A resource of Biophilia: Pittsburgh, #bioPGH is a weekly blog and social media series that aims to encourage both children and adults to reconnect with nature and enjoy what each of our distinctive seasons has to offer. 

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If you have been following our local bald eagle camera, you have probably spotted that as of posting time, there are currently two eggs in the nest!

Watching this little family year after year is a good reminder that in the bird world, as in the rest of nature, mate selection is an important decision. From the female’s perspective, reproducing is energetically costly, and you want to make sure your offspring will survive after all of that work! Thus, it’s in a female’s best interest to find a high-quality mate, as that will play a pivotal role in the survivorship of offspring. Reproductive strategies and family structures vary widely across nature, but sometimes mate selection just boils down to a contribution of genes, sometimes it is genes plus assistance with raising the offspring, and sometimes…sometimes it means assistance of raising offspring without the contribution of genes. That last option can be jarring idea. We often think of birds in monogamous, somewhat romanticized terms, but nature is a tough place and…well, sometimes a bird’s got to do what’s a bird’s got to do. From a very anthropomorphic perspective, there is a little more spice to bird love stories than it first seems. Let’s take a look!

Mostly Monogamous
The prevailing wisdom is that roughly 80-90% of bird species are monogamous. Mates often pair for life, depending on the species, and both parents will assist in caring for the young. This biparental care is very important for many bird species as their offspring are often altricial — virtually helpless at birth, as opposed to precocial, which refers to baby animals born with their fur or feathers and capable of movement like running or jumping shortly after birth or hatching. If you picture helpless, featherless robin nestlings as opposed to relatively capable ducklings who can swim and follow mom, it’s easy to spot who needs more parental care. That breakdown often correlates with monogamous pairings.

But…
At this point, I should make an important distinction between socially monogamous and genetically monogamous.* Socially monogamous refers to birds who pair together, raise offspring together, but may mate with each other and/or other birds. Genetically monogamous, on the other hand, refers to exclusively reproducing with a single individual. And it turns out that amongst our avant-garde avians (or perhaps libertine larks? Sultry sparrows?), genetic monogamy is actually fairly rare. In other words, to put it bluntly: “monogamous” birds are often cheaters. They engage in what’s called extra-pair copulation, mating outside their monogamous pair. Predictably, this leads to extra-pair paternity, when someone outside of the mated pair is the father. A single brood of nestlings can even have multiple fathers! (Australian fairywrens have one of the highest rates of cheating in the bird world – roughly 75% of nests contain eggs from at least two different fathers.)

Though we do have to be very careful not to interpret bird behavior through lens of human morality and social norms, it is still worth noting that cheating is a risky behavior for birds. If the cheater is caught by their partner, they risk “punishment” from their partner, which can range physical attacks, abandonment, or decreased parental involvement once chicks arrive. With such steep possible consequences, why would birds bother to cheat at all? Well, again, it comes down to survival of offspring. By mating with multiple partners, males are attempting to ensure that they have enough offspring that some will survive to adulthood. Females are mixing with more genes, again, to give as many of her own offspring the possible chance to survive.

*Note: In the strictest sense, there are a few variations on monogamous strategies; check out this table for a deeper dive.

And What About…
The best part about nature is that it’s full of exceptions. Whenever we humans think we have identified a “rule” in wild world, something out there will defy it. For example, we have spent most of our time today talking about monogamous birds (even when monogamy isn’t quite monogamy), but there are plenty of other birds out there with very different family. There are several species of tropical jacanas where one female will keep a harem of males with whom she lays clutches of eggs, and the males will each raise their little brood on their own. There is also a concept called “cooperative breeding” or “communal breeding” where multiple adult non-parental birds will help raise the babies.

Or…The Ultimate Single Parents
Back in November, many of us were stunned to discover that two critically endangered California condors had hatched to mothers but not fathers; the mothers had undergone parthenogenesis, a form of asexual reproduction! Eggs were never fertilized, but hatched chicks! Parthenogenesis is very rare among birds, but it has been observed occasionally in turkeys, zebra finches, quail, and chickens (as the Audubon Society notes here, all birds that are closely monitored or raised by humans, which makes the rare occasions of dad-free births easier to spot.) Nature indeed keeps us on our toes.

The Wild World
Again, we do have to keep in mind that the way human cultures and societies look at reproduction and family structure usually doesn’t translate to nature’s norms, so while we may say that many birds are “cheaters,” they are fundamentally just trying to survive and pass on their genes. It does add a bit of intrigue to bird watching, though, so at the end of the day, just enjoy nature’s drama!

Connecting to the Outdoors Tip: If you’re interested in some bird family research, check out the Cornell Lab of Ornithology’s Nest Watch. You can help collect valuable community science data and enjoying connecting the wild world!

Continue the Conversation: Share your nature discoveries with our community by posting to Twitter and Instagram with hashtag #bioPGH, and R.S.V.P. to attend our next Biophilia: Pittsburgh meeting.

Resources

Cooperation and Coordination in Socially Monogamous Birds: Moving Away From a Focus on Sexual Conflict

Genetic monogamy despite frequent extrapair copulations in “strictly monogamous” wild jackdaws

Brouwer and Griffith 2019 – Extra-pair Paternity in Birds

Valcu, Valcu, and Kempenaers 2021 – The macroecology of extra-pair paternity in birds

Images: Cover, Davie Govoni CC BY-NC-SA 2.0; Header, Pexels public domain