The question arose as to how nature stops songbird inbreeding (incest) when a new breeding female bluebird came into the scene – was she the previous female’s daughter? And who is her mate – could it be a new male or could it be her father since that was his territory for over four years.
There are several mechanisms that minimize songbird inbreeding risk. Two of these mechanisms, which are pretty strong are song discrimination and sex-biased dispersal. Despite these mechanisms, inbreeding can happen under certain situations.
So in this article, we’re setting out to answer whether this new female is an example of songbird inbreeding or not. And we will answer those questions by examining two strong mechanisms that reduce the risk of incest among songbirds. These two mechanisms are not the only ways songbirds prevent inbreeding, but they are one of the strongest and can be applied across many species of songbirds, not just bluebirds.
Why is it important for songbirds to prevent inbreeding in nature?
In nature, biology strongly selects against inbreeding, and that is because inbreeding can lead to more inherited diseases over time and also restrict genetic diversity.
By breeding outside of family members, organisms are able to adopt new variations and mutations that can offer new advantages such as breeding preference, adaptations against illnesses, or survival adaptations.
Dark-eyed juncos are a fun example to explore. Watch these winter birds in your backyard and take close note of the different shades and tones of their colors. Most juncos will be pretty dark grey with a very light gray breast. However, if you look close, you’ll find some lighter colored juncos, brownish juncos and a variety of others.
The Two Key Mechanisms Preventing Songbird Inbreeding/Incest
After extensive research, two primary mechanisms stand out that help minimize the risk of inbreeding among songbirds. These are song discrimination and sex-biased dispersal.
Both of these terms may seem pretty technical, but it’s actually really easy to understand, and we’ll break it down better in each section.
Song discrimination helps prevent songbird inbreeding among family members
Song discrimination is the technical term that simply means that a songbird can tell the difference between an individual’s song. We humans do this. We know the sound of our family members, our friends, and other people we’re regularly in contact with. Though it might be hard for us, when listening to birds, to tell one cardinal from another based on their call, a bird can do this.
With song discrimination, songbirds are able to distinguish their family members from non-family members. Researchers have studied in-depth. And a lot of research on this topic was conducted on western bluebirds. This might be because all three bluebird species are cavity nesting birds, which makes nesting habits and family relationships very easy to isolate and study.
One way of examining song discrimination was playing recordings of non-kin and kin bluebirds – meaning scientists played the song of family members and non family members near a male western bluebird’s nests. Researchers found that the male bluebirds responded more when they heard recordings of non-family members versus when they heard family members (Akçay, Swift, Reed, & Dickinson, 2013).
If you want to start to challenge yourself when it comes to differentiating songbirds by sound, cardinals are a great place to start. These bold singers can really get your attention. Listen to the cardinals in your community or subdivision. Then, if you visit a park or nature center, or somewhere else further out from home, you may notice some slight differences in how these other cardinals sound versus the ones near your home.
Sex-Biased Dispersal as a very strong way to minimize songbird inbreeding
Sex-biased dispersal is another very technical sounding term. But all it means is that in certain species, one sex, whether male or female, tends to disperse out further from it’s birth place (or natal territory).
For songbirds, the bias is for female birds to disperse further while male songbirds will form kin neighborhoods, which means related males will establish closer territories with each other. Their territories will usually be far enough away to meet the demands of their species but still rather close.
When it came to sex-biased dispersal, female bluebirds were shown to disperse several hundred meters, up to a few kilometers away, while male bluebirds may have remained just a few hundred feet from their birth place.
This long distant dispersal in female songbirds helps reduce the potential of songbird incest. As females disperse further, they wander into new territories and breed with the males there.
When songbird inbreeding still happens
Despite these two strong mechanisms that prevent songbirds from breeding within family members as well a few other mechanisms, incest among songbirds still happens. Fortunately, across several studies, the rates of songbird inbreeding events are very very low.
Inbreeding among mother birds and “helper sons”
One of the times songbird inbreeding could occur is between mother and helper son. A helper son is a male offspring bird who remains in the territory and helps with nesting. This especially happens among bluebirds. When the male offspring has not established his own territory, he may be permitted to remain around the territory and help out.
During this time, if the female is fertile, and her bonded mate is not around, the mother and son may mate, or copulate.
In songbirds, a phenomenon called extra-pair mating events happen, despite some songbird species forming strong mating bonds. Extra-pair mating is a mating event outside of the socially pair bonded birds. We have a lot more information on this in our article examining whether bluebirds mate for life.
We also have a video version about bluebird mating that goes into a lot more detail about extra-pair mating events and how often that happens.
Environmental pressures increases songbird inbreeding events
As you might expect, things like habitat loss, food scarcity and low breeding selection can increase songbird incest events.
And it makes sense. While biology selects against inbreeding, it still highly selects for making more of itself. When environmental pressures reduce choices to breed or thrive, biology still has its way, and that can open the door for inbreeding events to occur.
Researchers have still found inbreeding events to be very low, but environmental stress can have a big influence.
As backyard bird hosts, this gives us more reason to apply good conservation practices among the birds and wildlife we host in order to help them thrive and prevent diseases or other negative consequences.
References
A lot of research went into learning about incest and inbreeding within songbirds. Here are the references used:
Berg, M. L. (2007). Sexual selection and reproductive strategies in songbirds: Territoriality, mate attraction, parentage and parental care.
Bose, A. P., Koch, L., Dabernig-Heinz, J., Grimm, J., Sefc, K. M., & Jordan, A. (2022). Patterns of sex-biased dispersal are consistent with social and ecological constraints in a group-living cichlid fish. BMC ecology and evolution, 22(1), 21.
Çağlar Akçay, Rose J. Swift, Veronica A. Reed, Janis L. Dickinson, Vocal kin recognition in kin neighborhoods of western bluebirds, Behavioral Ecology, Volume 24, Issue 4, July-August 2013, Pages 898–905, https://doi.org/10.1093/beheco/art018
de Boer, R. A., Eens, M., Fransen, E., & Müller, W. (2015). Hatching asynchrony aggravates inbreeding depression in a songbird (Serinus canaria): An inbreeding–environment interaction. Evolution, 69(4), 1063-1068.
de Boer, R. A., Eens, M., & Müller, W. (2016). ‘Out of tune’: consequences of inbreeding on bird song. Proceedings of the Royal Society B: Biological Sciences, 283(1835), 20161142.
Dickinson, J. L., Akçay, Ç., Ferree, E., & Stern, C. (2016). A hierarchical analysis of incest avoidance in a cooperative breeder. Behavioral Ecology, 27(4), 1132-1140.
Leedale, A. E., Simeoni, M., Sharp, S. P., Green, J. P., Slate, J., Lachlan, R. F., … & Hatchwell, B. J. (2020). Cost, risk, and avoidance of inbreeding in a cooperatively breeding bird. Proceedings of the National Academy of Sciences, 117(27), 15724-15730.
Prugnolle, F., & De Meeˆus, T. (2002). Inferring sex-biased dispersal from population genetic tools: a review. Heredity, 88(3), 161-165.
Wheelwright, N. T., Freeman-Gallant, C. R., & Mauck, R. A. (2006). Asymmetrical incest avoidance in the choice of social and genetic mates. Animal behaviour, 71(3), 631-639.
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All images were taken by Nest Hollow and are copyrighted
