In India, habitat loss may force a once monogamous bird to form breeding trios

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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 a third member to raise chicks is no small feat, as cranes are very territorial birds,” Sundar said. “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 in pairs.

Besides India, the sarus crane is also found in Nepal, Pakistan, Southeast Asia, and Australia. According to the World Wide Fund for Nature-India, the global population of sarus cranes is estimated at between 25,000 and 37,000. But their population is now in decline with only 15,000 to 20,000 individuals found in India, with a majority in India. Uttar Pradesh. The birds are listed on Schedule IV of the Wildlife (Protection) Act 1972 and listed as vulnerable on the International Union for Conservation of Nature 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.

Researchers have encountered a few cases of breeding pairs accompanied by a young adult, all likely from the same herd. Photo credit: KS Gopi Sundar

Parenting threesome

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.

Map by Technology for Wildlife

All 15 extant species of cranes, including the sarus cranes, produce calls in unison or “duets”. Pairs are believed to help birds defend their territories and strengthen pair bonds. In the new study, Sundar called the unison call of the trios 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 calls made by a pair of sarus cranes,” Sundar said. “The frequencies are different, and that was a huge surprise. 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 said.

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 extra help needed to defend the larger territory and find food. Photo credit: KS Gopi Sundar

Diverse 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 like polyandrous acorn woodpeckers and polygamous mute swans,” says Sundar. “The owls would also have a third bird in their territory.”

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.

Habitat degradation is a serious threat to sarus cranes, often limiting the availability of, and access to, high-quality territory. Pairs in such territories where cultivated fields dominate are less able to raise chicks each year due to the complete removal of vegetation (crops) from the fields, making the chicks vulnerable to predation. Finding food also becomes difficult with only two parents, and an increase in these conditions may be the reason sarus cranes now allow “help” into their territory. Photo credit: KS Gopi Sundar

Behavioral changes

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 the chicks.

“Although it is poor quality territory, the Etawah trio have been able to raise at least one chick every two years,” Sundar said, adding that if an area has trios, it may indicate some level of habitat degradation. “And that was much more than breeding success in other territories.”

“The smaller the territory, the higher the quality,” he said. “So 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 said.

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 drainage of wetlands and conversion for agriculture,” Sundar said. “Our analysis shows that land use change is an important factor here. If conditions continue to deteriorate across landscapes or in individual territories, trios may well become much more common than they are now.

This article first appeared on Mongabay.

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