WING MORPHOLOGY AND FLIGHT BEHAVIOR OF SOME NORTH AMERICAN HUMMINGBIRD SPECIES

Auk, The, Jul 2005 by Stiles, F Gary, Altshuler, Douglas L, Dudley, Robert

ABSTRACT.-

We explored the relationship between wing morphology and flight behavior with respect to sex and age in five species of North American hummingbirds. We first measured the length, chord or "width," and area of entire hummingbird wing planforms. We then calculated additional parameters of wing shape and size, including aspect and shape ratios, degree of taper or "pointedness," wing loading, and wing disc loading (WDL). Wings of adult males are not only shorter but also more narrow and tapered than those of adult or immature females; immature males have larger wings and lower WDL, more like those of females. A proposed relationship between WDL and territorial behavior and dominance is not supported, given that adult and immature males show similar feeding territoriality outside the breeding season but females rarely do. The more extreme and divergent wings of adult males probably reflect sexual selection in connection with aerial displays that include species-specific sound effects given during the breeding season. North American species are unusual among hummingbirds in showing reversed sexual size-dimorphism (males smaller, with relatively shorter wings), a feature shared with some other small hummingbirds, notably the "Pygmornis" hermits. Attempts to explain hummingbird foraging and territorial behavior on the basis of differences in WDL have failed because many aspects of wing morphology, physiology, and flight behavior were not taken into account. Several wing parameters appear more related to other modes of flight than to strategies of nectar exploitation, and the morphology of any given wing represents a compromise between the often conflicting aerodynamic demands of different flight modes. Understanding hummingbird flight will require broad comparative studies of wing morphology and wingbeat kinematics in relation to flight behavior, and new theoretical models and experimental data will be needed to elucidate physiological and aerodynamic mechanisms underlying forward flight and maneuvering.

Key words: aerodynamics, flight behavior, hummingbirds, sexual selection, wing morphology.

Morfolog�a Alar y Comportamiento de Vuelo de Unas Especies de Colibr�es de Norteam�rica

RESUMEN. - Exploramos la relaci�n entre la morfolog�a del ala y el comportamiento de vuelo con respecto al sexo y la edad en cinco especies de colibr�es norteamericanos. Primero medimos el largo, la cuerda o "ancho" y el �rea de una planforma de cada ala, luego calculamos varies par�metros del tama�o y la forma de cada ala incluyendo las razones de forma y aspecto, el grado de "puntiagudeza", la carga alar y la carga del disco alar (wing disc loading o WDL). Las alas de los machos adultos son mas cortas, angostas y puntiagudas que las de las hembras adultas y inmaturas; machos inmaturos tienen alas m�s grandes con WDL mas bajas, mas similares a las de las hembras. Estos datos no corroboran una supuesta relaci�n entre WDL, comportamiento territorial y dominancia, puesto que machos adultos e inmaturos muestran comportamiento territorial similar fuera de la �poca reproductiva mientras las hembras rara vez defienden flores. Las alas m�s extremas y divergentes de los machos adultos probablemente reflejan la selecci�n sexual sobre despliegues a�reos que incluyen sonidos propios de cada especie, producidos por las alas. Los colibr�es norteamericanos son excepcionales entre la familia Trochilidae en su dimorfismo sexual invertido (machos m�s peque�os, hembras m�s grandes excepto en Calypte anna), una caracter�stica compartida con algunos otros colibries pequenos, especialmente los ermita�os "Pygmornis"). Los intentes de explicar diferencias en comportamiento de forrajeo y territorialismo con base en WDL fracasaron porque no se tomaron en cuenta muchos otros aspectos de la morfolog�a alar, fisiolog�a y comportamiento de vuelo. Varios par�metros alares parecen ser mas relacionados con otros tipos de vuelo que con estrategias de explotaci�n de nectar floral. La morfolog�a de cualquier ala representa un compromiso entre las demandas a veces contrapuestas de diferentes modos de vuelo. Para entender el vuelo de los colibr�es ser�a preciso llevar a cabo estudios comparativos amplios sobre la morfolog�a alar y la cinem�tica de los aleteos en relaci�n con el comportamiento de vuelo. Se requieren tanto nuevos modelos te�ricos como datos experimentales para entender los mecanismos subyacentes del vuelo hacia adelante y las capacidades de maniobra de los colibr�es.

HUMMINGBIRDS (TROCHILIDAE) ARE the second-largest strictly New World bird family and the most specialized avian nectarivores (Stiles 1981). The only birds capable of sustained hovering and backward as well as forward flight, they alone can probe flowers without perching. However, within that high degree of locomotor specialization, hummingbirds exhibit considerable diversity in wing sizes and shapes, both within and between species (Ridgway 1892, Stiles 1995). That diversity is only beginning to be understood in aerodynamic, ecological, and behavioral terms.