappeared.The ex- lost the same or a sibling bird again before it ceptionwas a nest from which 3 of 4 eggsdis- could fly. One suchbird lived for severaldays appeared.At this time I found two boas(ca. 1.5 in the water beneath its nest, but all of these m long, a male and a female) resting in a tree birds eventually disappeared,probably from where stork eggshad disappearedduring the attackby spectacledcaiman (Caimancrocodilus), preceding18 h. One of the snakeswas injured which are abundantin water surroundingstork and bled from a spot near its head. The follow- nests. ing year, closeto the same place, there were During the study, five nests(3%) were built additional lossesof stork eggs,and I began to on the tops of palm trees. Four of these nests monitor the clutchesof other speciesof water- (4-5 m high) were broken off by wind, with a birds nesting in closeproximity to the storks. minimum loss of l0 small nestlings.All nests Entire clutches of other birds disappeared in palms were small in diameter, made late in overnight:three 3-eggand two 1-eggclutches the season,and probably were made by inex- of the Bare-facedIbis (Phimosusinfuscatus) and perienced birds. The fifth palm nest was only one 3-eggclutch of a StriatedHeron (Butorides 3 m high, and it was the first nest of a 3-yr old striatus).The combined weights of the 3-egg banded male; one large nestling fell out and clutcheswere 104.5 g, 84.5 g, and 77.3 g, died below it, but the other nestling survived. whereas the weight of the clutch was The much heavier Jabiru (Thomasin press)al- 51.3 g. Thus, a clutch of these birds' eggs ways made its massivenest on the tops of the weighedless than a singlestork egg. I found a same palm speciesin the study area and suc- 1.3-m boa resting on an empty ibis nest that cessfullyraised as many as five young (Thomas had held three eggsand was 0.5 m below and 1981). The Jabiru, however, uses the top of a underneath a stork nest containing two fresh living palm, whereas the Maguari Storksused eggs. The next morning one stork egg was dead palms, which cannot support the weight missing,and the boa was still on the ibis nest. of the nest and young during strong winds. The secondstork egg was gone 4 h later, and the boa was restingas before on the ibis nest. DISCUSSION Snakepredation on eggsof colonial birds nest- ing over water has been reported by other Two morphologicaldifferences between Ma- workers (Dusi and Dusi 1968, Dickerman and guari Stork nestlings and other ciconiiformes GavinoT. 1969,Wiley and Wiley 1980). are the inability to grasp with the hallux and FewerMaguari Stork nestlings were lostthan the two black nestling plumages:these differ- eggs. Two hatchlingsof a very late nest dis- ences may be related. In sympatric colonial appeared,and the third hatchling may have ,egrets, and ibisesthat bred in the study been deserted.One 3-day-old chick fell out of area and in closely related specieselsewhere, its nest and apparently died while caught in the young leave the nest at an early age and brancheswithin the reach of its parents' bills. climb aroundin the nesttree long beforefledg- One medium-sizednestling (ca. 1,500 g) dis- ing (McVaugh 1972,Snow 1974,Kushlan 1977a, appearedfrom the top-mostnest of a colony on Maxwell and Kale 1977, Werschkul 1979). Pre- an extremelymoonlit night while its two sibs sumably, this early nest leaving is to elude remained unharmed. It may have been taken predators or to survive the destruction of flim- by either a Collared Forest-Falcon(Micrastur sy nests(Dickerman and Galvino 1969).In these 822 BETSYTRENT THOMAS [Auk, Vol. 101 speciesthe legsand feet developrelatively rap- changesto aggressivephysical and vocal de- idly. Comparedwith Wood Storks(Kahl 1962) fense. It must be pointed out, however, that and White Ibis (Eudocimus albus) (Kushlan Wood Storksbecome aggressiveat this age too 1977a),however, Maguari Stork nestlingshave (Kahl pers. comm.). slow-growingtoes, they are slow to stand on The timing of the rapid acquisitionof white their feet, and they are not able to cling to first basic plumage coincideswith the fledging branches using their hallux. Therefore, Ma- of Maguari Storks.White is no doubt more ad- guari Stork nestlingsare nest bound and un- vantageousfor socialsignaling (Kushlan 1977b, able to escapepredators. Mock 1980)such as keeping fledglingstogeth- Beingnest bound may be relatedto the two er and aiding adults in finding fledglings in black plumages of a bird that is white as an the marshesfor feeding. In addition, probably adult. In a review of the selectiveadvantages an appearancesimilar to more wary and expe- of blackversus white plumage,Mock (1980) has rienced adults could be of survival value to a listed five possiblereasons for the advantage young, inexperienced bird. A final reason for of one color over the other. The first, that black white plumage at this age has been demon- feathers are stronger, may be irrelevant for stratedby Mock (1980),who showedthat white nestlings.The second,for thermal regulation, plumage is lessconspicious to prey fish against remains controversial, evidence having been a bright sky. Thus, the color series,and timing found in favor of both black and white in dif- of Maguari Storkplumage (black for crypsisand ferent speciesunder different conditions(Hep- white for social signaling and hunting strate- pener 1970, Lustick 1971, Ellis 1980, Walsberg gy) support three of Mock's five selective ar- 1982). Maguari Stork nestlings do not appear guments. to need their dark color for thermal regulation. ACKNOWLEDGMENTS A few adults deliberately brought water back to the nest, which they drooled on to their It is a pleasure to thank the many people who young at midday. Nestlings are brooded or kindly assistedme during this long study. Ernst shadedcontinuously by their parentsuntil they Sch'tiz,John Ogden, and Nancy Wicker reviewed an are more than 5 weeks old, after which they early draft and contributed valuable suggestions; Philip Kahl, JamesKushlan, and Douglas Mock re- thermoregulateby gular flutter, panting, ptil- viewed the paper and made thoughtful and useful oerection, and urohidrosis. comments.Those who helped by identifying prey It seemsmore likely that the two black nest- species are Mark Staton, Dale Marcellini, Antonio ling plumagesare for crypsis.Unquestionably, Machado A., Peggy O'Connell, and Bill Mader. Ad- in the sharpchiaroscuro of tropicallight, a dark rian Warren sexed the boas. I am also indebted to bird is harder to see than a white one, both Max Bloesch,Lotte Sch'fifer-Bloesch,George A. Clark, from above and in a horizontal plane (Kahl Ramon Aveledo, Janet Hinshaw, and especially to 1971b, Kushlan 1978, Mock 1980), but it is the Tom•s and Cecilia Blohm on whose ranch I studied timing of the blacknestling plumages that most the storks and to John B. Trent who has continued strongly suggeststheir role in crypsis.All oth- to provide somefinancial support. er stork nestlings that have been described LITERATURE CITED hatch with white down, and all young storks probably have the same plumage successions BLAKE,E.g. 1977. Manual of neotropicalbirds, vol. during the first 3 months, even though the 1. Chicago,Univ. ChicagoPress. plumagesare not conspicuouswhen the series DICKERMAN, R. W., & G. GAVINO T. 1969. Studies is all white. In contrast both the second down of a nesting colony of Green Herons at San Bias, and the juvenal plumage of the Maguari Stork Nayrit, . Living Bird 8: 95-111. are black (ca. 10-75 days). Maguari chicks ex- Dusi, J. L., & R. T. DusI. 1968. Ecologicalfactors hibit akinesia only during their first 2 weeks, contributing to nesting failure in a heron col- when they are still mostlywhite, and the effect ony. Wilson Bull. 80: 458-466. ELLIS, H.I. 1980. Metabolism and solar radiation in is that they look like white eggs.Nestlings of dark and white heronsin hot climates.Physiol. the White Stork continue this presumably Zool. 53: 358-372. predator-avoidancebehavior up to the age of 7 GOLDSMITH,T. H. 1965. The red-yolked egg of the weeks (Schtiz 1942, 1943; Haverschmidt 1949). Touraco, Tauracocorythaix. Postilia 91: 1-7. After 2 weeks of age, when the Maguari chicks HAVERSCHMIDT,F. 1949. The life of the White Stork. are black, their behavior toward an intruder Leiden, E. J. Brill. October1984] MaguariStork Nesting 823

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