Interspecific Aggression of Ash-Throated Flycatchers on Cassin's Sparrows

Abstract

During the course of field studies in 1970 on the Santa Rita Experimental Range, Pima County, Arizona, we observed eight instances of interspecific aggression by Ash-throated Flycatchers (Myiarchus cinerascens) on Cassin's Sparrows (Aimophila cassinii). The former is a medium-sized tyrannid which nests in abandoned woodpecker holes in much of the southwestern United States and into M6xico. It forages mainly by hawking for insects from the upper limbs of trees. The sparrow is a ground-foraging species, feeding on seeds in winter and ground-inhabiting insects during the summer. Nests are placed near the ground in low vegetation. Its song is delivered in an upward arching flight from upper limbs of trees. The pattern of singing flights closely resembles the hawking maneuvers of the flycatcher. It was during the flight song that the sparrows were attacked by the flycatcher. The latter flew straight at the sparrow and usually knocked it to the ground. Apparently, the flycatcher was responding to a behavioral image which approximates its own foraging maneuvers. Selection against such behavior is probably weak since Cassin's Sparrow is rather restricted in its distribution and then only occurs in grasslands. On the other hand, strong selection pressures may operate to maintain this type of behavior in the flycatcher since it overlaps several potential competitors (Myiarchus spp., Tyrannus spp.). One other observation adds strength to the above suggestion. A Cactus Wren (Campylorhynchus brunneicapillus), flying about 3 m above the ground, suddenly made an upward jog in its straight line flight. This deviation was similar in form to a flycatcher foraging maneuver. Immediately, an Ashthroated Flycatcher, which was perched about 4 m away, flew at the wren and knocked it to the ground. These data were obtained during research supported by the US/IBP Desert Biome program under grant no. GB 15886 from the National Science Foundation.