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Why Robin Eggs Are Blue and Birds Build Nests: Statistical Tests for Amazonian Birds

1985; Issue: 36 Linguagem: Inglês

10.2307/40168303

1941-2282

Yoshika Oniki,

Ecology and Vegetation Dynamics Studies

Data on egg color, nest type, and nest location for bird species nesting at the Amazonian localities of Belem and Manaus, Brazil were examined for possible adaptations against predation and other negative environmental factors. White egg color and spotting was not related to sunlight, except in the negative sense that concealed eggs in any habitat lacked pigments. White eggs, often with spots, were also associated with thin nests in which eggs were visible from below; such eggs may resemble transparent holes in the canopy, in the fashion of transparent wings of certain understory butterflies. Blue eggs normally were found in thick, dark cup nests in dappled sunlight, apparently imitating spots of light on dark foliage in the manner of black-and-blue butterflies of similar zones. Buff (pink) color was most common among eggs placed against dull substrates, and when combined with spotting, resembles the tiger pattern known for certain butterflies of dull substrates. Oven-shaped nests were always thick, but evidently not for protection against weather; pouch nests, open above and to the weather, were also thick. More likely, such large nests had to be thick to discourage predator entry or detection of contents. Pouch nests were not necessary on thin supports (although a deep nest may secondarily keep eggs and young from rolling out); cup nests also occurred in such locations. Pale nests were more common on pale supports and in open areas, dark nests on dark supports and in dark locations, but hypotheses of heat gain or loss in relation to nest color were not supported. Small nests tended to be concealed from above, but none were oven-shaped; protection was against predators, not against sun or rain. Cavity nests were rare within 1 m of the ground, another fact explicable only if nests provide protection against predators rather than against sun or rain. A number of environmental factors, such as human interference (especially with ground or cavity nests) or low vegetation (grass, palms, second growth) limited nest sites. Limitation of nest sites may be as important as limitations of foraging niches in areas with limited foliage height diversity. No postulated anti-weather function for nests was demonstrated; my data supported only anti-predator functions as reasons why birds build nests or have pigmented (rather than white) eggs. RESUMEN. Se examino information respecto al color de los huevos, tipo de nido y ubicacidn de los mismos, para especies de aves que anidan en Belen y Manaos, en la regi6n amazonica de Brasil, para identificar posibles adaptaciones en contra de la depredacion y otros factores ambientales negativos. Ni los huevos de color bianco y ni los moteados, estuvieron relacionados con luz solar, excepto en el sentido negati vo que los ocultaba en cualquier ambiente carente de pigmentation. Los huevos blancos, generalmente moteados, tambien estuvieron asociados a nidos muy delgados en los que los huevos podian ser vistos desde abajo, esos huevos pueden asemejarse a agujeros transparentes en una boveda o techo, algo similar a las alas transparentes de ciertas mariposas del nivel bajo del bosque. Los huevos azules se encontraron normalemente en nidos gruesos oscuros con forma de copa salpicados de luz solar aparentemente imitando manchas de luz en follaje oscuro de la misma manera que las mariposas azul y negro de areas similares. El color ante (rosado) fue mas comun entre los huevos puestos contra substratos poco coloridos y cuando estuvieron combinados con manchas se asemejaron al modelo tigre, conocido en ciertas mariposas de substratos apagados. Los nidos en forma de homo, siempre fueron gruesos, pero evidentemente no para protection climatica, nidos en forma de bolsa, abiertos en su extremo superior y a los efectos climaticos, tambien fueron gruesos. Es mas probable que esos nidos grandes sean gruesos para desalentar el ingreso de los depredadores o que el contenido sea detectado. Los nidos en forma de bolsa no estuvieron necesariamente fijados a elementos delgados (aunque un nido profundo puede secundariamente prevenir que los huevos y los polluelos se caigan del nido); tambien se encontraron en esa posicion nidos en forma de copa. Nidos palidos fueron mas comunes en soportes palidos y