MAGUARI STORK NESTING: JUVENILE GROWTH AND BEHAVIOR BETSY TRENT THOMAS Apartado80844, Caracas1080-A, Venezuela ABSTRACT.--Althoughit has a wide range in tropical South America, the Maguari Stork (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 frogs and tadpoles,fish, 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 plumage, 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 Wood Stork served in 123 nests. The manner and size of (Mycteria americana)and the Jabiru (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 plumages,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 savanna 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 White Stork (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 Argentina (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 Storks 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 bird 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 birds 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 carotenoids, 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).