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In cooperatively breeding birds, more than a simple pair provide care to the young in a single brood (Brown 1987; Emlen 1997; Cockburn 1998). Breeding groups are often formed through natal philopatry of young previously reared by the group, and such groups usually consist of a dominant pair and related subordinates who provide alloparental care (Brown 1987; Stacey & Koenig 1990).
Although there are many potential benefits to helpers, the most widely cited is that they increase the reproductive success of relatives by increasing reproductive productivity of groups (Brown 1987; Emlen 1997; Mumme 1997). Mumme (1997), for example, notes that for both birds and mammals, there is both ‘extensive correlational evidence’ and experimental evidence that helpers increase the reproductive success of recipients. In some cases, larger groups can have more than double the success of pairs (e.g. Rabenold 1990; Emlen & Wrege 1991; Heinsohn 1992). This finding, in combination with evidence that helping is often preferentially directed to kin, suggests that indirect benefits are often crucial to explaining cooperative breeding (Emlen 1997; Mumme 1997).
Increasing a group’s reproductive success can also bring direct benefits to helpers, even if kinship is unimportant. Four of the seven potential direct benefits of helping listed by Emlen & Wrege (1989) require an increase in reproductive success with group size. Higher group success can be beneficial to helpers because: (1) increasing group size can enhance survival, (2) larger groups may allow territorial expansion and budding, (3) helpers may form coalitions with young produced in the group and (4) young produced in the group may later become helpers. Overall, it is important to quantify the relationship between group size and reproductive success in order to assess adaptive explanations for the evolution of cooperative breeding.
Although reproductive success is often higher in groups, this is not always true; about one-third of studies find no effect of group size (reviews by Cockburn 1998; Hatchwell 1999). The true proportion might be higher because correlations between group size and reproductive success are likely to be inflated, particularly in species in which groups form through natal philopatry (Brown et al. 1982; Brown 1987). Pairs on high quality territories may have higher reproductive success, leading to larger groups in later years, so that a correlation between group size and success may be confounded by territory quality, or the age or quality of breeders.
In addition to variation among species, there is also variation in the effect of group size on reproductive success within species. In an experimental study of Florida scrub jays Aphelocoma c. coerulescens Bosc, for example, Mumme (1992) found an effect of group size on reproductive success in only one of two years. Similarly, the effect of group size appears to be greater in poor years in some species (e.g. Acorn woodpeckers, Melanerpes formicivorus Swainson, Koenig & Stacey 1990; Discussion). Even within a population, there can be variation among groups in the same year. In Seychelles warblers, Acrocephalus sechellensis Oustalet, the effect of an additional helper depends on territory quality and the original group size; although a single helper raises reproductive success, further ‘helpers’ may depress reproductive success, especially on poor territories (Komdeur 1994).
One suggested cause of variation in the magnitude of group-size effects is that helpers could have a greater effect on reproductive success when nestling starvation is common (Emlen 1991; Magrath & Yezerinac 1997; Hatchwell 1999; Legge 2000). In this situation, the provision of food by helpers is likely to have a greater effect on reproductive success than when food is abundant. Hatchwell (1999) and Legge (2000) provide support for this idea, by showing that helpers increase the total rate of provisioning specifically in those species in which brood reduction is more common. Hatchwell also found some comparative evidence that helpers have a greater effect on fledging success in those species that suffer more brood reduction when breeding in pairs.
Here a general model is proposed for variation in group-size effects among and within species (Fig. 1a). In good breeding conditions, helpers may have little or no effect on the groups’ reproductive success, because there is no major limiting factor that they can ameliorate. ‘Good breeding conditions’ includes both environmental conditions, such as food supply and predator abundance, and the quality, age or experience of breeders. For example, if food is abundant, a pair may be able to provide easily for optimal growth of offspring; or when predators are absent, there may be no benefit to extra vigilance. In poorer conditions, however, provisioning by helpers could reduce the risk of starvation of young or vigilance might reduce the risk of predation, and so increase the reproductive success of the group. In extremely poor conditions, however, helpers may be of little benefit, since their activities may rarely be sufficient to allow successful breeding. This model suggests that the failure to find group-size effects in many species might be because studies have been conducted at benign locations, or because averaging effects over individuals of different age or quality may obscure important variation.
Figure 1. Models of the joint effect of conditions for breeding and group size on reproductive success of cooperatively breeding birds. (a) Group size has a larger effect on reproductive success in poorer conditions (except in very poor conditions), and no effect in the best conditions for breeding. (b) Group size has no effect on reproductive success under any conditions. (c) Group size increases reproductive success by a fixed amount regardless of conditions.
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An alternative model for failure to find an effect of group size on reproductive success is that there truly is no effect, regardless of conditions (Fig. 1b). Another possibility is that instead of having a reduced effect in benign conditions, groups might increase reproductive success by a constant increment, regardless of environmental conditions (Fig. 1c). In this case, groups will none the less have a proportionately smaller effect in better conditions, which might make it more difficult to detect an effect of group size.
One uncertainty in making predictions about group-size effects is that helpers might affect the survival and thus future reproductive success of breeders rather than, or in addition to, current reproductive success. Again, the magnitude of future benefits to breeders may depend on the conditions for breeding. Thus it is relevant to assess breeder survival as well as current reproductive success.
Testing these models requires examining the magnitude of group-size effects under different conditions for breeding. Because the demographic models that form the basis of cooperative breeding theory require detailed and long-term studies of marked individuals, it is rarely possible to conduct simultaneous studies at several sites differing in environmental conditions (Reyer 1990 provides an important exception; see below). However, it may be possible to examine group size effects in good and poor years (e.g. Woolfenden & Fitzpatrick 1984; Koenig & Stacey 1990), or on territories of different quality (e.g. Komdeur 1994, 1996a).
I suggest that focusing on breeder age may be a particularly powerful way to test these models, because individuals become more proficient at breeding as they age and gain experience (Sæther 1990; Forslund & Pärt 1995). Therefore, one can test the prediction that group size will have a greater effect on reproductive success in poorer conditions by comparing younger, inexperienced breeders with older, experienced individuals. Furthermore, an effect of group size on the reproductive success of younger individuals is important because effects early in life will have a disproportionate effect on lifetime reproductive success.
The white-browed scrubwren, Sericornis frontalis Vigors & Horsfield, is one of the species in which there appears to be no effect of group size on reproductive success Magrath & Yezerinac (1997). However, analyses were restricted to groups in which females were at least 2 years old, since the sample of yearlings was too small to include. In this paper, the effect of group size on reproductive success is examined for yearling females compared with older females, using data gathered over 7 years. Given that group size may covary with territory quality and breeder quality (above), these potentially confounding variables are also examined. It is concluded that yearlings do have greater reproductive success when breeding in groups rather than pairs, and the lack of benefit from breeding in groups for older females is confirmed. Finally, a survey of the literature shows that the pattern found in scrubwrens is of widespread importance in cooperatively breeding birds.
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The work presented here would not have been possible without the help and collaboration of many people. Beth Bobroff, Janet Gardner, Tony Giannasca, Ashley Leedman, Anjeli Nathan, James Nicholls and Linda Whittingham made substantial contributions to fieldwork in more than one year, and Camille Crowley, Megan MacKenzie, Helen Osmond, Amy Rogers, Derek Smith, Lynda Sharpe, Kate Trumper and Stephen Yezerinac also made valuable contributions. Sam Portelli and Belinda Mitterdorfer helped with data entry. The paper was largely written while on sabbatical with Jamie Smith, and I thank him and all of those in the Department of Zoology at UBC who provided a stimulating environment in which to work. Andrew Cockburn, Rob Heinsohn, Elsie Krebs, David Green, Jamie Smith, Liana Zanette and two anonymous referees provided insightful comments on earlier versions of the manuscript. This research was supported by grants from the Australian Research Council, and was carried out under permits from the Australian National University ethics committee, the Australian Bird and Bat Banding Scheme, the Australian National Botanic Gardens and Environment ACT.