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Müllerian Body Feeding by Andean Birds: New Mutualistic Relationship or Evolutionary Time Lag?1

THE MUTUALISTIC RELATIONSHIP BETWEEN CECROPIA PLANTS AND AZTECA ANTS is well studied (Janzen 1969, Schupp 1986, Duarte & Bergallo 1992). Cecropia plants produce Müllerian bodies from trichilia, pads located at the bases of leaf petioles (Rickson 1971). These bodies are rich in glycogen (Rickson 1971) and serve as the primary food source for Azteca ants (Janzen 1969). In return, the Azteca ants remove vines (Janzen 1969, Schupp 1986) and reduce damage from herbivorous insects (Schupp 1986, Duarte & Bergallo 1992). Species other than Azteca ants have been observed feeding on Müllerian bodies (Janzen 1973), including 15 species of birds (Skutch 1945, Isler & Isler 1987, Stiles & Skutch 1989). These authors, however, did not quantify their findings, and the extent of Müllerian body feeding by birds is obscure. Cecropia gabrielis (C. monostachya in Berg & Rosselli 1993; D. Neill, pers. comm.) produces Müllerian bodies like other Cecropia species although it is not inhabited by ants (Berg & Rosselli 1993; Naoki, pers. obs.). In western Ecuador, instead of Azteca ants, a few species of passerine birds have been observed feeding on Müllerian bodies (Naoki, pers. obs.). We decided to make further observations to (1) report the first quantitative observation of Müllerian body feeding by birds and (2) present hypotheses to explain the production of Müllerian bodies by C. gabrielis in the absence of Azteca ants. We quantified the foraging behavior of birds on C. gabrielis trees between 19 and 21 August 1998, for a total of 24 hours, in the vicinity of Mindo, Pichincha province, Ecuador (08029S, 788469W). The