106 J. Field Ornithol., 72(1):106–112 THE ‘‘PREDATOR EARLY WARNING SYSTEM’’ OF RED-WINGED BLACKBIRDS NICOLE BURTON AND KEN YASUKAWA Beloit College Department of Biology Beloit, Wisconsin 53511 USA Abstract.—Male Red-winged Blackbirds (Agelaius phoeniceus) are known to give alarm calls in response to the approach of a predator, and to encode information about the level of threat in their calling behavior. To determine whether such sentinel males alert females, we conducted a simple field experiment in which we measured the distances at which incubating females flushed from their nests in response to the approach of a human observer. Using a matched-pairs design, we measured flushing distances with a sentinel male present (mean 19.8 m), and when the same male was absent from his territory (mean 10.4 m). Female Red- winged Blackbirds flushed from their nests at significantly greater distances when males were present than when males were absent. These results and those of other studies support the existence of a ‘‘predator early warning system’’ in the Red-winged Blackbird. SISTEMA DE ALARMA TEMPRANA CONTRA DEPREDADORES POR PARTE DE AGELAIUS PHOENICEUS Sinopsis.—Los machos de Agelaius phoeniceus responden con llamadas de alarma a la pre- sencia de depredadores. Adema ´s e ´stas aves incluyen informacio ´n en sus llamadas sobre el nivel de peligro. Para determinar si los machos de e ´stas aves alertaban a las hembras, se condujo un experimento en donde se midio ´ la distancia a la cual la hembra dejaba el nido en respuesta al acercamiento de un observador humano. Utilizando un sistema de pareo medimos la distancia a la cual la hembra dejaba el nido cuando habı ´a como sentinela e un macho (19.8 m), y cuando el macho estaba ausente del territorio (10.4 m). Las hembras de la especie se salieron del nido a una distancia significativamente mayor cuando el macho estaba presente que cuando e ´ste estaba ausente. Estos resultados, al igual que el de otros trabajos, apoyan la existencia de un ‘‘sistema de alarma contra depredadores’’ en la especie estudiada. Numerous field studies of Red-winged Blackbird (Agelaius phoeniceus) reproductive success have identified predation as the single most impor- tant source of mortality of eggs and nestlings (Searcy and Yasukawa 1995; Yasukawa and Searcy 1995; Beletsky 1996). For example, during their 16- yr field study, Beletsky and Orians (1996) observed annual nest predation rates of 35–75%. Many reptiles, birds, and mammals are known to prey on redwing eggs and nestlings, and on average they produce nest failure at nearly one-third of all nests in this species (Beletsky 1996). Given the impact that predators have on nest success, it is not surprising that Red-winged Blackbirds have evolved anti-predator behavior (Searcy and Yasukawa 1995; Yasukawa and Searcy 1995; Beletsky 1996). Nests are concealed in vegetation, placed in proximity to each other, and initiated synchronously. Males and, to a lesser extent females, vigorously defend nests against predators. Finally, males use prominent perches within their territories and have an ‘‘alert system’’ of calls with which they can signal the presence of a nest predator (Beletsky 1996). Male redwings clearly communicate with each other using this call system (Beletsky et al. 1986).