Maguari Stork Nesting: Juvenile Growth and Behavior

Although it has a wide range in tropical South America, the Maguari Stork (Ciconia maguari) is poorly known. In an 11-yr study of its breeding biology in the llanos of Venezuela I found the mean clutch size for 3 yr to be 3.2 eggs. The young hatched asynchronously at 29-32 days and usually made their first flight at 60-72 days. Data on egg sizes, weights, and a probable dump egg are given. The weight and growth of two siblings, plumage succession, and nestling and fledgling behaviors are described. The fledging success of eggs from 55 nests, over 9 yr, was 61%. Loss of eggs (presumably from snake predation) was higher than nestling loss. Although the asynchronously hatched young in a clutch varied greatly in weight and continued this difference throughout nestling life, no case of brood reduction was found in 123 nests during the study. Nestlings were fed mainly frogs and tadpoles, fish, eels, and aquatic rats; food classes and food sizes brought by adults varied with the age of their young. Maguari Stork nestlings do not grasp with their hallux and so are confined to their nests until they can fly. I suggest that the two black plumages of nestlings, between the white hatching down and the white first basic plumage, are for crypsis of the nest-bound young. Received 29 August 1983, accepted 1 March 1984. THE Maguari Stork (Ciconia maguari = Euxenura galeata) is found only in South America and is less widespread than the other two storks of the Western Hemisphere, the Wood Stork (Mycteria americana) and the Jabiru (Jabiru mycteria), both of which range farther to the north (Blake 1977). Although the three species are largely sympatric in South America, the wide ranges indicated on distributional maps are somewhat misleading, because these storks are ecologically restricted to tropical, low-elevation wetlands and adjacent nonforested areas. In Venezuela the three storks are found together in the interior of the country, a vast savanna called the llanos. Mayr and Amadon (1951) and Kahl (1971a) have suggested that this stork is closely related to the White Stork (Ciconia ciconia), and this paper gives a broader basis for a comparison of the young with those of the better known White Stork (Schiuz 1942, 1943, 1944; Haverschmidt 1949). The principal published information about the Maguari Stork is from 2 months of observations of nesting birds in Argentina (Kahl 1971b). For 11 yr I followed the seasonal breeding of all Maguari Storks on a ranch in the llanos of Venezuela. I give data on the eggs and on the asynchronously hatched young from the I Present address: 1 Wetsel Road, Troy, New York 12182 USA. day of hatching to the postfledging period. Perhaps because of abundant food on the study site, no incident of brood reduction was observed in 123 nests. The manner and size of food presented to the young and sibling growth data, however, suggest that brood reduction may not occur in this species or may be rare. Loss of eggs, presumably to snakes, was higher than nestling losses from all causes combined. One puzzling aspect of Maguari chick development is the succession of two different black plumages, between the white hatching down and the white first basic plumage (Humphrey and Parkes 1959) acquired at fledging. I suggest that the black plumages are for crypsis because of their timing and the fact that Maguari Stork nestlings, like adults, do not grasp with their hallux and thus cannot leave their nests to escape predators. STUDY SITE AND METHODS The study site (08?31'N, 67035'W) was a cattle ranch located roughly in the center of the Venezuelan Ilanos, about 45 km south of Calabozo in the state of Guarico. Troth (1979) provided an account of the vegetation of this ranch and Thomas (1979a) an annotated list of its bird species. The llanos are grassland savanna with scattered palms and clumps of other trees. During the rainy season (generally May to November) Maguari Storks nest, usually from June to October (Thomas in press), when much of the sa812 The Auk 101: 812-823. October 1984 This content downloaded from 40.77.167.14 on Tue, 20 Sep 2016 04:59:58 UTC All use subject to http://about.jstor.org/terms October 1984] Maguari Stork Nesting 813 TABLE 1. Maguari Stork hatching and fledging success.