A pair of sarus cranes (Antigone antigone) was observed preying on a grey-headed swamphen (Porphyrio poliocephalus) chick at the Dhanauri Wetlands, in west Uttar Pradesh, in northern India, beating the chick, and feeding it to their own young.
Researchers feel this extremely rare behaviour is probably a result of altered conditions in the crane habitat caused by climate change.
While literature on the crane diet does not exclude the possibility of waterbird chicks, sarus cranes mostly feed on insects, aquatic plants, frogs, and crustaceans. Among the world’s tallest flying birds, males can grow up to a height of 180cm.
Multiple studies have shown changes in behaviour among sarus cranes on the Indian subcontinent, linked by researchers to changed land use and a warming climate.
The swamphen chick was probably killed to meet the crane chicks’ huge nutritional needs. Sarus crane chicks need a diet high in calcium and protein that may not have been available in the territory, says KS Gopi Sundar, chair of the IUCN Stork, Ibis and Spoonbill Specialist Group, in a paper slated to be published in Current Science.
Sundar says this feeding behaviour is likely “culture behaviour” — such as that chimpanzees exhibit where a small group or an individual demonstrates a new behaviour, which is then imitated or adopted by immediate contacts.
Wetlands and marshes, sarus cranes’ preferred habitats, are increasingly being lost to human impact. Sarus cranes also inhabit mixed agricultural mosaics with small marshes, canals and crops. Pairs in rice fields often synchronize their nesting and breeding with farm activities like the flooding of rice fields.
Kandarp Kathju, a naturalist who studies the species around Thol bird sanctuary in Gujarat, says changing rainfall patterns is one of the most important reasons behind the tweaks in the birds’ nesting and feeding behaviour.
Kathju has observed unseasonal nesting, renesting and increased clutch sizes — three eggs instead of the usual two. One study says “altering cropping patterns associated with increased artificial irrigation and changing rainfall patterns appear responsible for unseasonal nesting in sarus”. The hypothesis is that nesting beyond the monsoon increases in response to the changes in cropping patterns and altered rainfall conditions.
Renesting has been observed mostly in cases where nests were lost to flooding. “Cranes have been documented to re-nest only if the eggs or nest are lost, but there have been no records of successful renesting once the eggs have hatched,” the study says.
Other anomalous behaviour is a crane pair allowing a third crane in their territory to help in raising the chicks. Sarus cranes are known to maintain pair bonding for life. The inclusion of a third crane is unheard of.
Since the behavioural changes were observed in low-quality territories, it is evident that cranes are being forced to adapt. “In the cases we observed, the pair bonds didn’t break up; the third one (mostly female) stayed away during breeding and came into the scene only to ensure brood success,” says Sundar.
He has also observed sexual role reversal, where an intruder female kicks out the pair female to mate with the male. The third females were trying to get hold of better territories occupied by crane pairs, he says.
“In such cases, breeding success is observed to be late with the newly formed pair not able to produce chicks for a few years,” Sundar says.