As a rule, corvids like crows tend to be monogamous, though extra-pair copulations do occasionally occur (McGowan 2001).  This pair bond can be extremely strong: in one longitudinal study, only one divorce was noted in over 100 test pairs (Verbeek and Caffrey 2002).  Though courtship rituals are limited (Verbeek and Caffrey 2002), the male may feed the female, who may display submissive behavior (Niemann 2002).  Pair-bonded individuals may also preen each other (Verbeek and Caffrey 2002).  During the breeding season, these monogamous pairs are usually strongly territorial, though pairs some populations occupy more of a large home range (Niemann 2002).  Territory size can range from .72 hectares in Davis, California to 278 hectares in parts of Saskatchewan; pairs in many populations focus more energy in defending a core area (Verbeek and Caffrey 2002).

Because crows are not sexually dichromatic, we should not expect polygyny or sexual promiscuity to be the rule.  But why monogamy?  Monogamy has also evolved in a small African antelope, the dik-dik.  Male dik-diks can only provide resouces for one female in his territory, because all available territory has been saturated.  Moreover, female dik-diks do not allow extra pair copulations because the male can provide her no resources; moreover, the male defends the female against competitors.  Extra-pair copulations in American Crows likewise fail because the paired male with drive his competitor away (Verbeek and Caffrey 2002).  Because crow pairs usually defend a core area at the least, and males will physically drive other males away, this arrangement as some combination of resource- and female-defense monogamy.  My suspicion is that crows abandon the home range-flocking behavior for a more territorial system during the breeding season because they are exploiting seasonal resources that are distributed differently.

While some crow populations are “solitary nesters” (Niemann 2002), many populations are cooperative breeders.  A bonded pair’s helpers are usually their fledglings from the previous year who stick around (Verbeek and Caffrey 2002); though much variation likely exists, family groups in Florida average 7.2 individuals.  There may be a number of advantages to such cooperative breeding.  Research done in Cape Cod has shown that nests with at least 4 helpers raised more young successfully than those that have fewer helpers.  Verbeek and Caffrey have written (somewhat contradicting themselves, see below) that helpers may feed the female while the male guards against extra-pair copulations.  Interestingly, cooperative breeding has also evolved within another bird lineage, the Harris Hawk.  However, the crow system features monogamy, while the Harris Hawk system centers around polyandry or polygynandry; moreover, the diets of American Crows (omnivorous) and Harris Hawks (strictly carnivorous) differ significantly.

Other authors, however, have questioned the adaptive value of cooperative breeding.  Caffrey in particular has found that helpers do not significantly increase a pair’s breeding success.  So why have helpers?  Perhaps the young from previous years stick around because of territory saturation, i.e. they cannot stake out a plot of their own because all available areas are taken.  By sticking around to “help” their parents, they can increase their fitness slightly.  At the same time, they can take advantage of mom and dad’s resources (like a college grad moving in with their folks) and practice feeding themselves and young, in a way practicing to be a parent.  Similar patterns actually occur within the lesser apes, namely gibbons.  Before establishing their own territory, juvenile gibbons live in their parents’ territory, presumably as “helpers.”

One must also keep in mind the behavior may be more adaptive in some crow settings than others.  Crows inhabit many different habitats, each with its own suite of conditions and factors to affect nesting success.  Because cooperative breeding seems to optimize reproductive fitness in most cases, crows in different habitats may retain this strategy because evolving a new reproductive system may be more evolutionarily expensive than simply keeping their old, tried-and-true system.

Howcver, this elaborate social system has recently come under the threat in the past few years.  The deadly West Nile virus affects many bird, but seems particularly deadly to crows; birds may even transfer the disease directly to one another.  McGowan in particular has expressed concern that this disease may destroy the social fabric of the American Crow (Boyce 2002).  Though ornithologists have barely scratched the surface of this question, both McGowan and Caffrey have begun to explore the relationship of the West Nile virus and the crow’s cooperative breeding structure (Boyce 2002).