Coexistence and Behavior Differences among the Three Western Hemisphere Storks

Abstract

The only storks in the New World, the Wood Stork (Mycteria americana), the Maguari Stork (Ciconia maguari), and the Jabiru (Jabiru mycteria), were studied in the llanos of Venezuela. Stork weights and linear measurements are given, and behaviors are described and compared. The storks use both different foraging-techniques and intraspecific foraging-associations; these may result in resource partitioning among the storks and with other waterbirds. Although their diets of mostly aquatic organisms overlap broadly, available data suggest that the percent of each food class taken, and prey-sizes, are not alike. The three storks also differ in morphology, resident status, pair bond length, breeding months, nest-site preference, and nesting dispersion. The timing and amount of annual rainfall influences the storks' breeding months through the availability and abundance of food. RESUMEN. En los llanos de Venezuela fueron estudiadas las unicas especies de ciguenas del Nuevo Mundo, la Ciguena Gaban (Mycteria americana), la Ciguena Americana (Ciconia maguari) y el Jabiru (Jabiru mycteria). Se brinda informaci6n sobre pesos y medidas lineales de las ciguenas y se describen y comparan comportamientos. Las ciguenas usan tanto tecnicas de alimentation y diferentes como asociaciones de forraje interespecincas; esas pueden resultar en la repartici6n de los recursos entre ciguenas asi como con otras aves acuaticas. Aunque sus dietas son mayormente de organismos acuaticos y se superponen ampliamente, la informaci6n disponible sugiere que el porcentaje de cada clase de comida consumida y el tamano de las presas, no son similiares. Las tres ciguenas tambien son diferentes en sus aspectos morfoldgicos, situacidn como residentes, duraci6n de parejas, meses de reproducci6n, sitios preferidos para anidacibn y dispersi6n de anidaci6n. La epoca y la cantidad de lluvias anuales, influyen en los meses de reproducci6n a traves de la disponibilidad y abundancia de comida. Only three of the world's 17 living species of storks are native to the Western Hemisphere (Kahl 1971a); they are the Wood Stork (Mycteria americana), the Maguari Stork (Ciconia maguari = Euxenura galeata), and the Jabiru (Jabiru mycteria). Probably because of their large size storks are the socially dominant species of the waterbird guild in the Venezuelan llanos. Current ecological theory holds that no two species occupy exactly the same niche (Moreau 1 966); thus, it is of interest to examine some of the differences among these sympatric storks. In this paper I review some of the morphological and behavioral differences that may facilitate their coexistence. When he revised the taxonomy of the world's storks, Kahl (1971a) maintained these three species in separate genera, although he changed the Maguari Stork to Ciconia as an indication of its close relationship to the White Stork (Ciconia ciconia). Even though the three species share many familial traits of morphology and behavior, their placement in separate genera suggests, correctly, that distinctions also exist. STUDY AREA AND METHODS Beginning in 1972, and continuing for 1 1 years, I studied storks in the llanos in Venezuela. My main study area was a cattle ranch, Fundo Pecuario Masaguaral, which lies roughly in the center of the llanos, in the state of Guarico. This area is classified, under the Holdridge system, as tropical dry forest (Ewel and Madriz 1968). The habitat consists of open grassland savannas with occasional clumps of trees and gallery forests. The area is generally below 100 m in elevation; thus, the climate is tropical with one wet season and one dry season each year. Vegetation of the study site is described in Troth (1979), and the species of birds found there are listed in Thomas (1979). I also observed storks in other parts of the llanos, mainly in southern Guarico and throughout