|Common Bulbul nesting in Arusha|
Bulbuls are fairly generalist- they eat invertebrates very happily (often feasting early morning on moths around security lights) but probably their biggest impact is through their fruit eating. Now as is often the case I could find little specific to Tanzanian bulbuls, but the species is very widespread and is found in the Middle East as well as across Africa. There its role in seed dispersal has been studied, and it turns out to be rather important. Obviously, plants grow fruits to encourage birds and other animals to eat the fruit, swallow the seeds and then drop them elsewhere with a nice helping of dung to start them off. I'd always assumed that with the exception of some special fruits eaten by large animals like elephants, the bird dispersal was a nice optional extra for plants - without birds I assumed the seeds would germinate perfectly well, just rather closer to tteamsother tree than if a bird had carried the seeds away. Well, it turns out I was wrong, and there are two reasons why this is so. Firstly, it seems like the process of passing through a bird's digestive system is extremely important to many seeds, without which they barely germinate. In the Israeli study, for example, one of the trees studied has a 2% germination rate of fruit that hasn't been through a bird (i.e. 2 of 100 seeds germinated), but 50% of those that had passed through a bulbul germinated, a much more acceptable rate. The same was true for several other tree and bush species studied.
|Bulbul at Olduvai Gorge|
Now, of course, I have to confess that for both of these important roles, it doesn't seem to matter too much what species of bird does the eating, so this story could have been told for many other fruit eating birds so you might suggest that I'm cheating by using it as an illustration of the Common Bulbul. But as I mentioned to start with, common species, by being abundant, have a very important role to play in many of these processes. Moreover, another study from Israel showed that Bulbuls are much more likely cross open gaps between trees, meaning they're most important at spreading fruit seeds to new locations, so I'd suggest it is a really good species to choose to illustrate this important process...
Sharam, G., Sinclair, A., & Turkington, R. (2009). Serengeti Birds Maintain Forests by Inhibiting Seed Predators Science, 325 (5936), 51-51 DOI: 10.1126/science.1173805