Sociable Weaver Research Project

Coordinators

Dr Rita Covas & Prof. Phil A.R. Hockey

Members of the research project

Dr Claire Doutrelant (CEFE-CNRS, France)
Mr Matthieu Paquet (PhD Student, University of Montpellier, France)
Ms Margaux Rat (PhD Student, University of Cape Town)

Research collaborators

Dr Res Altwegg (SANBI, SA)
Dr Mark Anderson (BirdLife South Africa, SA)
Prof. Ben Hatchwell (Sheffield University, UK)
Dr Claire Spottiswoode (Cambridge University, UK)
Dr René van Dijk (Sheffield University, UK)

Sociable weavers Philetairus socius are highly cooperative passerines that live in large colonies that are built and maintained communally. A breeding pair within the colony is often assisted by non-breeding helpers that bring food to the nestlings. Understanding the costs and benefits of cooperative breeding in this species has been the central focus of our research programme. We also use our long-term data set to investigate questions about population dynamics and the benefits of coloniality.

About the Sociable Weaver Research Programme

The Sociable Weaver study was initiated in 1993 by Mark Anderson (currently executive director of Birdlife South Africa) to collect basic demographic data on a fascinating colonial species with intriguing cooperative behaviour. In addition, Mark, then an ornithologist at Northern Cape Nature Conservation and based in Kimberley, was interested in getting people involved in bird ringing. He set up the study at Benfontein Game Farm, 15 Km outside of town. For five years Mark and an enthusiastic group of volunteers regularly captured the birds at some 20 colonies on the farm. In 1998, Rita Covas initiated a PhD project on the cooperative breeding behaviour and life history strategies of the sociable weavers, starting a new line of research that continues to this day.

Cooperation: why help?

Why do the ‘helpers’ assist other birds with their breeding attempts instead of breeding on their own? What are the benefits of this behaviour for both breeders and helpers? And what are the costs? Over the years, our research has shown that it is often the older offspring of the breeding pair that are retained as ‘helpers’. This suggests that kin selection (i.e. increasing one’s fitness indirectly by assisting the reproductive effort of close relatives) may play an important role in the evolution of this behaviour. This, however, does pre-suppose that the helpers do indeed contribute to increasing the reproductive success of the breeding pair: in the case of the weavers, this is not immediately obvious. Although our results have shown that the breeders do benefit from the presence of helpers under some conditions, this is not always the case. We have, however, shown that parents reduce their work loads when helpers are present, which may improve parental survival. We have preliminary results supporting this effect for females, but not males. This suggests that females who have helpers may in some way be able to make a ‘saving’ on their reproductive effort. As of now, the evidence points towards females decreasing their investment in producing eggs, thereby saving energy for future reproductive events and enhancing their own survival. The additional food provided to the chicks by the helpers may compensate for the smaller or poorer quality eggs. This possibility has sparked an interesting new line of research being pursued by a PhD student, Matthieu Paquet.

Cooperation and conflict

Whilst helpers may increase female survival, it appears that this does not necessarily translate into increased juvenile survival. To our great surprise, juveniles raised in the presence of helpers may be worse off than juveniles raised by pairs that lack helpers. This unexpected discovery may be a result of competition between helpers and juveniles. Because remaining in the natal colony confers several benefits, older helpers might force juveniles to disperse away from the study colonies and the study area (even though they are siblings). The possible effect of sibling competition on dispersal patterns is now being investigated.

More generally, in group-living species, where individuals collaborate in communal tasks there is also conflict around sharing the resources or workload. For example, how much effort do helpers put into their feeding effort or why do some group members obtain breeding positions while other don’t? Conflict may result in dominance hierarchies inside the group. Understanding dominance in a given society is therefore an important key to understanding conflict resolution and cooperation. Differences in resource-holding potential, aggressiveness or motivation can influence the outcome of these conflicts. Dominance hierarchies can be settled through behavioural interactions, but can also be assessed through morphological signals, providing a way to resolve contests without the costs of potential injuries. Plumage ornaments, called 'badges of status', might be used for such an assessment. These patches might evolve through intra-sexual selection (to deter rivals), social selection (e.g. to reflect dominance in a group when there is competition for resources) or sexual selection (mate choice). A new student, Margaux Rat, has now joined the project to study the role of dominance in mediating cooperation and conflict in the weavers.

Demography and population dynamics

One of the greatest benefits of long-term projects is to have variability in population parameters spanning several years and hence encompassing natural fluctuations in population trends. This provides unique opportunities to understand the factors that affect population dynamics and determine population trends. Data on the weaver population at Benfontein now span 17 years, during which time we have detected a slow but steady decrease in the population. To explore the reasons behind this trend, we collaborated with Dr Res Altwegg, a specialist researcher at SANBI. The results indicate that part of the decrease may be the result of climatic changes in the area, in particular a trend of decreasing rainfall. Decreasing numbers, whether driven by climate or not, may in itself start a snowball effect. As colony size decreases, survivorship falls and emigration rates rise: these combined responses can together result in an accelerating colony decrease. We are continuing this research to get a more detailed understanding of how different demographic parameters respond to different environmental drivers.

Another benefit of long term-studies is that one is able to follow individuals throughout their lives. This can provide insights into animal life histories that cannot be gained in any other way. By way of example, at the end of 2010 we captured two sociable weavers that had been ringed by Mark Anderson in the mid 1990s: one of these (ringed as a juvenile) was 14 years old, and the other (ringed as an adult) was at least 16 years old, making it the oldest Sociable Weaver on record.

Cooperative nest building

The massive nests of sociable weavers are believed to be built communally by all colony members, but how is this cooperation maintained? Who builds where and when and, more interestingly, how is cheating avoid? The weavers’ communal nest structure is a ‘common good’ that requires constant work to repair and enlarge. It therefore provides an ideal setting to investigate how the ‘tragedy of the commons’ can be avoided. The tragedy of the commons arises when common goods cannot be maintained because individuals acting selfishly do not contribute to the maintenance of the common good, even though in the long run everyone benefits from having it. Post-doctoral Fellow René van Dijk and Professor Ben Hatchwell from Sheffield University started working on the sociable weavers at Benfontein in 2010 with the specific objective of investigating this conundrum.

More about the team

Dr Rita Covas (CIBIO, University of Porto, Portugal and Percy FitzPatrick Institute)

Rita's homepage | E-mail: rita.covas@mail.icav.up.pt

I have a broad interest in the evolution of life history strategies and sociality in birds. My research activity focuses on three main topics: cooperative breeding (see below), the evolution of family living across species (in collaboration with Michael Griesser) and adaptation in island birds. My research on sociable weavers has focused essentially on the costs and benefits of cooperative breeding. The effect of sociable weaver helpers on reproduction is intriguing because the helpers do not appear to have a strong effect on reproductive success and helpers that are unrelated or more distantly related to the breeders bring more food to the nest than closely related helpers. These results suggest that i) there might be ‘cryptic’ helper effects and ii) that some helpers gain direct fitness benefits from helping (i.e. as opposed to simply kin-selected benefits where an individual benefits from increasing the fitness of a close kin). I am particularly interested in understanding the relative role of direct and indirect (kin-selected) benefits in maintaining cooperation and, specifically, which direct benefits are involved. For example, we have shown that sociable weaver helpers tend to go in to the nest to feed when there are other birds around that can see them doing it. Is this because cooperative behaviour can be used to convey information about individual quality and performance? Is that information used by other individuals for mate choice or grouping decisions?

Claire Doutrelant (CEFE-CNRS, France)

Claire's homepage | E-mail: claire.doutrelant@cefe.cnrs.fr

Besides understanding the costs and benefits of helping for both helpers and recipients, I am interested in the consequences of helping on individual strategies. How do parents behave in the presence of helpers? Do they modify their investment in reproduction to gain increased survival? Is there a conflict between parents and helpers and can maternal effects influence this conflict? In addition, I’m interested in establishing whether there are any direct benefits of helping and whether this might lead to conflicts between relatives around helping. I’m also very interested in the links between cooperation, signalling and sexual selection. Are there any benefits in signalling cooperative behaviour? Can high social and sexual competition in cooperatively breeding species explain signal evolution?

In addition to these questions, I also work on Blue Tit behaviour and reproductive phenology in Europe. More particularly, at the moment I’m designing experiments and collecting data to test the hypothesis that social and sexual selection act on female phenotypical traits.

Matthieu Paquet (PhD Student, University of Montpellier, France)

Matthieu's homepage | E-mail: matthieu.paquet@cefe.cnrs.fr

Cooperative breeding is a fascinating system were sexually mature individuals help other individuals breed by providing care to their offspring. However, several studies have found no positive effect of helpers for offspring fitness. This apparent paradox could be explained if parents are able to reduce their reproductive investment when helped. By providing additional care helpers may allow the parents to reduce their workload and thereby save energy for the same result in terms of current offspring fitness. Parents may then have a better chance of survival and more breeding opportunities.

Studies on the Sociable Weaver have found that this load lightening strategy is likely to occur at the nestling stage, with parents providing less food when assisted by helpers. Our preliminary results show that load lightening can also occur at the egg stage, Sociable Weaver females reducing the weight of their eggs when the breeding group size increased. Preliminary results also suggest that females may modify offspring phenotype by varying hormonal contents in their eggs. The aim of our project is to better understand how parents (males and/or females) modify their investment when assisted by helpers and what the consequences of these strategies are in terms of offspring, helpers and parental fitness (survival and reproductive success).

Margaux Rat (PhD Student, PFIAO, University of Cape Town - to register in 2012)

Margaux's homepage | E-mail: margaux.rat@uct.ac.za

In the course of my higher education I have focused mainly on the fields of evolutionary biology and behavioural ecology. I am particularly interested in investigating the processes and patterns of social and sexual selection and the emphasis of my masters thesis was on the latter. Under the supervision of Adeline Loyau (CR Biological station CNRS of Moulis, France), and in collaboration with Jeremie Cornuau (PhD student, Biological station CNRS of Moulis, France), I designed experiments to test whether female newts (Lissotriton helveticus) use multiple signals (behavioural and morphological) to choose their mates. In my PhD project on sociable weavers I get the opportunity to intertwine sexual and social selection theories.

Sociality often implies costs around sharing resources. Sociable weavers, for instance, have to share access to the communal nest and food which increase the potential for the occurrence of costly conflicts. The establishment of dominance hierarchies as a means to diminish the costs associated with sharing resources is often observed in group-living species. Understanding how dominance is settled, and how does it influences social organisation and mate choice, is therefore an important key to understanding conflict resolution and cooperation. This is the core of the PhD project. Using intensive behavioural observations, photo analysis, kinship analysis and benefiting from skills and techniques of the different collaborative teams, I wish to investigate the following: 1. Who are the dominants and subordinates within and between breeding groups in a colony?; 2. How do dominant individuals signal their status and, in particular, does the melanin-based bib patch act as a badge-of-status?; 3. How does kinship influence the outcomes of dominance interactions?; 4. How does dominance status affect mate choice?; and 5. How much do dominant individuals contribute to cooperative behaviour (nest building, cooperative breeding, predator defence) compared to subordinates?

In answering these questions I hope to shed light on some of the proximate and ultimate implications of dominance in the hypotheses proposed to explain the evolution of cooperation: Is dominance used to maintained kin-based structure thereby favouring kin-selection? Is it a mechanism to punish ‘cheaters’, i.e. lazy individuals that contribute less to the public good? Is dominance associated with direct benefits in mate choice? Does it convey information on individual qualities?

Key co-sponsors

The DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, the Portuguese Science and Technology Foundation (FCT) and the French Research Agency (ANR)

Sociable Weaver project publications

Doutrelant, C., Dalecky, A. & Covas, R. (in press) Age and relatedness have an interactive effect on the feeding behaviour of helpers in cooperatively breeding sociable weavers. Behaviour IP.

Covas, R., Deville, A.-S., Doutrelant, C., Spottiswoode, C. N. & Gregoire, A. 2011. The effect of helpers on the postfledging period in a cooperatively breeding bird, the sociable weaver. Animal Behavior, 81, 121-126.

Spottiswoode, C. N. 2009. Fine-scale life-history variation in sociable weavers in relation to colony size. Journal of Animal Ecology 78, 504-512.

Covas, R., du Plessis, M. Doutrelant C., 2008. Helpers in a colonial cooperatively breeding bird help to counteract the effects of adverse breeding conditions. Behavioral Ecology and Sociobiology, 63, 103-112.

Gimenez O., Viallefont A., Charmantier A., Pradel R., Cam E., Brown C.R., Anderson M.D., Brown M.B., Covas R., Gaillard J.-M. 2008. The risk of flawed inference in evolutionary studies when detectability is less than one. American Naturalist 172, 441–448

Schwager M., Covas R., Blaum N., Jeltsch F. 2008. Limitations of population models in predicting climate change effects: a simulation study of sociable weavers in southern Africa. Oikos 117:1417-1427

Doutrelant, C. & Covas, R. 2007. Helping has signalling characteristics in a cooperatively breeding bird. Animal Behaviour. 74, 739-747.

Spottiswoode, C.N. 2007 Phenotypic sorting in morphology and reproductive investment among Sociable Weaver colonies. Oecologia 154, 589-600.

Covas, R., Dalecky, A., Caizergues, A., & Doutrelant, C. 2006. Kin associations and direct vs. indirect genetic benefits in sociable weavers Philetairus socius. Behavioral Ecology and Sociobiology, 60, 323-331.

Gimenez, O., Covas, R., Brown, C.R., Anderson, M.D., Brown, M.B., & Lenormand, T. 2006. Nonparametric estimation of natural selection on a quantitative trait using mark-recapture data. Evolution, 60, 460-466.

Covas, R. & du Plessis, M.A. 2005. The effect of helpers on artificially increased brood size in sociable weavers (Philetairus socius). Behavioral Ecology and Sociobiology, 57, 631-637.

Covas, R., Doutrelant, C., du Plessis, M.A .2004. Experimental evidence of a link between breeding conditions and the decision to breed or to help in a colonial cooperative bird. Proceedings of the Royal Society of London B., 271, 827-832.

Covas, R., Brown, C.R., Anderson, M.D., Brown, M.B.. 2004. Juvenile and adult survival in the sociable weaver (Philetairus socius), a southern-temperate colonial cooperative breeder in Africa. Auk, 121, 1199-1207.

Doutrelant, C., Covas, R., Caizergues, A., Plessis, M.A.. 2004. Unexpected sex ratio adjustment in a colonial cooperative bird: pairs with helpers produce more of the helping sex whereas pairs without helpers do not. Behavioral Ecology and Sociobiology, 56, 149-154.

Covas, R., Doutrelant, C. & Huyser, O. 2004. Nest-dependant Pigmy Falcons predate on their host, the sociable weaver. Ostrich, 75, 325-326.

Brown, C.R., Covas, R., Anderson, M.D., Bomberger Brown, M. 2003. Multistate estimates of survival and movement in the sociable weaver. Behavioral Ecology, 14, 463-471.

Covas, R., Brown, C.R., Anderson, M.D., Bomberger Brown, M. 2002. Stabilising selection on body mass in the sociable weaver. Proceedings of the Royal Society of London B, 269, 1905-1909.