Where eagles defy the odds

Researchers studying breeding patterns of Verreaux’s eagles in the province have found the top predators had more chicks in agricultural areas than in conservation areas.

When scientists from the University of Cape Town began monitoring the large eagles at two sites, one in the potato-growing region of Sandveld and anotherin a conservation area in the Cederberg, they expected to find the Sandveld population would be struggling to breed.

Their findings, published in the February 10 edition of the journal The Condor: Ornithological Applications, showed instead the Sandveld population was thriving, while the Cederberg population was under strain.

Lead author Megan Murgatroyd, a postgraduate student from the Animal Demography Unit and the Percy Fitz Patrick Institute for African Ornithology at UCT, said agriculture often had negative impacts on biodiversity.

“Raptors are often thought to be sensitive to environmental change,” she said. “Therefore, we expected that eagles in the Sandveld would be negatively affected by habitat loss via potato farming.”

To gather data, scientists monitored occupied eagle nests over four breeding seasons between 2011 and 2014.

Of the 112 records, 64 were in an area of the Cederberg Conservancy and 48 in the largely agricultural region of the Sandveld.

Pairs of Verreaux’s eagles, previously known as the black eagle, can produce up to two eggs per breeding attempt, said Murgatroyd.

But following siblicide – where the stronger chick murders its weaker brother or sister – one chick only survives.

As the researchers couldn’t closely approach the eagle’s nests on cliffs, they conducted their observations with binoculars and a spotting scope.

They checked on nests every two to three weeks through the breeding season, lasting from late May to early November.

When the birds appeared to be sitting, rather than standing, researchers could assume they were on an egg or two.

When an adult bird was seen feeding a nestling, researchers could tell a chick had hatched.

Lastly they confirmed a hatchling survived 90 to 100 days to become a fledging, which meant the breeding was successful.

When the data was crunched, the research team found that “all measures of breeding performance” were greater in the Sandveld than in the Cederberg.

In fact “overall breeding productivity” – which means the number of young fledged each year as a proportion of the monitored population – was almost three times higher in the Sandveld.

While in the Cederberg only 28 percent of pairs produced a healthy fledgling, in the Sandveld, 76 percent did.

Equally surprising for researchers was that the Sandveld was found to be the “most productive area for Verreaux’s eagles currently known”.

As the study’s authors state, this was the “direct opposite” of their initial assumptions that top predators – such as the eagles, would thrive in natural and undisturbed ecosystems rather than on farms.

According to Murgatroyd, the finding could be explained by more prey being available in the Sandveld, or the “low-energy consuming lifestyles” of the birds there.

However, she said researchers could not conclude that simply because the Sandveld eagle population was thriving, all birds of prey on farms across the country were in good shape.