- A new study has discovered a new social unit among sarus cranes, where these once monogamous birds now form trios to raise their chicks.
- The study authors note that these breeding trios have mostly been observed in more degraded habitats.
- Highly territorial cranes might be forced to alter their mating behaviors to increase the chances of offspring survival, as habitat loss in India is on the rise.
A new study by KS Gopi Sundar, Swati Kittur and Suhridam Roy at the Nature Conservation Foundation details the discovery of a new social unit among sarus cranes. The study found that during breeding, sarus cranes, which usually mate for life, form trios to rear their chicks, to improve brood survival.
“Bringing in a third member to raise chicks is no mean feat, as cranes are very territorial birds. They have synchronized behaviors like unison calls and behavioral repertoires that can only be performed in pairs. That’s why this is such great news! A bird that we thought was monogamous now allows a third bird among the pairs,” says Sundar.
Besides India, the sarus crane is also found in Nepal, Pakistan, Southeast Asia, and Australia. According to WWF-India, the global population of sarus cranes is estimated at between 25,000 and 37,000. But their population is now declining with only 15,000 to 20,000 individuals found in India, with a majority in Uttar Pradesh. The birds are listed on Schedule IV of the Wildlife (Protection) Act 1972 and listed as vulnerable on the IUCN Red List.
The state bird of Uttar Pradesh, the sarus crane stands about 1.5 meters tall, boasts an eight-foot wingspan and weighs about seven to eight pounds. It is mainly found in marshes and wetlands.
Parenting Trios and Unison Calls
Sundar’s appointment with the Sarus cranes began in 1998 in Etawah district of Uttar Pradesh when he encountered an unusual sight during one of his patrols.
“I saw three sarus cranes calling in unison. I stopped dead because it was unheard of,” he said. noticed that one of the females had disappeared early in nesting, and the remaining pair had nested, incubated eggs, and provisioned young chicks. The female joined the pair immediately after the chicks were one month old, or if the eggs were lost.
Sundar realized he was onto something new and intriguing and continued to observe the behavior of a trio, unmarked, but assumed to be the same over the years, in the southwest of Uttar Pradesh, between 1999 and 2015. The lifespan of sarus cranes is not known due to a lack of long-term study of the species.
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All 15 extant crane species, including the sarus cranes, produce unison calls or “duets”. Pairs are believed to help birds defend their territories and strengthen pair bonds. In the new study, Sundar referred to the unison call of the trios as a “triet.”
Like duets, triets also had a typical structure with the female beginning the call, the additional cranes joining in a series of repeated notes, followed by a synchronized displacement smoothing.
“The trios emit a unison call that is acoustically distinct from the calls made by a pair of sarus cranes. The frequencies are different, and that was a huge surprise,” says Sundar. “The trios presented a low frequency call. This was unusual as most of the trios were women, who call more frequently.
Low-frequency calls typically occur in animals such as elephants, which attempt to transmit their sound very far. Whether the low frequencies of triets have any functional significance needs to be investigated, he says.
A diversity of social systems
Although monogamy is the dominant social system in birds, more complex social systems such as polyandry (where a female animal has more than one male mate), polygyny (where a male animal has more than one female ), polygynandry (a multi-male and multi-female mating system), and cooperative breeding (when more than two birds of the same species are involved in rearing the young of a nest) occur in all avian species, a behavior that scientists believe is linked to improved chances of offspring survival.
“Trios are found in other species such as polyandrous acorn woodpeckers and polygynous mute swans. Owls are also reported to have a third bird in their territory,” says Sundar.
Sundar and his team also assessed the seasonality and distribution of the trios using data collected from several agricultural landscapes in South Asia, as part of a multi-year monitoring program from 2013 to 2020.
A total of 11,591 groups of sarus cranes in pairs, families, flocks and trios were sighted, of which 193 trios were widely distributed across all surveyed locations.
Behavioral changes in an uncertain future
The reproductive success of the sarus trio observed over several years was high despite nesting in a territory facing anthropogenic pressures. This suggests that polygynous trios potentially improve fitness. More trios have been observed in these human-inhabited territories and during summers, suggesting that poorer habitats and harsh environmental conditions may force pairs to compensate for potential risks of low reproductive success by employing a ” assistant” to raise chicks.
“Although it is poor quality territory, the Etawah trio have been able to raise at least one chick every two years. And that was way more than breeding success in other territories,” says Sundar, adding that if an area has trios, it can indicate some level of habitat degradation.
“The smaller the territory, the higher the quality. Thus, in a low-quality area, the territory would be quite large. Birds must spend a considerable amount of time flying, ensuring that neighboring birds do not invade their territory. So having a third bird is also likely to help with that,” he says.
The study indicates that it is important to understand whether changing habitat conditions are linked to trio formations, given that behaviors such as monogamy in long-lived birds can be altered by the impacts of climate change.
“The main threats to the sarus crane in India are habitat loss and degradation, due to wetland drainage and conversion to agriculture. Our analysis shows that land use change land is a big factor here,” says Sundar. “If conditions continue to deteriorate across landscapes or in individual territories, it may well be that trios become much more common than they are now.”
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Banner image: A trio of adult sarus cranes. The three cranes work together to defend the territory, issuing calls in unison to warn other cranes. A third crane can provide the additional help needed to defend the whole territory and find food. Photo by KS Gopi Sundar.