Thursday, 14 March 2013

Common birds: Red-billed Quelea, commonest bird in the world?

1000s of quelea at a dam on Manyara Ranch
I've been struggling to think of the next common bird to do something interesting with, until the obvious solution came to me, possibly the world's most abundant bird, the red-billed quelea.

So, first the identification. The most obvious thing about red-billed queleas are, as the name suggests, a large red beak! Apart from that feature, females and non-breeding males are rather nondescript, small sparrow-like birds. Breeding males are rather brighter, with the red bill surrounded by a black face and variable amounts of orange on the top of the head and breast, with otherwise sparrow-like brown streaks on the back and wings. Perhaps the most useful identification feature though is the fact that you almost never see just one, but flocks of tens, hundreds or thousands of busy quelea all searching for grass seeds or drinking at waterholes.

Red-billed Quelea on right in non-breeding plumage. What else do you see?!
So, what are the most interesting things to say about red-billed quelea? I think the most interesting things about quelea are to do with their numbers - if you've a good enough internet connection, have a look at the clip here for a typically impressive flock. There are likely to be well over a billion pairs of these little birds: estimates vary greatly, but I tend to trust the 1.5 billion pairs quoted here as a reliable minimum estimate. (If you like back-of the envelope calculations, you'll be impressed to hear that at 20g each that's around 60,000 tons of red-billed quelea across Africa!) With nesting colonies reported in the millions, you can imagine the food that these birds need to consume: 4 million adults, feeding a further 8million young. Each bird needs around 3.3g of grain per day (a figure I'm sure is an underestimate - I also found 10g online, but doubt that too...), giving a daily demand of nearly 40 tons of grain per day. If they happen to nest near your crops, I'm afraid the impact could be disastrous. It's no surprise, therefore, that governments and farmers across Africa control these birds. There's a range of control mechanisms in use, from large-scale spraying with fenthion (which obviously kills large numbers of non-target species too, to say nothing about the risk to humans when they are inadvertently sprayed too) to the extremely spectacular (but rather less effective) fire-bombing of nesting colonies (I know there's a video of it online, but I can't find it now. Anyone help?). This massive impact alone surely makes these birds one of the most interesting things out there, but I find it interesting to think about why there are so many in the first place.

Quelea always roost together at night - an chance to exchange gossip?
The answers to this are quite interesting and can be tackled at a number of levels. Most simply, there's a lot of grass seed out there, and if you can eat human grass seed like sorghum and millet as well as wild grass seeds, you're certainly going to do very well. But finding grass seeds isn't always easy - rains can be patchy, meaning grass seeds ripen in patchy spots: one field might be good for a few days, then another field somewhere else. Finding this patchy and briefly-available type of food is pretty tricky. To solve this problem, red-billed quelea are a very good example of birds that appear to support the "Information Centre" theory. This theory suggests that the optimal strategy to find food under these conditions is to scatter into small groups, search about, and then come back together, share information about what you find, and then follow any other individuals that have found food. Quelea are pretty good at this - they roost together overnight in huge flocks, and also roost during the heat of the day in smaller groups, often visiting water holes. During these roosts, not only are they fairly well protected from predators (if you are one in a flock of 100s of 1000s, you're very unlikely to be the one that gets eaten this time) but they can exchange information with one another. Why should a bird that has found food share this information with other birds though, and not just sneak off to eat it's fill on it's own? Well, if the food supply once found is much more than one or a few birds can eat alone there's little cost to sharing this information. And with seeds, before long they fall off (or get harvested) and lost - so there's only a short period of time when ripe seeds are available, and when they are, there's far more than any individual bird can eat. So with little cost involved in sharing information, the benefit of sharing with your relatives (helping relatives is generally good for your genes too), or even with any other individual likely to share similar information with you in the future, is certainly going to be a good idea. It becomes much more likely to evolve when individuals are identifiable to each other (a sort of ;you scratch my back, I'll scratch yours)- and this is one of the reasons currently being investigated as to why male quelea are so obviously different in how red they are, not (as often the case) simply to attract females, but to make themselves identifiable when in a vast flock. There's lots more work to do in this area, to test the theory completely, but I think that there's some good evidence for quelea that it might really be an important reason why the species is always found in such large flocks.

Another level to explain the large flocks at, is to look at their breeding biology. Quelea are really rather interesting as Africa birds, because they have relatively large clutches: four eggs are fairly common, though two and three are more usual. That's a somewhat higher than average clutch size for similarly sized African birds, where small clutches are the rule (see the earlier blog on African birds doing this slowly to understand why that is generally the case). And even more remarkable is the speed with which they breed. Once adults build up the correct protein reserves they start to nest: incubation is 9 or 10 days, fledging is at 16 days and independence a day or so later, start to finish in 5 weeks and the population has doubled! With more than one breeding event per season, possibly in distant locations, and first breeding at only 9 months old, the species has the potential to rapidly increase in numbers when conditions are good.

There are lots of other things we could talk about weavers too, of course - most recent research on them has focused on trying to explain the red colour in males, which is certainly an interesting topic (I'm saving it for another species of common bird though!). But I think that's enough for tonight!

Main References:

ResearchBlogging.orgBayer R.D. (1982). how important are bird colonies as information centres? The Auk, 99 (1), 31-40

No comments:

Post a Comment