Clutch Size, Nesting Success, and Predation on Nests of Neotropical Birds, Reviewed

Abstract

A survey of the clutch size of 2 1 7 species of passerines of the humid neotropics shows that two is the prevailing number of eggs, sets of one and three are less frequent, and larger sets are rare. Contrary to what we should expect from the theory of maximum reproductionthat birds rear as many young as they can adequately nourishunaided females commonly have broods as large as those attended by both parents, sometimes with helpers. Failure to find consistent correlation between clutch size and number of nest attendants, diet, habitat, or type of nest (other than the well-known tendency of hole-nesters to rear larger broods) leads us to seek some factor, or factors, that profoundly influence the reproduction of most birds of the humid neotropics. Not to be neglected is the high percentage of nest failures, greater in forest than in neighboring clearings and plantations, and greater at low than at high elevations. Available evidence leaves the effect of human visits on nest losses uncertain; hatching failure due to infertility, faulty incubation, or other intrinsic factors appears to be no greater in the tropics than at higher latitudes; predation is certainly responsible for most losses. The major factor responsible for the small clutches of tropical birds of many kinds appears to be, as Cody and Ricklefs have argued, the less strongly contrasting seasons of the humid tropicsa measure of which is the annual march of evapotranspirationas compared with northern lands. The restrained reproductive effort of tropical birds is adjusted to their low annual mortality in a climate that does not force birds to confront a season of scarcity and stress unless they undertake hazardous migrations. Moreover, the high incidence of predation on nests makes it advantageous to limit the energy expended on a brood, so that, if this fails, strength remains for repeated trials. Also, the smaller the brood, the fewer the feeding visits that may reveal the nest's location to predators. Because ornithology was born in the north temperate zone where broods tend to be large, we ask why the broods of tropical birds are so small. If more ornithologists had grown up in the tropics, we would be asking why birds at high latitudes lay so many eggsa question easier to answer. RESUMEN. Un estudio del tamano de las nidadas de 217 especies de passeriformes de las regiones humedas neotropicales, muestra que el numero prevaleciente de huevos en una nidada es dos; siendo menos frecuentes nidadas de uno o tres huevos y son raras las nidadas mas grandes. Contrariamente a lo que deberiamos esperar, si consideramos la teoria de maxima reproductionque las aves crian tantos polluelos como les es posible alimentarlas hembras que no tienen ayuda cuidan nidadas tan grandes como aquellas nidadas que son atendidas por ambos padres, que algunas veces tienen ayudantes. El no encontrar una correlaci6n consistente para la relaci6n entre el tamano de la nidada y el numero de encargados del nido, dieta, habitat o tipo de nido (otra que la tendencia conocida para los anidadores en huecos que crian grandes nidadas), nos hace considerar ciertos factores que influyen profundamente la reproduction de la mayoria de las aves de las regiones humedas de los neotripicos. Algo que no debe ser descuidado es el alto procentaje de fracasos de anidaci6n, los cuales son mayores en el bosque que en las zonas abiertas o plantaciones cercanas y mayor a baja que a altas elevaciones. No esta claro que efecto tienen las visitas humanas en las peYdidas de nidos; fracasos de eclosion debido a infertilidad, incubaci6n defectuosa, u otros factores intrinsecos que parecen no ser mas importantes en los tr6picos que en otras latitudes mas elevadas; la depredaci6n es por cierto la mayor responsable en la mayoria de las perdidas. El factor mayormente responsable por el tamano pequeno de las nidadas de las aves neotropicales de cualquier tipo parece ser, tal como lo discutiesen Cody y Ricklefs, el menor contraste entre las estaciones en los tr6picos humedoslo cual puede ser medido por la marcha anual de evapo-transpiracionsi se compara con tierras septentrionales. El esfuerzo reproductivo moderado de las aves tropicales, se ajusta a la baja mortalidad anual en un clima que no fuerza a las aves a enfrentar una estacidn de escasez y stress, a no ser que participen en migraciones riesgosas. Mas aun la gran incidencia de depredacidn en los nidos hace ventajoso limitar el gasto de energia en una nidada, de