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Scholars are still debating the origins and causes of cooperative breeding. For a few species it appears that cooperative breeding is an “ancestral” trait, appearing in the earliest period of its evolution and maintained throughout the species’ history. But for most cooperative breeding species, including raptors, the trait seems to have entered and/or exited the phylogenetic history one or more times.

Delayed dispersal from its natal area by a young bird is the most common mechanism by which a related helper ends up assisting at the nest of related kin. The reasons that a bird may delay dispersal instead of seeking its own mate and territory or spending time as a floater are not always clear. And while nests with related helpers make up the majority of cooperatively breeding cases, many birds remain in their natal territory yet do not become helpers at their kin’s nest. And for floaters or unsuccessful breeders, routes other than delayed dispersal lead them to become helpers.

As with all behaviors among wildlife, the benefits of cooperative breeding must outweigh its costs, and benefits must be advantageous to both the helper(s) and the dominant pair. Direct fitness benefits contribute to the immediate survival and well-being of individuals in a breeding season; these result from proximate causes, short-term immediate circumstances that guide choices about breeding behavior in a particular season. Indirect fitness benefits contribute to the long-term strength and survival of a species; these result from ultimate causes, factors that affect the evolutionary history of a species, whether in its ancestral past or emerging sometime later in its phylogenetic history.

• Direct benefits to the dominant pair can include assistance with nest building and maintenance, defense of nest, territory, and nestlings, food provision, and incubation and brooding of nestlings. Members of the pair also have the opportunity to assess the helper as a possible future mate.
• Direct benefits to the helper can include gaining experience in foraging, nest-building, and nestling care, access to copulation and breeding, formation of alliances, and a favorable position for inheriting a mate, nest, or territory.
• Indirect fitness benefits include strengthened genetic makeup of the species resulting from increased survival of nestlings.
• Costs to the dominant pair can include conflict over territory, reproduction, or food, unexpected danger to offspring, and uncertainty of genetic improvement or compatibility.
• Costs to the helper(s) can include increased energy outlays and delay in establishing an independent territory, acquiring a mate, and producing offspring.

Although monogamy is the prevalent breeding system among birds, genetic studies show that most birds engage in extra-pair copulation (EPC). Scientists make a distinction between genetic monogamy, in which female and male copulate only with each other and raise offspring together, and the more common arrangement of social monogamy, in which female and male bond and raise offspring together, but one or both engage in EPC.

Whether genetically or socially monogamous, about 81% of birds care for their young as a male-female pair (Cockburn 2006). So what induces a mated pair to allow a third adult (or more) to help raise their offspring, especially the member of the pair of the same sex as the helper? And what induces a breeding-age adult to forego the opportunity to produce its own offspring and help raise someone else’s?

Scientists have developed two general hypotheses to answer these questions:

1. Ecological constraints hypothesis. Ecological challenges can make the benefits of cooperative breeding offset the costs for both a dominant pair and one or more helpers. The problems can be short-term, such as inclement weather or habitat destruction, or long-term, such as prevailing climatic conditions that species have adapted to over time.
   • Limited territory (habitat saturation), a shortage of suitable mates, and diminished food resources can lead offspring or floaters to offer alloparental care to an established pair instead of spending the breeding season dispersing or floating. In some cases an adult bird who has had an unsuccessful breeding attempt may determine that the costs of helping a nesting pair are fewer than the costs of floating.
   • Habitats with low precipitation but a high precipitation variability, or with high temperatures and low temperature variability, can make the food supply unpredictable. Helpers can provide some stability in food acquisition and other aspects of caring for young, leading to greater reproductive success during bad seasons, as well as helping maintain successful breeding in good years (Rubenstein & Lovette 2007; Jetz & Rubenstein 2011; Koenig 2017; Griesser et al. 2017). These conditions might help explain the concentration of regularly cooperative breeding species in the tropics, subtropics, and neotropics.
2. Life history hypothesis. Adaptive traits and behaviors that contribute to a species’ fitness and survival over time, especially in its reproductive strategies, might include elements that are conducive to cooperative breeding.
   • There are many more cooperative breeders among altricial species than precocial species. Most scholars believe that the behavior evolved early in altricial species because of the longer period of parental care needed by the young in the nest, making the aid of extra(s) advantageous to the dominant pair. (Ricklefs & Starck 1975; Ligon & Burt 2004; Cockburn 2006; Hatchwell 2009).
   • Cooperative breeding is more common among species that are sedentary – non-migrating and maintaining year-round territories – because this reduces the availability of uncontested nesting territory.
   • Low mortality, small clutch sizes, high survivability of offspring, and longevity of adults all contribute to a stable population with low turnover of territories or mates, leading to increased numbers of helpers (Arnold & Owens 1998).

These hypotheses are not necessarily mutually exclusive, but aspects of both may be at play in a given species or situation. And often it is not clear whether a circumstance is a cause or an effect of cooperative breeding (Arnold & Owens 1998). The theories are based on direct observation; past history cannot be observed, and no hypothesis yet proposed perfectly predicts the behavior before it occurs. No theory explains why species under the same conditions may or may not engage in cooperative breeding (Koenig & Dickinson 2004). Some researchers point out that most species encounter ecological challenges but not all respond by breeding cooperatively (Hatchwell & Komdeur 2000; Koenig 2017). Likewise, many species have life history traits favorable to cooperative breeding but do not practice it. Scholars continue to debate why some species are regular cooperative breeders while in others cooperative breeding occurs only occasionally, rarely, or never.

Why do raptors cooperatively breed?

Most raptors, including Bald Eagles, are considered occasional or rare cooperative breeders. Most species appear to have had many transitions to and from cooperative breeding in their evolutionary histories (Cockburn 2006). Because more helpers among raptors are mature adults than younger birds and are less likely to be genetically related to the dominant pair, delayed dispersal and kin selection by fledglings is not a factor in general (Kimball et al. 2003). Although raptors, like most birds, are philopatric, remaining in or returning to their natal territory for breeding, helpers even if not related might have some familiarity with nesting birds in the area. These factors might suggest that circumstances leading to the behavior may ebb and flow over spans of time, and that proximate causes connected with direct benefits for individuals are more influential than ultimate causes that relate to indirect benefits to the species.

Ecological challenges that lead to cooperative breeding among raptors probably include some of those that are typical of other birds, including limited food resources, scarcity of territory, and shortage of available mates. Shrinking habitats due to human encroachment might lead to more instances of cooperative breeding among raptors, who require comparatively large territories for survival. Climate changes such as increasing variability in precipitation might lead raptors in temperate zones to engage in cooperative breeding to help mitigate the unpredictability of food availability (Jetz & Rubenstein 2011).

Life history factors include the fact that cooperative breeding is far more common among altricial birds than precocial birds. The nestling period of raptors is especially long, which might encourage a nesting pair to welcome a helper, especially if ecological circumstances are challenging. The curious fact that raptors have more mature than immature helpers might be because raptors depend on capturing live prey as their main food source, and adults are more skilled at capturing prey than younger birds (Kimball et al. 2003).