what songbirds can teach us about divorce and moving on
- Written by Frigg Janne Daan Speelman, PhD Candidate in Behavioural Ecology, Macquarie University
In humans, it’s very common for the traditional family structure of two parents raising children to change abruptly. Usually, this happens when the parents decide to separate.
Many separated couples are able to navigate this change well to ensure their children are not overly affected. But this isn’t guaranteed. Parental separation can lead to behavioural, developmental and health issues in children. The effect can be long-lasting.
That’s humans. But many other species have a similar arrangement in choosing a long-term mate to raise offspring. What effect does it have when these couples split up?
In our new research, we examine what happens after a family split in a tropical bird species known for its long-lasting partnerships. To our surprise, we found the separation had no long term effect on the health of their offspring.
Family drama on a tropical island
Most birds form strong partnerships to raise offspring as a couple. Raising baby birds is an intense job. Having two parents at the nest is often essential for the survival of the chicks.
The Seychelles warbler is no exception. These small songbirds live only on the Seychelles, northeast of Madagascar.
They often pair for life and defend a territory together. The longest documented partnership is 15 years.
Still, family life is not always perfect for these warblers. Of all warbler couples, about one in seven (14%) end in a separation.
In these cases, one of the parents leaves their territory and finds a new mate elsewhere when there is still an egg, young chick or fledgling in their original nest.
How do you know if a bird is stressed?
For decades, a team of international researchers has studied the population of warblers on Cousin Island nature reserve. The dataset they have gathered includes information about the relationship status of all birds, as well as measures of their wellbeing, lifespan and how many offspring they produce.
Using this data, we looked at what happened to the offspring of partners who had split up.
We measured the stress levels of chicks who hatched just before the ending of their parents’ relationship up to the moment they reached adolescence.
In humans, it’s common to measure stress by looking at the levels of cortisol in saliva and hair. But measuring stress in birds is a little different. To do this, we used three biomarkers which indicate physiological stress experienced by animals.
The first is telomere length, the protective “caps” on the end of chromosomes which protect DNA against damage. The shorter the telomeres, the higher the stress.
The second is the percentage of red blood cells in blood, which shows how well a warbler chick can use oxygen. The third is body condition, which indicates a bird’s fat reserves. These three markers let us gauge the stress levels and health of young warblers.
We also wanted to know whether parental separation would have lifelong consequences for the chicks. To find this out, we compared chicks from separated parents to those with more stable nests and assessed how long they lived and how many offspring they produced.
Warbler separation doesn’t stress chicks
What we found was surprising. There was no evidence the separation of parents affected the stress levels of chicks or their long-term survival and breeding success.
Why might this be?
One reason is that these birds don’t linger on the past. They move on very quickly.
When a partnership ends through separation or death, it opens up a vacancy – which can be quickly filled.
When separation happens, one bird leaves. The remaining partner can find themselves left with offspring to take care of themselves.
But parenting chicks is only half the story – nesting is also about territory. Cousin is a very small island of just 0.3 square kilometres with over 300 Seychelles warblers living on it.
That means the space to breed is very limited. As a result, when vacancies arise they’re often filled just hours after separation.
For the chicks, this means they now have a step-parent who can help raise them almost immediately. This means they don’t face a future with less parental care.
That’s not all. The Seychelles warbler is known for an unusual arrangement in which breeding couples sometimes receive help from other warblers, known as cooperative breeding.
On Cousin Island, there are more Seychelles warblers than there are breeding vacancies, meaning many adults can’t secure a partner and territory. These adults can join couples in established territories and help raise the offspring. This extra help may offset any reduced parental care after the ending of a partnership, meaning the offspring are still well off.
What we can learn from this
Our research shows Seychelles warbler chicks are better able to weather their parents separating than we expected.
Importantly, this humble bird is just one of a huge group of species who form socially monogamous partnerships where both parents raise the offspring.
Whether a parental relationship ending affects other species remains to be seen.
Authors: Frigg Janne Daan Speelman, PhD Candidate in Behavioural Ecology, Macquarie University
