CHAPTER SUMMARY

Cooperative breeding, in which more than a pair of individuals cooperates to produce young, is found in a small proportion of birds, mammals, and fishes. Cooperative breeding encompasses a variety of alternative tactics. Some of these, such as mate sharing by males and joint nesting by females, explicitly concern reproduction, while others, such as cooperative courtship and helping at the nest by nonbreeders, involve activities leading to indirect reproduction through kin. In addition to these behavior patterns, cooperative breeding systems less commonly exhibit traditional alternative reproductive tactics (ARTs), including sneaking by males and parasitic egg laying by females. In virtually all cases, cooperative breeding behavior is conditional, facultative, and potentially frequency dependent, at least within groups. However, few attempts have been made to understand the expression or diversity of reproductive tactics observed in cooperative breeding systems using the theoretical framework provided by ART theory. Viewing alternatives as ARTs may help to clarify the selective forces that promote helping at the nest and mate sharing and help to explain the infrequent occurrence of parasitic reproductive behavior patterns in cooperative breeders.

INTRODUCTION

Cooperative breeding is a phenomenon in which more than a pair of individuals shares the tasks of producing young in a single nest or litter. It is known to occur in about 3% of birds and mammals (Brown 1987, Arnold and Owens 1998, Russell 2004), as well as some fishes (Taborsky 1994, 2001), but may be considerably more frequent given that the mating systems of many species remain to be determined (Cockburn 2003).

Cooperatively breeding birds are species in which social groups comprise at least three individuals that share parental care at a single nest. Although same-sex group members are sometimes non-relatives (Davies 1992, the majority of cooperative breeders exhibit delayed dispersal of offspring, which subsequently forgo reproduction and become non-reproductive helpers at the nests of parents or other close relatives. Group formation, social interactions, and reproduction within groups are characterized by both cooperation and competition among family members (Mumme 1997). In many cases, helpers derive indirect fitness benefits by increasing the productivity of their parents' nest. However, because helping at the nest only partially compensates helpers for failing to breed independently, helpers usually pay a net fitness cost by helping instead of breeding on their own (Stacey and Koenig 1990a).

Because most helping appears to represent a “best of a bad job” strategy, rather than an adaptive peak, it has become clear that we can understand the evolution of kin-based helping only by investigating why offspring forgo personal reproduction in the first place, and the associated question of why offspring, once they postpone personal reproduction, remain on their natal territories (Emlen 1982a). The decision to delay dispersal should be the key to the formation of family units, hence the aim of this chapter is to explore the selective basis of delayed dispersal. The selective factors favoring postponement of reproduction and helping are discussed in Chapter 3.

The route leading to cooperative breeding in multigenerational family groups involves a series of decisions.

Cooperatively or communally breeding birds are species in which individuals live in groups of three or more breeding-aged adults, all of which care for young at a single nest (Brown 1987; Stacey and Koenig 1990a). Most cooperative breeders retain young that delay breeding and help their parents raise siblings. Additional forms of cooperative breeding include polygamous groups with multiple cobreeders of one or both sexes and, more rarely, groups with unrelated helpers. Cooperative breeding is rare, occurring in only about 3% of avian species worldwide, and is particularly common in Australian birds (Brown 1987; Russell 1989; Arnold and Owens 1998). Its prevalence in Australasia can be accounted for phylogenetically due to a particularly high frequency in the Corvida (23%) (Russell 1989; Edwards and Naeem 1993; Clarke 1995).

Theoretical and comparative treatments of avian cooperative breeding have usually dealt with the full range of avian social systems (Brown 1987; Koenig et al. 1992; Hartley and Davies 1994; Arnold and Owens 1998). This practice has demonstrated that cooperative breeders share many important characteristics, such as year-round residency, high survivorship, small clutch sizes, and, in many cases, constraints on independent breeding (Brown 1987; Stacey and Koenig 1990a; Arnold and Owens 1998, 1999). Specific limitations on independent breeding vary from one species to the next, and involve a variety of resources, including food, territories, suitable nest or roosting sites, and a lack of skill or mates (Smith 1990).

The primary focus of this chapter is helping at the nest by retained offspring.

Cooperative breeding continues to engender considerable interest among behavioral ecologists. However, the players and issues have changed dramatically since the publication of the first Cooperative Breeding in Birds volume (Stacey and Koenig 1990a). Back then, a series of long-term demographic studies were coming to fruition, opening the door for a synthetic volume that would “search for common themes and patterns” while illustrating “the great diversity that exists among cooperatively breeding birds” (Stacey and Koenig 1990b). At the time it appeared that the “common themes and patterns” would outstrip the “great diversity” and that a general understanding of the main issues raised by the phenomenon of cooperative breeding was about to be achieved (Emlen 1997a).

Such optimism concerning a general answer to the paradox of helping behavior was quickly dismissed (Cockburn 1998), and it has continued to elude our grasp. Instead, new theoretical approaches and studies have emerged to reinvigorate the field. Three stand out in particular. First is DNA fingerprinting, which was just getting started in the late 1980s and was only minimally represented in the 1990 volume. Multilocus minisatellite fingerprinting and its descendant, microsatellite fingerprinting, provided the long-sought-after ability to determine parentage and estimate relatedness. Fingerprinting allowed those who were continuing long-term studies or who had been fortunate enough to collect and save blood samples either to confirm prior inferences regarding patterns of parentage (as in Florida scrub-jays and acorn woodpeckers: Quinn et al. 1999; Dickinson et al. 1995; Haydock et al. Haydock et al. 2001) or to turn all prior inference on its head (as in the splendid fairy-wren: Brooker et al. 1990).