Wednesday, June 9, 2010

Social Structure in Superb Fairy-wrens

 Here is a sample chapter from a book I am working on about the Superb Fairy-wren, comments and criticisms are always welcome.

This is the Superb Fairy-wren of Southeastern Australia...

Social Structure
Superb Fairy-wrens have attracted much attention among researchers in recent years due to the intricate social systems they have in place. It is actually hard to separate out breeding behaviours from general social behaviour, as in the Superb Fairy-wren the whole social dynamic is governed by breeding structures.

Superb Fairy-wrens are territorial, and resident throughout the year with a group of from 2 to 7 occupying an area from 0.8 to 2.5 hectares, depending on its suitability. A more habitat rich environment obviously needing less land area, although as always there are exceptions and much depends on the unique environmental factors of the location.

During the breeding season a socially monogamous pair will assume control of the territory, reserving up to half of that area for themselves. The dominant pair can be recognised by such actions as counter-singing, close association and territorial defence. This territory will also contain up to approximately four sub-dominant males who assist the pair in raising young. This is in addition to any fledglings during the breeding season.

Juvenile females are forced to leave the territory to an adjacent one, the dominant female reacts aggressively to sub-dominant females forcing them out of the territory as young females mature, leaving them to find a new breeding territory. In fact as is so far known a female will never breed on the territory on which she is born, even if the dominant female dies.

The social structures of the Superb Fairy-wren while fairly well understood are extremely complex and often leave more questions than answers. The dominant pair will tolerate subordinate males on the territory and these subdominant males will essentially wait in line to become the next dominant male in the following season. This queuing for dominance seems to be based on age. During this period they will assist in the raising of the dominant pairs young.

A male can rise to dominance if the dominant male dies, in which case the senior of the sub-dominant males will become the dominant male, essentially the next in line in the hierarchy of males. A sub-dominant male can disperse to a neighbouring territory or found a new one. Or as is often the case towards the end of the breeding season females expelled from other territories will join the group for the winter and pair with a sub-dominant male in the group, essentially splitting the group in two with the next breeding season. This process of splitting the territory is termed fission.

Most males whether they have achieved dominance or not, will also die on their territory, if they do move to a new territory it will be in the majority of cases to a neighbouring territory. Females will on the whole range further from the original natal territory – the one in which they were born. Although in general males and females will not range to far from the territory in which they were born.

It is important to realise with the Superb Fairy-wren, not just the importance of the natal territory, but the interrelationships between territories. It could almost be pictured like a chess board with females moving between territories and sometimes forming new ones through fission as mentioned. This aspect is important in regards to conservation and preservation measures which may be undertaken.

This interrelationship of territories and transfers between them results in a high rate of promiscuity, the male often raising young that are not his, assisted by his own “sons” that at least in a high percentage of cases are not his either.

On the converse side the males in adjacent territories will be raising his genetic young. In fact with this flux between territories combined with a queue based social structure. That is with males essentially waiting in line to become the next dominant male, incestuous relationships are not unheard of.

Sub-dominant male feeding on leaf litter. Note the black bill. Ourimbah NSW.

The pressure on males in particular to achieve dominance, and for females to gain a territory, will mean if given the opportunity, a sub-dominant male that has left his natal territory may return to his birth territory if the chance arises to become the dominant male there. In which case, he may mate with his mother, who is still the original dominant female of that territory.

Although these cases are minimal, they are not unheard of, however the general promiscuity combined with the high turnover and gene flow between territories seems to make this approach one of minimal risk, compared to the benefits of a male gaining dominance or a female losing a territory.

The females for their part will mate not only with the dominant male, but with dominant or sub-dominant males from their own or adjacent territories. Indeed there seems to be a short period every morning during which mating seems to occur. The dominant males suffering from the classic male problem of being unable to mate with the females of adjacent territories, and at the same time guard his own female from suitors.

On other occasions sometimes territories will be fused, for example if a dominant male or female in adjacent territories lose a mate. In some cases females will “divorce” a mate, but will very rarely move more than a few territories away. Rarely a dominant male can be evicted from his territory.

Sub-adult males will assist in defending the territory; this actually makes perfect sense, since an intruding male is effectively queue jumping. With most sub-dominant males eventually taking over the natal territory – or at least aspiring to this, they have as much to lose as the dominant male in terms of maintaining the territory.

Group size within a territory seems to be influenced by a number of factors. A more fertile territory allows for a greater number of young to be raised, and a larger resident population to be maintained. Although in the Superb Fairy-wren habitat saturation seems to play a large part in determining a territories population size.

For a sub-dominant male, if there are no suitable nearby habitats to which he can move and become dominant, then it makes sense for the bird to stay in the current territory and effectively wait his turn to become the dominant male. If however there are either suitable nearby habitats in which he can form a new territory with a female, or he has the opportunity to gain dominance in an adjacent territory then he will leave his natal territory.

So there are many factors at work in determining the population size of a territory. In short though it seems that population sizes of a territory tend to be larger when suitable habitat nearby is unavailable for colonisation.

The interesting aspect, is that the territoriality and social hierarchy is combined with explicit promiscuity. Over half of the females young are sired from males outside of the female’s social group. That is the young are fathered by males from adjacent territories, with approximately 25% of young being fathered by sub-dominant males. Some of these issues are what attracts researchers to these little birds, since the questions are applicable across broad areas of biology.

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