South Africa’s national bird, the blue crane, is struggling to raise chicks in parts of the Western Cape and scientists warn the region might be becoming an “ecological trap” for the species.
A new study published in the ornithology journal Ostrich, comparing breeding success, found that blue cranes in the Overberg and Swartland wheatlands were producing far fewer fledglings than birds in the Karoo and eastern grasslands.
It was led by researchers from the FitzPatrick Institute of African Ornithology at the University of Cape Town and the African Crane Conservation Programme, a partnership between the Endangered Wildlife Trust and the International Crane Foundation.
The blue crane is listed as vulnerable on the International Union for Conservation of Nature Red List because its population has declined by an estimated 19% from 2011 to 2019. The researchers noted that the reasons for the recent decline were unclear.
“Historically powerline collisions, poisoning and habitat transformation led to declines in the Karoo and eastern grasslands,” the study said. “Now the decline is largely occurring in the Western Cape’s Overberg wheatlands, where blue cranes occur in intensively farmed cereal croplands and planted pastures.”
Nonetheless, the Overberg wheatlands were “a stronghold for the species, with the highest densities of blue crane occurring in this artificial habitat, as opposed to natural habitats in the Karoo and grasslands. There are concerns that the Western Cape may be
an ecological trap for the species”.
Breeding productivity, which is measured by how many chicks survive per nesting attempt, was almost twice as high in the Karoo and grasslands as in the Western Cape’s intensive agricultural areas.
In the grasslands, cranes produced an average of 0.95 fledglings per attempt and in the Karoo about one chick per attempt. In contrast, productivity in the Overberg was just 0.55 fledglings per attempt and even lower in the Swartland, at 0.48.
The proportion of breeding pairs that managed to raise at least one chick also sharply differed. In the grasslands and Karoo, about two-thirds of pairs successfully fledged chicks each year. However, in Overberg and Swartland, the figure dropped to about 40%.
The study also found worrying signs in winter flocks. Between 2019 and 2021, juveniles made up only 4% of cranes in winter flocks in the Overberg and 3.6% in the Swartland. That is roughly half the proportion recorded in the Overberg 30 years ago.
Historically, blue cranes were drawn to agricultural habitats. However, modern farming practices might be undermining breeding success.
The study identified known causes of nest failure in the Overberg including predation, infertile eggs, nest destruction by machinery and disturbance. Wheat harvesting in October and November could unintentionally crush nests if birds bred before harvest or between cutting and collection.
Predation, particularly by pied crows, accounted for about half of clutch failures in some areas. Higher temperatures and longer periods when adult cranes were off the nest also increased the risk of eggs being lost.
Less is known about why chicks die after hatching but the authors said anecdotal reports from farmers pointed to drownings in farm water troughs, entanglement in fences, leg injuries or starvation.
The Swartland presented additional challenges. Summers were generally drier than in the Overberg and many farm dams dried out in summer. Blue crane chicks needed accessible water within walking distance of nests and farm dams drying out, particularly in drought years, might be affecting survival.
Research indicated that Swartland farmers experience more crop damage from cranes than Overberg farmers, resulting in negative attitudes towards cranes. That might hinder pro-conservation behaviour towards cranes, contributing to the poorer breeding success in the region, the authors said.
They noted that while some historical data showed fluctuations in breeding success over time, the “consistently low productivity in the Western Cape wheatlands compared to other regions is concerning”.
“Our findings indicate lower breeding success and recruitment for blue cranes in the Western Cape wheatlands than in their traditional range, highlighting a potential ecological trap. This is concerning for this stronghold population and might explain why numbers in the Overberg have been declining for the past decade.”
Practical interventions could help. “It would be beneficial to implement interventions that improve breeding success, such as engaging with landowners to reduce nest disturbance, promoting chick-safe water troughs (stacking rocks to allow chicks to climb out) and reducing fence entanglements with improved fence designs.”
The authors said their findings highlighted the challenges and complexities with conserving remaining biodiversity in transformed or novel landscapes, noting that the species often required sustained and intensive management to maintain their populations.
A new study has warned of a sharp drop in blue crane chicks in Overberg and Swartland
