Sunday, 6 January 2013

Common Bulbul and frugivorous birds

Common Bulbul nesting in Arusha
Thanks to doubtful comments from a colleague, next up in the common bird series is going to be the Common Bulbul. These birds are probably the most widespread birds in Tanzania and should be a familiar sight to everyone, with their dark blackish heads, brown back and tail, and dirty white underparts with yellow under the tail. They're typical garden birds, and often ignored. However, it is often the common birds that we know most about, because they are so easy to study. And common species, simply because there are so many individuals, often have a very irritant role to play in ecosystems - the common bulbul is no exception!

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
Secondly, it seems that many Tanzanian riverine forests need birds to eat their fruits to protect them from beetles. This work, carried out by Tony Sinclair's group in the Serengeti, studied what happened to seeds from riverine forest trees if they were eaten by birds, and if they weren't.The story is simple - if the seeds fell with the fruit still intact virtually all were predated by beetles. If, however, birds had eathen the fruit and the seeds fell uncoated, the beetles ignored the seeds and the trees germinated. So without fruit-eating birds in riverine forests there'd be no riverine forest. And one of the problems with a lot of riverine forest at the moment is that too frequent fire has removed much of the understory, leaving grasses that the baby trees strugly to compete with and removing the undergrowth that many fruit-eating birds nest in,a nd once the birds are gone, the forest follows. So frugivores like the Common Bulbul are an absolutely essential part of the riverine forest habitat too.

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...

Main reference: 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

1 comment:

  1. Birds are very important for plants in their seed dispersal but i never knew that passing through the digestive system was so important for the plant. It is sorta like how plants in colorado need a cold winter to germinate. I think it would be very cool to find out chemically what happens to the seed when it passes through the digestive system.