Pompadours in the Palms

A rare bird’s elaborate mating habits help a tropical tree disperse its seeds.

female umbrellabird

The female sports only a nominal wattle, and her head crest is tamer than that of the male.

Murray Cooper

My team and I hypothesized that the mating behavior of umbrellabirds might have important consequences for chapils and other plants whose fruits they disperse. Males, we thought, might bring a high proportion of the seeds they eat back to their lek sites. To test that idea, we trapped umbrellabirds of both sexes, attached lightweight, temporary radio transmitters to their tails, and released them unharmed. We then noted their locations over the course of several months as they traveled in search of food or displayed at their leks. A few of the birds we held in aviaries and fed chapil fruits, recording how long it took them to regurgitate the seeds—that’s how we arrived at the figure of one hour. Armed with those new data on movement patterns and seed-retention time, we were able to estimate where umbrellabirds of both sexes deposit the seeds they ingest.

Sure enough, we determined that males deposit roughly half their seeds in the lek sites. Females, on the other hand, distribute their seeds more evenly throughout the forest. In other words, the different reproductive behaviors of males and females translate into different dispersal patterns. To address how that affects chapils, we compared the rate of survival to the seedling stage for seeds deposited within and outside leks. We found no difference in the probability of survival. That came as a bit of a surprise: we had anticipated that high seedling density in the leks, and the resultant competition, would be a disadvantage. That it was not suggests that, for reasons we could not determine, the leks may be unusually favorable sites for chapil seedlings relative to the forest at large.

Working with Victoria L. Sork, a plant ecologist at the University of California, Los Angeles, my team and I are now using genetic markers to identify which adult trees the seeds arriving in umbrellabird leks came from. In addition to illuminating how umbrellabirds promote gene flow among chapil palms, that approach will tell us something about how forest fragmentation and deforestation affect seed dispersal patterns. The challenges are great, but between our research and our efforts to involve local residents in conservation work, we hope to preserve the age-old pact between a palm and its conspicuously plumed dispersal agent.

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