Mobbing Behavior of a Pair of Elegant Trogons

Abstract

Mobbing behavior among birds is considered a mechanism of predator deterrence; however, the benefits of mobbing to small or passively mobbing species is not clear (Cully and Ligon 1976, Curio 1978). Most species mob by giving loud calls combined with wing and tail flitting that, together, appear to attract additional birds of many species. I report here my observations the mobbing behavior of a pair of Elegant Trogons (Trogon elegans) to a Great Horned Owl (Bubo virginianus). Although the trogons maintained a substantial distance between themselves and the owl, their calls quickly attracted other species, some of which mobbed very aggressively. By attracting more aggressive species to mob, small birds may be able to have predators driven away from their territories and derive the same benefits as the more aggressive species (the move on hypothesis of Curio [1978]). On 4 July 1970 at the South Fork Campground in the Chiricahua Mountains, Chiricahua County, Arizona, I tethered a live Great Horned Owl 60 cm above the ground at the base of a small tree. It was immediately mobbed by a pair of Elegant Trogons. (I later discovered that the tree contained their nest.) I observed these birds for approximately 30 min before I terminated the session by moving the owl away from the area. After the owl was removed the trogons freely entered their nest. I watched the birds from behind some shrubs at a distance of about 10 m. Calls were recorded a Uher 4000 Report-L tape recorder at 3.5 IPS using a Uher M514 microphone. The calls were analyzed a Kay Electric Company Sonagraph 6061B. I did not attempt to count calls or flits, and I did not attempt to measure the duration of the response. Within 1 min after I exposed the tethered owl, a male trogon began to mob from a distance of about 15 m. He was perched a branch approximately 5 m high in full view of the owl and me. Within 2 min after the male trogon began mobbing he was joined by his mate. Although their calls were loud and attracted various other birds, no other trogons joined the mob. The male trogon flew to a branch in view of the owl about 15 m away. He called and occasionally flitted his wings and tail. There was very little jumping or flying between perches. When the female arrived she took a perch with more cover about 6 m from the owl, and also began to call. Both trogons changed their perches during the mobbing session, but they never approached the owl closer than 6 m. Elegant Trogons used a wing-tail flit while mobbing. The trogons first rapidly flicked the wings (duration about 0.1 sec), and then slowly raised and lowered the tail (duration approximately 1.0 sec). There was no bowing of the body associated with the flit. Trogon mobbing calls consisted of a rapid series of loud staccato chucks given in phrases of 4 to 10 notes (Fig. 1). The male's calls sounded higher pitched than the female's, but otherwise they were similar. Harmonics ranged in frequency from 1.0 to 2.5 kHz in the male and from 0.5 to 6.0 kHz in the female. The dominant harmonic of the male was centered at 1.5 kHz, that of the female at 0.8 kHz. Since high-frequency harmonics attenuate faster in woodlands than low frequencies (Martin and Marler 1977), the shorter frequency range recorded for the male may have resulted from my recording his calls at a greater distance. Mobbing call phrases ranged from approximately 0.5 to 1.2 sec in duration. The individual notes were regularly spaced at 0.1 to 0.12 sec intervals. Each note of the phrase was of similar duration (0.08 sec) and frequency range. The energy of the calls was given in a rapid burst. The frequency range and amplitude characteristics of the calls of many species of mobbing birds are well suited to facilitate locating the calling birds, which makes it easy for more birds to join the mob (Marler 1955, Marler and Hamilton 1967, Martin and Marler 1977). The calls of Elegant Trogons are similar. High frequencies attenuate quickly, making a wide frequency range useful for determining the distance to the source (Martin and Marler 1977). The calls were repetitive and began and ended abruptlycharacters that enhanced direction finding. Trogon mobbing was passive compared to that of many other species. Gray-breasted Jays (Aphelocoma ultramarina), which joined the mob within 1 min after the male trogon began calling, jumped and flew constantly, often diving within 5 cm of the owl's head. Passerine mobbing is characterized by loud calls, numerous jumps and flights from perch to perch, and frequent tail flits (Hinde 1954; Cully and Ligon 1976, in press). In passerines, the tail flit