Wednesday, 10 April 2013

Common birds: Rattling cisticola and why birds hold territories?

Rattling Cisticola, near Arusha, March 2011. Something of a birder's bird?
Returning to my recent theme of common birds, what could be more suitable as the rains begin than a look a Cisticola chiniana? Whilst small, streaked and brown might make this something of a 'birder's bird', I'm happy to think there's plenty to interest everyone in this species too.

First the identification. Let's be honest, Cisticolas can be something of a challenge to identify! It doesn't help that there are seven pages of nearly identical looking small, streaky brown birds in the fieldguide! Happily, there are better ways to identify Cisticolas than their looks - the key is always to listen. Most Cisticolas, and rattling is no exception, have fairly distinctive calls and once you know it their 'tee, tee, churrurrurr' call is a constant sound in the bush (click the link to find a recording on xeno-canto), especially during the rains when they breed. In the unlucky event that none are making any noises, you can usually be fairly confident in your identification of any moderately sized, streaky cisticola present in the drier bush regions as rattling simply because they're so common! They are surprisingly variable in size (sometimes appearing really rather small) and colour (from very grey to warmer brown - but never with bright chestnut on the wing or head) though, so don't be too taken in my any one feature if they're not calling.

This is a bright individual from Serengeti (thanks to
So, what is so interesting about this common bird? Well, the most interesting things I know about them concern their communal territories. We're so used to birds (and other animals, of course) holding and defending territories, that we often don't think to ask why they do it? Not all birds do, of course: our last common species, the red-billed quelea, is a colonial species that benefits from information sharing with others in the flock and never defends territories. Other species that we see in Tanzania (like willow warblers) show no territorial behaviour here, but are intensely territorial at other times of year. And most of the birds we think of as territorial usually defend a territory as a pair with one adult male and female (sometimes with sons and daughters from previous years sticking around to help out in cooperative breeding). Yet the rattling cisticola often holds communal territories involving two or three males and several females (and multiple nests within the same territory) where the males can be unrelated to each other. Watch carefully those males you see singing at the moment (and take the opportunity to make sure you learn the song, because it really is the easiest way to identify them!) and you'll soon spot that some are working together to defend territories against others. So why is this? Why should rattling cisticolas (and possibly other cisticolas that you see in groups) choose to not only defend territories, but defend communal territories with unrelated males?

Obviously, this problem needs breaking down into two parts: why defend a territory in the first place - all that singing and fighting is pretty time and energy consuming. And if defending a territory is a good thing, why let others into yours and defend together? The former problem has been studied for a long time, and the answers are fairly obvious: territories are all about securing access to resources, be they food, females, nest sites or other scarce things. As such, there are two very important requirements that need to be met, before territoriality of one form or another can evolve: competition, and defendability. If some resource is extremely abundant, it's obvious that there's no use wasting effort fighting over it: red-billed quelea's grass seed food is so super-abundant when ripe, they have no need to fight each other for access, and as we saw before, there are real advantages to sharing information. If, however, you eat insects that are hard to find and that live in a rather harsh semi-arid environment, there's more incentive to fight over your spot - especially when you've worked hard to identify the best places to find those insects within you local patch. So competition is very important to the evolution of territoriality. Equally, there are some foods that are so spread out, that even if competition is very high, there's no feasible way for you to defend them. For example, if you're a seabird fishing over large areas of sea, you have to come all the way back to shore to nest and feed your chicks, meanwhile you obviously aren't able to defend your resources, so again there's no point in wasting energy trying to do so.

Obviously, food availability varies throughout the year. Equally, a bird's requirement for food varies during the year- when feeding young, the amount of food an adult has to find is much greater than when only worrying about it's own needs. Consequently, the impact of competition is likely to change during the year, being greatest when trying to feed young. At some point, for some species, this might mean that the advantages of defending a territory may increase, and may outweigh the costs- leading to a pattern of seasonal territoriality like we see in the willow warbler. Of course, birds also have some options available to them to reduce the costs associated with territory defence - it's hardly efficient to constantly fight your neighbour over resources, so neighbours may 'agree' to leave each other alone, or simply identify accurate indications of how likely they are to win fights, without having to it all the time. Which, as I discussed on the post on why birds sing,  is one of the functions of song. So, these ideas work well to explain why birds might defend territories in the first place, but why should the Rattling Cisticola form communal territories?

A much duller individual from Tarangire (Daudi Peterson)
A number of ideas have been suggested to explain this behaviour. Firstly, there's the idea that you may well already be familiar with from knowing something about lions:  that without friends helping to defend a territory, there's no way you'll be able to do so on your own. Equally, there's the rather interesting idea that when competition gets particularly tough, any 'agreement' that may normally exist is torn up. For example, it's all very well agreeing not to constantly fight your neighbour, if you know that there will be a suitable territory for you somewhere else - but if all the territories are full, and you have no chance of breeding, nor much of surviving without one, you'll keep fighting even much bigger neighbours than you for the slim possibility of winning, against the certain possibility of death if you don't try. Now consider what options you have if you are a territory holder and these agreements are torn up. Suddenly, all the little males around who previously left you in peace because they had their own patch (maybe not as good as yours, but still something to scratch by on) will come picking fights all the time. They have nothing to lose, and all to gain. And although you can keen chasing them off, the cost of doing so is going to grow and grow until, at some point, it might be more sensible to make some new agreement with just one or two of them that you'll stop chasing them off, if they help chase off all the other males who want a part of your territory. So here are two slightly different ideas: one that suggests it is simply impossible to hold territories of sufficient size without being in coalition, the other that says that the cost of defending against highly persistent but obviously poorer quality males is so great that 'non aggression' pacts might be the way forward. Of course, both may be involved, but the two hypotheses make slightly different predictions that are testable. If coalitions are necessary, just as with lions, we'd expect larger coalitions (and particularly larger coalitions of larger males) to have larger territories, whereas if it's just persistence being rewarded, we'd expect primary males and secondary males to be identifiable - probably by size - and not necessarily expect territory size to change much with coalition size. In fact, Allan Carlson who looked at all this in the 1980s in Kenya found very good evidence indeed that there are primary and secondary males in territories, that size doesn't necessarily increase with coalition size, and even observed the 'persistence pays off' formation of new territories.

All of which means that I think rattling cisticolas, though perhaps not much to look at, are every bit as fascinating little birds as their brighter looking cousins!

Main Reference:

ResearchBlogging.orgCarlson, A. (1986). Group Territoriality in the Rattling Cisticola, Cisticola chiniana Oikos, 47 (2) DOI: 10.2307/3566044

No comments:

Post a Comment