- Scientists can now tell nearly identical male and female sarus cranes apart, by decoding their duet, a coordinated performance of movements and calls.
- A recent study found that the duet’s main notes are reliable indicators of sex, offering a non-invasive tool for studying and conserving the bird across India’s farmlands.
- It could provide the tools to study the understudied non-breeding flocks, asses the role of duets in signalling habitat quality and even hint at broader social and ecological dynamics.
It always begins the same way: the female sarus crane lifts her head and looks to the sky. She calls — a sharp, clear note that signals the start of a duet.
The male, close by, looks up and joins her, too.
Together, they perform a tightly coordinated sequence — a series of pulsed notes followed by continuous calls. The male spreads his wings and, bobbing them rhythmically, his notes are longer but fewer, modulated, with a low frequency. The female calls often, but the notes are shorter, flatter, and of a higher frequency.
After about 15 seconds of synchronised vocal and visual display, adding to the sound effects of farmland, the duet ends.
These vocal patterns emerged when scientists deconstructed the famed unison calls of sarus crane pairs for the first time in the wild across diverse north Indian agricultural landscapes.
A sarus crane duet in a paddy field. A duet is a synchronised, unison call produced by mating pairs of sarus cranes. Video by Suhridam Roy.
Why the noise about acoustic sexing?
The duet is more than just a show. In the study published in April, the scientists found that the duet holds a distinct vocal signature of each sex, hidden in plain sound. For a scientist, it solves a puzzle. Males and females look nearly identical, red-headed, grey-bodied giants standing five to six feet tall. However, the differences in vocal signatures are substantial enough to help in acoustic sexing — identifying the sex from sounds — without disturbing it.
Distinguishing between sexes is crucial for research and conservation. Knowing who is who, is necessary for calculating the population sex ratio, understanding social dynamics, conservation breeding, and more, of this species, which is threatened by habitat loss and urbanisation, and distributed across diverse farmlands.
All 15 species of cranes worldwide, like the sarus, are singers of their genre and perform duets. Each unison call, as explained in a Mongabay India podcast, is like a unique thumbprint for each pair assumed to mark territories and reinforce bonds.
Armed with a shotgun microphone and a digital recorder, the study’s co-author Suhridham Roy collected and analysed 215 duet recordings from 136 breeding bird pairs over six months from Gujarat, Haryana, and Uttar Pradesh. The duet, a daily “audio-visual extravaganza,” consists of an introduction, trill, and the main section, explains Roy, who is pursuing his PhD in sarus vocalisations and behaviour from the Nature Conservation Foundation (NCF).
The study converted sound to sight, visualising the recordings into graphs called spectrograms. The analysis found that the main notes are most suitable to reliably sex the sarus cranes using duets.
An annotated spectrogram of a sarus crane duet. Recording by K.S. Gopi Sundar, visualisation by Suhridam Roy and animation by Kartik Chandramouli.
From the different calls the sarus makes, the unison calls were most suited for this exercise because they are coordinated, sex-specific, and easily observable as compared to alarm calls, flight calls, or other contact calls, explains Sundar, adding that some of these calls need further studies for sexing.
One of the few moments when an observer can identify the large waterbird’s sex visually is when the male spreads his wings during a duet. Traditional sexing methods, including behavioural observations and genetic analysis, are time-consuming and depend on the observer’s skill, while physically handling the world’s tallest flying bird poses risks for it and the handler. “You don’t want to be kicked or pecked by this bird,” says the study co-author and Editor-in-Chief of the journal Waterbirds, K.S. Gopi Sundar, from experience.
Oceans apart in Australia, ecologist John Grant says the methods to develop acoustic sexing in India can be translated to all four existing subspecies in the world.
The sarus crane is also found in wetlands of Pakistan, Nepal, and Myanmar, as well as forested habitats in Southeast Asia and Australia. Some findings, like the female always taking the lead and initiating the duet, may hold across the range. Both Sundar and Grant, frequent collaborators but not for this study, believe there could be regional variations in duets, which future research can explore.
For Grant, who has studied cranes for about three decades, the ability to identify the sex of recorded birds opens up many new possibilities since acoustic data is widely used to supplement field observations in Australia. Of particular interest to him is examining the roles of the sexes in the recently documented hybridisation between sarus cranes and brolgas, a large Australian crane species.
One duet, many clues
“Identifying sex is a bigger problem in non-breeding flocks that don’t perform duets but are constantly vocalising and form 50-70% of a population in an area,” explains Sundar, Roy’s PhD mentor, who has studied these cranes since 1998. He adds that the study creates a baseline for training machine-learning models to identify sexes in non-breeding birds, addressing a gap in sarus crane research that is mainly limited to breeding populations.
Grant sees similar limitations and scope for immature cranes in Australia. “All of this requires further work on sexing cranes by calls other than the unison calls, and this work gives a good foundation for more research.”
Previous work by Sundar, Roy, and Swati Kittur discovered unusual trio formations, performing ‘triets’ mostly in low-quality habitats. A revelation in species assumed to be monogamous and to raise chicks in pairs.
The researchers expect that the duets could also offer clues to the health of crane territories. Could better duet coordination signal a better habitat? “Besides sexes, what else is communicated in these duets? That’s something we’re still exploring,” says Sundar.
Sarus and science
This study was built on a decade of field monitoring, during which Sundar’s team mapped nearly 1,000 sarus crane pairs. The long-term tracking helped the researchers choose where to go across three extensive states.
Unlike songbirds with their vocal organ called the syrinx and a large variety of calls, sarus cranes have a more primitive vocal system called the pharynx and a limited number of vocalisations. The complex and sweet songs of songbirds have been extensively studied. However, the vocalisations of non-passerines like cranes that are not as melodious remain largely unexplored, says Roy, as he points to and speaks of the spectrograms of sarus calls as “a well of insights”, over a video call.
“The sarus system is just like a trumpet. They have a tube, and they blow air through it,” laughs Sundar. “But even with a basic trumpet structure, there is a whole bunch of signatures in those vocalisations.”
Banner image: A sarus crane pair performing a unison call in Nepal. Image by Maya.Xu/Wikimedia Commons (CC BY-SA 4.0).
