- Wintering harriers contribute to the seed dispersal of 137 plant species within India’s savannah-grasslands, a study found.
- In secondary seed dispersal, carnivores such as harriers, that prey on seed predators, unintentionally consume the seeds in the digestive tracts of their prey and later expel them intact and viable for germination.
- With grasslands increasingly becoming fragmented across the Indian subcontinent, this functional role by harriers potentially enhances the resilience and recovery of these fragile habitats, according to the paper’s authors.
Harriers are best known for their low, buoyant flight in a zig-zag, back-and-forth pattern while hunting for prey, and their spectacular ability to pass food to each other in mid-air. They are diurnal raptors, birds of prey active during the day, that have uniquely adapted to open natural ecosystems such as grasslands and wetlands. Part of the Accipitridae family, these birds feed on small birds, rodents, reptiles, and insects.
India’s fragmented savannah-grassland ecosystems serve as the wintering grounds for six of the 16 harrier species present globally. The subcontinent is home to the world’s largest roost of the near-threatened pallid harrier (Circus macrourus) and Montagu’s harrier (Circus pygargus).
Most ecological studies on harriers in Asia have been limited to mapping their migration routes, roosting sites, population trends, and food preferences. In a recent paper published in Oikos journal, scientists from the Ashoka Trust for Research in Ecology and the Environment (ATREE) have demonstrated that migratory harriers contribute to the regeneration of India’s savannah-grassland ecosystems through a secondary seed dispersal method known as diploendozoochory.
Diploendozoochory is an indirect seed dispersal mechanism where carnivores that prey on seed predators unintentionally consume the seeds in the digestive tracts of their prey and later expel them intact and viable for germination.
Between October 2016 and April 2023, the research team collected 3,216 pellets (regurgitated mass of undigested food) from prominent wintering sites of harriers and recorded viable seeds of 137 plant species from these pellets.
Seed dispersal is a fundamental process that shapes the composition and resilience of grasslands. So what does the discovery of this missing dispersal pathway mean for India’s fragmented grasslands?
Finding viable seeds in harrier pellets
As part of the study, ATREE researchers visited seven prominent wintering harrier sites across the western, Deccan and southern regions of India. Pallid, Montagu’s and occasionally western marsh harriers (Circus aeruginosus) were the three species found at these sites. The team collected 3,216 harrier pellets over a period of nearly seven years.
Without type specimens for comparison, the researchers spent considerable time discerning prey items and seeds from the pellets. The study recorded seeds in 670 (21%) of the pellet samples and counted a total of 9,603 seeds. A detailed analysis of the seed-containing pellets revealed viable seeds from 137 plant species across 23 families.
Although they had previously noticed seeds in harrier pellets, it was a paper by Roger Clarke and team in 1997 that inspired them to develop the study, says Arjun Kannan, co-author of the paper and a Ph.D. researcher at ATREE. “Clarke’s paper looked at the hen harrier diet in England through pellet analysis, and that is when we realised that harriers can secondarily disperse seeds,” he said.
“[Our study] is a pioneering work from Asia looking at secondary dispersal by a migratory raptor species in grassland ecosystems,” Kannan said. There have been previous studies, but they have focused on island ecosystems, such as the Canary Islands.
To confirm the viability of the seeds collected from the pellet samples, the researchers planted 6,253 seeds in a carefully controlled environment at their field station located in the plains of Tamil Nadu. The experiment showed that 674 (11%) of the seeds successfully germinated, indicating that seeds of many species can survive the digestive enzymes in harriers’ guts and suggesting a crucial role for harriers in seed dispersal.
The vast pastures of the Deccan
The study found that the seed diversity in harrier pellets from the Deccan region was the highest, followed by the south, and then the west of India. It is a function of the dietary preferences of the harriers, says Kannan. “Birds were the most abundant prey in the harrier diet across seasons in the Deccan and southern regions, while grasshoppers and insects were more common in western India,” he said.
The Deccan region is a stronghold of the pallid harrier, a specialist bird and rodent predator. “Pallid harriers, which prey on larks, pipits and other small birds, are the major secondary seed dispersers,” said Ganesh Thyagarajan, lead author of the paper and a senior fellow at ATREE. In contrast, Montagu’s harriers primarily consume grasshoppers (acridivorous predators), which limits their role in seed dispersal.
The arrival of migrant pallid harriers to India’s grasslands alongside the greater short-toed lark, a migrant seed predator, assists in the secondary dispersal of seeds by harriers, Thyagarajan said.
The Deccan region also recorded the highest crop seed diversity in the harrier pellets, with seven crop species, followed by five in the south, and four in the west.
Grassland management and restoration
With grasslands becoming increasingly fragmented across the Indian subcontinent, secondary seed dispersal by harriers becomes crucial, says Perumal Ravichandran, head of the department of plant science at Manonmaniam Sundaranar University, Tirunelveli. “Migratory harriers move across hundreds to thousands of kilometres, linking distant grassland patches and facilitating genetic exchange and recolonisation of degraded habitats,” said Ravichandran, who is not a part of the study.
While such seed dispersal mechanisms enhance landscape connectivity and climate resilience, it also introduces management risks. Alongside seeds of crop species, the study also identified three invasive plant species, including Lantana camara, in the harrier pellets. Dispersal of invasive species and agricultural spillover pose significant management challenges, according to Ravichandran.
However, the paper’s authors consider the dispersal of invasives to be negligible. “Out of the 137 species found in the pellets, only three were invasive species, and the number of pellets that contained these species was negligible (< 1%),” Kannan said.
Ashish Nerlekar, who leads the savannah ecology lab at IISER, Pune, says that it is too early to predict whether harrier-dispersed invasive seeds can pose a serious problem. “Especially considering that many of the invasive species are already lurking around the margins of old-growth grasslands,” Nerlekar said. He is more curious to know the persistence and survival ratio of seedlings dispersed by harriers in the wild.
Ultimately, the discovery of secondary seed dispersal by migratory harriers reveals the complex pathways involved in grassland sustenance in the subcontinent, says Ravichandran. He thinks that grassland policies must integrate predator conservation with invasive species control and evidence-based restoration planning.
“Effective grassland policy must move beyond species-centric protection to process-based, adaptive management, balancing predator conservation with proactive habitat stewardship,” he said.
Read more: The long way might just be the right way for Montagu’s harriers
Banner image: A western marsh harrier near Nal Sarovar Lake in Gujarat. Image by Hari K Patibanda via Flickr (CC BY-NC 2.0).
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