Definitions

Pair bond: association that is at least moderately long term between male and female

Monogamy: 1 male pair bonds with 1 female

• may be long term, over many seasons (e.g. swans, geese) or just for one breeding attempt (most species)
• monogamy is the most common mating system in birds; occurs in most bird species
• in other animal groups, monogamy is much less common than other mating systems

• next most common mating system in birds after polygyny, but much less common than monogamy
• in other animal groups polygyny often the most common mating system

• less common than monogamy or polygyny in birds
• more common in birds than in other animal groups
• a number of examples in the Charadriiformes

Promiscuity: >1 male mates with >1 female: little/no pair bond

Are mating systems species typical?

• some species: virtually all show the same mating system (ex: monogamous gulls, polygynous red-winged blackbirds)
• some species: different individuals have different mating systems (ex: indigo buntings: most monogamous, 15-20% polygynous; dunnocks: many different possible mating systems for different individuals.)

•  young birds energetically costly to raise; requires both parents.  Support: in some altricial species, if remove one parent, decrease survival of young.  Not supported for some monogamous precocial species: removal of males does not affect survival of young
• both parents required to successfully raise young for some reason other than the energy cost of raising young. Ex: many seabirds: if one parent removed, young lost to predation.
• male-male and female-female interactions limit possibility of being polygynous or polyandrous. Support: Willow Ptarmigan (Galliformes): females defend larger territories from other females; males cannot successfully defend areas large enough to hold more than one female from other males. "Spaced-out female effect."

• high "environmental potential for polygyny"; female spacing patterns  may make it easier for a male to defend >1 female. Ex: nest predation may favor female clumping in favorable, predator free areas.  A male can more easily defend several clumped females than several spaced out females.
• the polygyny threshold model: in patchy habitats, late-arriving females may benefit by mating with an already mated male on a high quality territory (polygyny) than by mating with an umated male on a low quality territory (monogamy) if even by sharing resources with another female on the high quality territory she will get more resources than she would in the low quality territory.

• many polyandrous birds are in the Charadriiformes, birds with a small  number of precocial young per clutch (e.g. jacanas, spotted sandpipers, phalaropes)
• double clutching evolves first: female lays eggs in a nest for a male, then lays another set of eggs in nest that she incubates.  This would evolve because it increases the reproductive success of male and female.  Only one parent needed to raise young.
• once the male has evolved the behavior of doing all parental care at one nest, the female can maximize her reproductive success if she mates with more than one male and having each male take care of one nest.  Polyandry thus evolves after double-clutching

Examples:

• forced extra-pair copulations: ducks
• indigo buntings: young males sire fewer young
• white-crowned sparrows: old males sire fewer young
• red-winged blackbirds: males with many females on their territories sire more young on & off territories