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Trios–A novel social unit in Sarus Cranes

Read time: 5 mins
29 Apr 2022
Trios–A novel social unit in Sarus Cranes

 Picture Credit: Suhridam Roy

Studying the social behaviour of birds, bees, or animals is often full of surprises as we discover or know more about them. For instance, the social behaviour of tigers is very much different from that of lions. The tigers are mostly solitary, while the male and female tiger hunts on their own; in lions, the lioness hunts and allows the lion to savour first. Or the small green-bee eaters, after they have bred, laid eggs, and the eggs have hatched, will allow another younger, non-breeding individual to help feed and be around.

And among insects, especially ants, bees and wasps, they are fascinating to study social behaviour and are already subject to several doctoral studies. A recent study found out who inherits the wasp colony when the queen dies or is removed from the colony.   

Now, a new study by researchers Dr KS Gopi Sundar and his team at the Nature Conservation Foundation (NCF) published in Ecology has discovered a novel social unit in Sarus cranes.

Sarus Cranes

Sarus cranes are among the most popular resident cranes found in most parts of north India and in parts of Southeast Asia and Australia. They are also the tallest flying birds and are mostly found in and around marshes and shallow wetlands.

It is a striking large bird with grey wings and body, a characteristic red on its head and upper neck, and a pointed bill. Although in terms of plumage, both sexes are alike, the male of the species is slightly larger and can be as tall as 180 cm.

Sarus cranes are known to be distributed along the Gangetic plains from coastal Gujarat in the west to West Bengal and Assam in the east. But they are limited to the Godavari River in the south. However, in the recent past, they seem to be restricted to Gujarat, Madhya Pradesh, Rajasthan, Uttar Pradesh, Haryana, Punjab, and some parts of Uttarakhand, Himachal Pradesh, Jammu and Kashmir, and northern Maharashtra. They are no longer found in Bihar and are extremely rare in West Bengal and Assam.

The trios

Monogamy is the prominent social system among birds. However, to increase the survival of the brood and improve their fitness, some species are known to exhibit polyandry (when a female animal has more than one male mate), polygyny (when a male animal has more than one female mate), polygynandry (when both females and males mate with multiple males and females) and cooperative breeding (where both males and females are helpers).

For instance, the Acorn Woodpeckers are known to be polyandrous, and the Mute Swans are polygynous, says Gopi Sundar.

The cranes are known to be monogamous with a characteristic long-term pairing and playing stereotyped and synchronised behaviour of producing unison calls, known as duets. The calls are assumed to help them mark their territories and reinforce their pair bonding.

However, in their recent study, Gopi Sundar and his team found a novel social unit — groups of three Sarus cranes named 'trios’. All the three birds sing in unison, which was named by the researchers as ‘triet’.

Interestingly, the chicks raised by trios displayed behaviour identical to that of the pairs.

Gopi Sundar adds, "We only know that trios were more successful in raising chicks, suggesting that the third crane was helping with provisioning and perhaps also defending the territory.

Duets and triets

In one of the first observations of the polygynous Sarus crane trios, the researchers found that they produced triets when other Sarus cranes came closer to their territory.

“As they commenced nesting, one of the female cranes disappeared while the remaining pair nested, incubated eggs and provisioned the young chicks. The second female would join after the chicks have fledged and participate in provisioning for chicks. However, when the eggs were lost, or the breeding pair were unsuccessful in nesting, the second female immediately joined back,” explains Gopi Sundar.

The researchers found that, like duets, the triets had a coordinated structure with the female commencing the call. Then other cranes would join in to create a series of repeated notes, followed by synchronised displacement-preening where the birds would clean their feathers together.

The researchers used spectrograms to compare the triets and duets. They found that the triets had a lower minimum frequency in relation to the duets. In addition, the triets were acoustically distinct from duets.

Field observations from 1998 to 2020 that documented this unique behaviour among Sarus cranes were used in the study. The researchers also documented the videos and call recordings of duets and triets. In addition, they assessed the seasonality and distribution of trios using data collected as part of a multi-year monitoring programme from 2013 to 2020.

Territorial quality and trios

In all, the researchers had 11,591 sightings over the years of pairs, flocks, and trios. Of these, only 193 were trios, but the occurrences of trios were widely distributed.

The researchers documented the territorial quality of breeding sites as well. The trios were found mostly in low-quality territories (with less than 20% of wetlands or marshes in the territory) and mostly during summers.

This has raised questions among researchers on the evolutionary significance of the formation of trios in Sarus cranes, which were, until now, known to be monogamous. One of the reasons could be to improve breeding success and fitness, particularly when territorial quality is poor.

The climate change we are witnessing, in all likelihood, might have hit the territorial quality of the breeding sites, thus also impacting the breeding behaviour of Sarus cranes. Are we seeing its effects already? Future studies and genetic analysis can throw more light on this.

Editor’s note: An earlier version of this article was published in the Deccan Herald.

This article has been run past the researchers, whose work is covered, to ensure accuracy.