© elfruler 2018

Research on cooperative breeding among birds has exploded since the 1980s. These investigations have broadened what scientists know about the behavior and have produced promising theories about how it develops. But the studies also have revealed numerous unknowns and variables that have introduced greater complexity than previously believed and have left many questions unanswered.

Let’s begin with what scientists agree on, cooperative breeding’s essential components. It is a parental care system with a group of 3 or more full-grown birds attending a nest during breeding season: a mated female-male pair, and 1 or more “extra” birds who contribute in some way to the rearing of young. The contribution by the helper is referred to as “alloparental care” (care by an adult “other than the parents”).

Beyond this fundamental definition, two major factors help differentiate various types of cooperative breeding:

1. Is the helper(s) genetically related to at least one member of the dominant pair?
   • Most instances of cooperative breeding (perhaps as many as 81% of species; Hatchwell 2009) involve a helper who is kin to at least one member of the dominant pair, usually offspring from a previous season. A choice by a third adult to help a pair to which it is related is called “kin selection,” a type of natural selection that enhances the genetic line.
   • Unrelated helpers might be “floaters,” young birds or adults who have not yet established a territory or a mate, or adults who have been unsuccessful in breeding attempts.

2. Does the helper(s) engage sexually with a member of the dominant pair?
   • “Pair nesting” describes a group in which copulation occurs exclusively between the mated female and male, and the helper(s) engages only in alloparental care. This is the most common type of cooperative breeding, comprising about 74% of birds who engage in cooperative breeding (Riehl 2013).
   • “Cooperative polygamy” is a system in which the helper acts as a cobreeder, engaging in copulation with the opposite member of the mated pair, called “extra-pair copulation” (EPC). Polygamy is less common among cooperative breeders than pair nesting, encompassing about 26% of cooperative breeders (Riehl 2013). EPCs do not always result in production of offspring. In some cases there may be competition and even conflict within the group. (Some researchers view cooperative polygamy as a mating system rather than alloparental care system.) Polygamy can take one of three broad forms:
     • Polyandry (pronounced POL-ee-an-dree): 1 female and 2 or more males. (Adjective polyandrous is pronounced pol-ee-AN-druss.) This is the most common type of cooperative polygamy.
     • Polygyny (pronounced puh-LIH-juh-nee): 2 or more females and 1 male. (Adjective polygynous is pronounced puh-LIH-juh-nuss.) This is a less common type of cooperative polygamy, possibly because multiple females might increase the size of the clutch in the nest, and it can instigate conflict between the females and among offspring (Riehl 2013).
     • Polygynandry (pronounced pol-ee-JIN-an-dree): 2 or more females and 2 or more males. This is the least common form of cooperative polygamy.
     • Genetic studies have shown that related helpers avoid incestuous relationships within the group, although scientists do not yet know how birds can tell if they are related to one another.

These 2 factors are found in different combinations: pair nesters with related helpers, pair nesters with unrelated helpers or a combination of related and related helpers, cooperative polygamists with related helpers, or cooperative polygamists with both related and unrelated helpers.

Several other variables contribute to the wide diversity of avian cooperative breeding:

• How many helpers are involved. In most cases there is only 1.
• Sex of helpers(s). Most are males.
• Age of helpers(s). Most helpers are recent fledglings who later go on to become independent breeders.
• Type and amount of alloparental care. This can include incubation, brooding, nest-building and maintenance, foraging and feeding, and defense of nest, territory, and nestlings.
• How many nests are used. Cooperative polygamists often use 2 nests, especially polygynous groups, where females lay their eggs in separate nests.
• Longevity. Cooperative breeding may occur in a single isolated season or persist for more than 1 season. Subsequent seasons may or may not see the return of the same extra(s).

Cockburn 2004 describes 22 distinct systems among 34 species of cooperative breeders for which genetic analysis (which can establish kin relationships among group members) is available.

How common is cooperative breeding?

Species may be considered regular cooperative breeders if the behavior occurs in a certain percentage of a particular population; scientists do not always agree on the percentage threshold (Cockburn 2006 defines a species as cooperatively breeding in cases where at least 10% of nests in a population are cooperative breeders). Cooperative breeding is considered occasional in some species and rare in others. A tiny number of species are obligate (necessary) cooperative breeders, in that reproduction is impossible without it, but most cooperative breeding is facultative (optional), a result of a choice made under given circumstances. (See Hatchwell 2009).

Current estimates categorize about 9% of all bird species worldwide as regular cooperative breeders (852 of 9456 species; Cockburn 2006, who omits 150 species that he categorizes as occasional cooperative breeders). Cooperatively breeding species are found in about 84 out of 188 avian families (45%), but are concentrated in only about 32 families (Arnold & Owens 1998; Ligon & Burt 2004). Most cooperative breeders are concentrated in tropical, subtropical, and neotropical zones, especially sub-Saharan Africa and Australia, with very few in temperate climates (such as most of North America). The reasons for this remain a topic of lively debate among scholars.

Cooperative breeding among raptors

Breeding behavior among raptors has always been challenging to study because of their large territories, sensitivity to the proximity of human observers, relative lack of tagged or banded birds to trace movements, and paucity of genetic studies to establish genetic relationships within groups. The increase of nest video cameras in recent years has helped expand the universe of information.

By any measure, cooperative breeding is highly unusual among raptors, and in most species where it occurs, it is occasional or rare. Kimball et al. 2003 is the only published review to date of cooperative breeding among diurnal raptors worldwide, and they report that instances of cooperative breeding have been recorded in only 42 of 304 species, or about 13.8% (Cockburn 2006 estimates about 15.4%, or 46 of 298 species). Cooperative breeding has been observed in only 5 of 170 species of owls (Cockburn 2006). Falcons and kites are more frequent cooperative breeders than hawks and eagles. The few North American species that engage in cooperative breeding regularly are Mississippi Kite, Harris’s Hawk, and Merlin. Occasional or rare cooperative breeders in North America include Osprey, Snail Kite, Bald Eagle, Cooper’s Hawk, Red-tailed Hawk, Red-Shouldered Hawk, Ferruginous Hawk, Swainson’s Hawk, Golden Eagle, Prairie Falcon, Peregrine Falcon, American Kestrel, Northern Harrier, Eastern Screech Owl, Great Gray Owl, Burrowing Owl, and Long-eared Owl. Worldwide, cooperative breeding has been observed in 8 species of eagles: Bald, Golden, Lesser Spotted, White-tailed, Bataleur, Bonelli’s, Madagascar Fish-Eagle, and Javan Hawk-Eagle. (Kimball et al. 2003; Cockburn 2006; Birds of North America Online. See References for individual raptor species.)

There is wide diversity in the types of cooperative breeding among raptors. As with most cooperatively breeding birds, raptors have more male helpers than female. (But it should be noted that the lack of plumage differences between male and female in many raptor species makes it difficult to determine the sex of helpers.) But raptors buck many of the trends (Kimball et al. 2003):

1. Over half of species for which some genetic information is available have helpers who are not kin of the dominant pair.
2. Helpers are more often mature adults than juveniles or sub-adults, especially among hawks and eagles. Adults are not likely to be kin to either member of the dominant pair.
3. Cooperative polygamy occurs more often than non-polygamous pair nesting.
4. 2 or more helpers are relatively common among raptors. Such groups can include kin and non-kin, cobreeders and non-cobreeding helpers.
5. Alloparental behavior of helpers is not always apparent or confirmed.
6. Cooperative groups with adult rather than immature helpers often persist for several years.
7. Many species inhabit temperate zones in contrast to the concentration of most cooperative breeders in tropical, subtropical, and neotropical zones.