Form, Foraging Behavior, and Adaptive Radiation in the Tyrannidae

Abstract

Relationships between foraging behavior and external measure- ments of the bill, wings, tarsus, and tail are analyzed for 94 representative species of tyrant flycatchers (one quarter of the family). Results are presented in three parts: (1) Allometric relationships are summarized for external measurements, followed by general correlations between measurements (corrected for body size) and foraging mode. Upward-strikers have wide bills, short wings, long tarsi, and reduced tails; perch-gleaners and groundforagers both have narrow bills, and with the latter also showing substantially longer legs; aerial-hawkers have triangular bills, extremely long wings (as do long-distance migrants), and short legs. Generalists, using many prey capture techniques, have intermediate structural features. (2) Using three sample analyses I show that relative sizes of certain characters vary continuously among species along a quantitative scale of foraging behavior. The morphology-behavior functions suggest adaptive interpretations for much of the observed structural vari- ation. Evolutionary change in structure appears to accompany fine-scale changes along behavioral spectra. Extreme structural peculiarity characterizes the most behaviorally specialized forms, and even could limit evolutionary flexibility at the end-points. These relationships support the notion that evolution proceeds most easily from generalized form and behavior, through intermediate conditions, to the most extreme behavioral and morphological specializations. (3) I speculate on the directions along which increasingly specialized descendants evolved from generalized ancestors in the Tyrannidae, suggesting that these pathways are still visible in present-day forms. The great diversity within the Tyrannidae results, in part, from the unusually competitor- free environment within which the nonoscine lineages may have evolved. This en- vironment allowed various flycatcher species to stop along certain routes toward specialization, remaining or even proliferating at stable intermediate points while certain others continued to specialize further. In this way, today's family Tyrannidae may illustrate pathways of morphological and behavioral specialization long since obliterated in most other modern bird families. RESUMEN. Se analiza la relation entre el comportamiento de alimentation y medidas externas del pico, alas, tarso y cola para 94 especies representativas de atrapa moscas tirano (un cuarto de la familia). Los resultados se presentan en tres partes: (1) Se resumen las relaciones alometricas para medidas externas, seguido por una correlation entre medidas (corregidas para el tamano del cuerpo) y el modelo em- pleado para alimentarse. Las especies que picotean hacia arriba (upward-strikers) tienen picos anchos, alas cortas, tarsos largos y cola reducida; aquellas aves que picotean posadas en ramas (perch-gleaners) asi como las que se alimentan en el suelo (ground-foragers) tienen picos angostos y estas ultimas aves muestran patas singularmente largas; las que cazan en el aire (aerial-hawkers) tienen picos trian- gulares, alas extremadamente largas (tal como las que migran grandes distancias) y patas cortas. Aquellas aves que usan tecnicas diferentes para la captura de sus presas (generalists) presentan componentes intermedios. (2) Utilizo analisis de tres ejem- plos para mostrar que los tamanos relativos de ciertos caracteres varian continua- mente entre especies a lo largo de una escala cuantitativa de comportamiento de forraje. Las funciones de comportamiento-morfologia sugieren interpretaciones adap- tativas para la mayoria de las variaciones estructurales observadas. Los cambios evolucionarios en la estructura, parecerian acompanar cambios muy pequenos en los espectros de comportamiento. La presencia de peculiaridades estructurales externa caracteriza a las formas con comportamientos mas especializados e inclusive podrian limitar la flexibilidad evolucionaria en los puntos de los espectros. Estas relaciones soportan la idea de que evolution avanza mas facilmente desde formas y compor- tamientos generalizados a traves de las condiciones intermedias hacia las especiali- zaciones mas extremas de comportamiento y morfologia. (3) Especulo sobre las di- recciones a lo largo de las cuales evolucionaron los descendientes mayormente especializados de ancestros generalizados en los Tyrannidae, sugiriendo que esas direcciones aun son visibles en formas de la actualidad. La gran diversidad en Ty-