|Rattling Cisticola, near Arusha, March 2011. Something of a birder's bird?|
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 tanzaniabirds.net)|
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)|
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!
Carlson, A. (1986). Group Territoriality in the Rattling Cisticola, Cisticola chiniana Oikos, 47 (2) DOI: 10.2307/3566044