Through a complicated experiment, researchers unraveled how flycatcher chicks could identify the calls of others of their species even if they hadn’t heard them before.
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
A male songbird’s singing prowess attracts females. When birds of many species sing at the same time, how do the young identify which one is theirs? Do the melodies imprint when the chicks hear their parents – or are they hard-wired in their DNA? Or do mothers’ hormonal profiles affect their chicks’ learning in some way?
Daizaburo Shizuka, a professor at the University of Nebraska-Lincoln, discovered American golden-crowned sparrows as young as six days old recognise the songs of their own species. Creatures belonging to the same species are called conspecifics. The chicks chirped when they heard their species even though a related species, white-crowned sparrows, lives nearby. And even when raised in isolation and devoid of any song, zebra finches hopped more when they heard conspecific warblings.
How did the young birds know?
The island of Öland in the Baltic Sea, off the coast of southeastern Sweden, hosts two related species of warbler-sized songbirds: pied and collared flycatchers. David Wheatcroft and Anna Qvarnström, a postdoctoral researcher and professor, respectively, from Uppsala University, Sweden, conducted an experiment here to unravel this mystery.
Like the American golden-crowned sparrows, nine-day-old collared flycatcher nestlings recognise conspecific songs. So the researchers serenaded 10- to 12-day-old nestlings of both species with recordings of warblings. A remote camera fit on the ceiling of the nest box allowed them to observe the chicks’ behaviour. When they heard tunes sung by adult conspecific males, they looked at the entrance of the nest box, shifted around and begged for insects. The researchers interpreted these reactions to mean the young could differentiate between the songs of two species. This formed the baseline of the experiment.
Some nestlings reacted to the tunes of both species. The researchers think hunger clouded their judgments. For instance, instead of remaining quiet when their parents shrieked in alarm at predators, hungry scrubwren nestlings begged loud and long. This doesn’t mean they couldn’t differentiate: ravenous flycatcher young begged and moved more in response to their own species’ songs.
Wheatcroft swapped the eggs from 11 nests of both species. Although raised by foster parents, the 92 youngsters reacted to the tunes of their biological parents. These melodies spoke to them as they begged, moved and looked at the entrance. In fact, they reacted more vehemently than chicks that hadn’t been separated from their parents. This, the researchers say, is because the nestlings were in poorer condition than ones raised by their own parents, and therefore begged more.
Being raised by foster parents, the young couldn’t have learned the songs of their biological parents. How did they know it then? Could mothers’ hormonal profiles influence youngsters’ song recognition? For instance, the order in which females lay their eggs can change nestlings’ traits. They deposit more testosterone in the yolks of eggs they lay later, making younger chicks demand food much more than older ones. Testosterone also enhances the alertness of nestlings and affects the development of the neural network involved in the song system.
To test if testosterone could play a role, the researchers came up with an innovative experiment. They cross-bred the two species in an aviary. After eggs were laid, the researchers had collared flycatchers incubate 14 clutches. It would have been better to distribute these eggs to pied flycatchers too.
“But there are very few pied flycatcher nests, only around 15%, on Öland then,” Wheatcroft told The Wire. “To avoid manipulating too many pied nests, we fostered them all out to collared nests. In addition, we knew from the cross-species swapping experiment that early experience wasn’t so important. So we reasoned that the collared parents should have little influence on the hybrid nestling responses.”
Since the two flycatcher species hybridise in the wild, the researchers also tested two such clutches. They cross-fostered these with other mixed-species parents. For instance, a pied father-collared mother pair incubated the eggs of a collared father-pied mother pair.
When the nestlings were old enough, Wheatcroft played a song recording of one species and after a minute played the other. No matter to which species the mother belonged, all the young responded to pied flycatcher songs. Wheatcroft wondered if the collared flycatcher-raised nestlings found pied flycatchers’ songs novel. But in the baseline experiment, chicks didn’t favour songs other than their own, ruling out this possibility.
If it wasn’t early exposure to parental warbling or maternal influence on eggs, the only remaining possibility was genes. After all, female birds inherit the ability to recognise mates of their kind through the Z chromosome bequeathed by their fathers. Could this chromosome be the vehicle of song recognition?
Male and female hybrid chicks showed no preference for the tunes of their paternal lineage. This meant song recognition is not tied to gender-associated chromosomes. Since offspring inherit genes from both parents, the researchers say the song trait (allele) from pied flycatcher parent is dominant. This would explain why chicks, irrespective of gender and paternal species, prefer the warbling of pied flycatchers.
But nailing the specific genes that play a role would remove any shade of doubt. “Unfortunately, we are somewhat limited in how we can proceed with this goal,” says Wheatcroft. “The typical way to find genes controlling a trait would be to repeatedly breed hybrid individuals with adults from one of the pure species.” This would produce individuals with most of the genome of one species but the traits of the other. This course of experimentation is impossible in flycatchers because hybrids are sterile. “Another way to proceed would be to use neurobiology approaches, such as looking for neural activity in the brains of nestling birds when they listen to songs,” says Wheatcroft. “These approaches are used often in laboratory species, mostly zebra finches, but could potentially be used on wild birds like flycatchers as well.”
For a long time, researchers suspected song recognition is hereditary, and now this study demonstrates it for the first time. Even though the two species separated recently, about half a million to 1.5 million years ago, the chicks’ fidelity to conspecific tunes maintains the distinctness of species.
“What makes this study strong is the use of a great study system,” Shizuka told The Wire. “The collared/pied flycatcher system is well-known as a model for understanding the evolutionary processes of speciation and hybridisation. It has a lot of logistical advantages – they readily breed in nest boxes, they can be cross-fostered and they can be hybridised and bred in captivity. The study design definitely makes great use of this system.” He wasn’t involved in the study.
Pulling off this complicated experiment was not without difficulties. Predators gobbled up eggs and one female turned out to be a hybrid so her eggs were infertile. “The biggest challenge was conducting the study in nature,” Wheatcroft says. “For all of the experiments, nestling responses to songs are highly variable. Eventually, we were able to figure out some of the factors underlying this variation, such as the condition or weight of the nestlings. But very often nestlings simply didn’t respond. This doesn’t mean that they don’t have a song preference, rather that we couldn’t measure it easily. Each nest could only be tested in a two- to three-day window, since after the nestlings are 12 days old, they can fledge at any time.”
Even though nestlings are born with a song template and the pied flycatcher song alleles are dominant in hybrids, songs aren’t immutable. As adults, male pied flycatchers integrate riffs from their neighbour’s songs into their own.
“We would like to know more about how these [genetic] mechanisms interact with experience,” Shizuka says. “What other experiences are songbirds exposed to during their young lives that influences the choices they make in later life? These are the ultimate questions that we need to answer to fully understand both the origin and collapse of species.”
The study was published in the journal Nature Ecology & Evolution on June 12, 2017.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.