Happy snappers unwittingly gather valuable Martial Eagles data

20 February 2019 | Story Supplied. Photos Riaan Marais. Read time 5 min.
Adult female Martial Eagle (Polemaetus bellicosus) with rock monitor kill. Photographed in Kenya.
Adult female Martial Eagle (Polemaetus bellicosus) with rock monitor kill. Photographed in Kenya.

University of Cape Town (UCT) researchers have used Google Images to track the dietary habits of the Martial Eagle, Africa’s largest eagle, gaining new data from regions where the species has never previously been studied. This input from citizen scientists could shape many aspects of future research.

A recent paper in the international journal The Condor detailed how the scientists used photos sourced from the web to reveal the bird’s main prey types, information that may help conservationists protect this threatened species.

“Very little research has been done on the diet of Martial Eagles, leaving huge gaps in our understanding of how prey abundance is impacting their populations and what we can do about it,” said lead author Vincent Naude, a doctoral student at the Institute for Communities and Wildlife in Africa, in the Department of Biological Sciences.

“Meanwhile, hundreds of online users who post their photos have inadvertently been collecting intense field data over many years that could help answer these conservation questions.”

The Martial Eagle is Africa’s largest eagle, weighing up to 6.5 kg. In 2013 it was up-listed to “vulnerable” on the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species because of population declines throughout its range.

UCT app used

The study used the free app MORPHIC, which was also developed at UCT, to search for photographs of Martial Eagles with prey that have been uploaded to websites, social media and photography platforms.

The researchers trawled through 4 872 photos, 254 of which were used to extract data on eagle location, age, feeding position and type of prey.

Happy snappers unwittingly gather valuable Martial Eagles data
Adult male Martial Eagle (Polemaetus bellicosus) with a white-tailed mongoose kill. Photographed in Kenya.

The photos were taken from across southern and eastern Africa, including South Africa, Kenya, Namibia, Botswana and Tanzania. The pictures revealed that reptiles, birds and mammals each make up one-third of the Martial Eagle’s diet. They also showed that the proportions of prey types varied between the different regions, with mammals dominating in eastern Africa and reptiles in southern Africa.


“We’ve been able to paint a picture of what Martial Eagles feed on over a huge geographic scale without many of the biases of previous methods.”

“We’ve been able to paint a picture of what Martial Eagles feed on over a huge geographic scale without many of the biases of previous methods,” said Naude.

“And we’re doing it at a fraction of the time and expense that would be required using traditional methods.”

This is the first research exploring Martial Eagle diet beyond the borders of South Africa, where earlier studies were based. It is also the first time that information on their diet has been obtained without looking at prey remains, a practice that requires scientists to find nests in the field and physically examine and identify prey remains, such as bones or feathers.

“Prey remains studies are costly and limited by the low density of these huge eagles. They also limit our understanding of diet to during the breeding season when males are the primary hunters,” Naude explained.

“These previous approaches also exclude information about non-breeding eagles which could be important for understanding why this eagle is in decline.”

Shortage of prey

A reduction in available prey is one explanation for the decline.

Associate Professor Arjun Amar, an avian conservation biologist at UCT’s Percy FitzPatrick Institute of African Ornithology, who supervised the research, said the information gathered in this study can help researchers investigate whether prey shortages might be contributing to the decline. This will help guide conservation efforts in the different regions.

“For example, we now know that reptiles are the eagle’s most important prey in eastern South Africa. Conservationists can use that information to investigate what factors affect the abundance of reptiles in that region and what can be done to protect them,” he said.

The scientists say Google Images could also help inform conservation efforts for other species under threat, from raptors to big cats, if they are photographed widely over their range.

Naude stressed that, with very little financial outlay, “scientists can tap into this online resource and contribute to the global conservation of a species”.

“It’s a powerful take on citizen science that could shape many aspects of future research.”

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