With the breeding season fast approaching, I thought it would be fun to write a post dedicated to some of the finer details of growing up in a crow family. From territory selection to the function of helper birds I hope this post illuminates some of the less familiar aspects of what goes on between crow parents and kids.
Crows reach sexual maturity between 2-4 years, with females generally maturing faster. Once they’ve bonded with a mate it’s time to secure a territory. After they’ve settled on a territory they’ll hold on to it for years, if not the rest of their lives, providing a great opportunity for some in depth crow watching! With few exceptions, crows generally nest in areas similar to where they were raised1. Meaning, rural crows settle down in rural areas, suburban crows in suburban areas and so on and so forth. Dispersal distance ranges (generally) from 0-60 km, and some birds will settle down right next door to their natal territory.
As far as mating goes, we refer to crows as being socially monogamous but genetically promiscuous, as is the case with most birds. This means mated pairs will typically stay together for life, but extra pair copulations are not unusual, at least in some populations. For example, in one study in New York, breeding males sired 82% of their offspring, while the rest of the clutch resulting from extra pair matings.2 A separate study showed that among 25 families, 36% experienced extra pair attempts, and of 252 offspring, 19% were not sired but their mother’s permanent partner.³ Whether females are soliciting these extra pair matings is unclear, but most data suggest that females do not have complete control over their extrapair partners, if any.2 Loss of paternity increases dramatically if the male has been non-fatally injured. Although his partner won’t leave him altogether, he is generally at risk for smaller brood sizes and paternity loss jumps to about 48%, likely due to more difficultly mating and guarding his mate, as well as lower sperm counts as a result of stress3. In these cases, females may be more willing to accept fertilization from extrapair males, though this is poorly supported.
The mechanics of mating
For those that want the biological details on crow sex, male crows, like the majority of birds, do not have an external penis. So mating is generally preceded by solicitation from the female, followed by the male mounting and rubbing his cloaca against hers, which transfers the sperm. This is known as the cloacal kiss (which, bleh, saying reminds me of Tina Fey in 30 Rock talking about the word ‘lovers’). The whole process takes a couple of seconds.
Crows start to nest in mid to late march with both males and females participating in nest building. Nests are constructed from sticks and lined with grass, fur, feathers or other soft materials. The building process is pretty conspicuous if you know what you’re looking for (and very fun to observe) but be warned; I have little doubt that crows will build fake out nests if they believe they are being watched by someone they don’t know or trust. Females lay a clutch of usually 3-4 blue and brown speckled eggs. Although males will occasionally (rarely) sit on the nest while the female is away they can’t really incubate because they lack the necessary brood patch to transfer heat. Chicks hatch after about 19 days of incubation followed by another 30-45 days in the nest before fledging. Once they fledge they spend 1.5-2 months dependent on their parents before they’re ready to strike it out on their own.
I dedicated another post to talking about the behavior and survival rates of hatch-year offspring which you can find here. Let me use this space to pick up where that post left off addressing helpers. Crows, like a number of other birds including nuthatches and kingfishers, engage in cooperative breeding, though they are not obligate cooperative breeders meaning they can successfully fledge young without helpers. Cooperative breeding is defined as when more than two individuals contribute to the care of young in a single brood. For crows this means that, in addition to the mated pair, there can be up to 10 additional birds helping to raise this year’s brood.1 Generally, these are young males that are related to the male breeder.2 The motive behind cooperative breeding is somewhat mysterious since there are costs to both breeders and helpers. Costs to parents including diversion of food provisioning towards helpers and, for males, threat of paternity loss to helpers. Costs to helpers are more straight forward; they’re delaying their own breeding efforts to rear offspring which only share some of their genetic identity. So why do crows bother?
Let me preface this by saying that quantifying the helpfulness of helpers is surprisingly difficult. A central roadblock can be; how do you tease out the effects of helpers, vs the underlying quality of parents that have lots of helpers? One study that controlled for differences among parents, found that helpers were’t hardly helpful at all, begging the question if “helper” is really the most apt word choice here.5 Still they are fact of life for many crow pairs, so let’s attempt to understand what’s going on here, based on the available evidence.
Why parents keep them around:
One insight comes from studies done on carrion crows (a species native to Western Europe and Asia). In these birds, differences in helper helpfulness are so obvious that some individuals are scientifically referred to as “lazy” group members. Amazingly, however, if one of the parents is injured it’s these lazy group members that pick up the most slack to compensate for the reduction in effort by another group member.6 So they’re kinda like your State Farm policy. Most of the time they feel like an annoying burden that will never come in handy, until that one time they do. Alternatively, in other birds, it’s been shown that although helpers don’t make much of a difference as far as each individual breeding attempt goes, they do increase the breeding female’s overall life expectancy and therefore pay off in the long run.7
Why helpers help:
For the helpers themselves, there are more potential benefits. Whether it’s by choice or force, helper males can sire as much as 7% of offspring, so breeding is in fact not out of the question.4 Although it’s a small number, some of that percent is even from mother/son relationships, though this isn’t consistent among corvids and some species do a better job avoiding incest than others. Even if they don’t breed with the female, they’re still helping to raise siblings that share some of their genetic material so it’s not a complete loss from the standpoint of their DNA (something known as inclusive fitness). They also stand to inherit the territory, should something befall the territorial pair. Finally, though this has been especially difficult to quantify, it’s possible that sticking around and observing a breeding attempt offers valuable insights that helper birds will go one to use when they raise their own brood.
How did such a behavior evolve?
That really is the million dollar question. It may have to do with delayed dispersal, which is basically a fancy way of saying there wasn’t always places for these young juveniles to go so they just stuck around at home (ah ha! See, millennial are just doing what comes naturally). It’s notable that the majority of birds that partake in cooperative breeding do so in kin based groups, which seems to suggest that perhaps this is a product of the fact that kin tend to settle down close to each other and as time went on a more loose, flexible system of helping became the more formal cooperative system we see today.
1McGowen, K. 2001. Avian ecology and conservation in an urbanizing world. Kluwer Academic Press, Norwell, MA. p 365-381
2Townsend, A.K., Clark, A.B., McGowen, K.J., and Lovette, I.J. 2009. Reproductive partitioning and the assumptions of reproductive skew models in the cooperatively breeidng American crow, Animal Behavior 77(2)
³Townsend, A.K. (2009). Extrapair copulations predict extrapair fertilizations in the American crow. The Condor 111: 387-392
4Townsend, A.K., Clark, A.B., and McGowen, K.J. 2011. Injury and paternity loss in cooperatively breeding American crows. J. Field Ornithology 82(4): 415-421
5Caffery, C. (2000). Correlates of reproductive success in cooperatively breeding crows: If helpers help, it’s not by much. The Condor 102: 333-341
6Baglione, V., Canestrari, D., Chiarati, E., Vera, R., and Marcos, J.M. 2010. Lazy group members are substitute helpers in carrion crows. The R. Soc. Proc. B: 282(1804)
7Wright, J., amd Russell, A.F., How helpers help: Disentangling ecological confounds from the benefits of cooperative breeding. British Ecological Society 77: 427-429