The Overberg region of the Western Cape is one of the best areas to go to spot our national bird, the blue crane (Anthropoides paradiseus). In fact, the sprawling agricultural fields of the region now support the largest population of blue cranes in the country. This means that the Overberg is home to the “population stronghold” of the species.
This was not always the case. Historically, it was the grasslands of Gauteng that hosted the most cranes, as the expansive open plains provided the ideal breeding habitat for these ground-nesting birds. By contrast, the Overberg region was historically characterized by dense renosterveld vegetation, and only supported a small blue crane population.
As these habitats were transformed by humans, the population dynamics of blue cranes shifted. Gauteng grasslands were transformed and degraded, and the number of blue cranes the region could support drastically declined. In the Overberg region, renosterveld was cleared for the cultivation of cereal crops and the creation of livestock grazing pastures.
The loss of renosterveld was devastating to many species in the region, with remaining individuals confined to small, fragmented patches of natural vegetation. Interestingly however, the Overberg population of blue cranes began to grow as the landscape was transformed.
Grazing pastures provide the cranes with their ideal nesting habitat: short, sparse vegetation cover with a wide field of view for spotting approaching threats. Following harvest, which coincidentally overlaps with the crane’s breeding season, cereal crop fields also provide ideal breeding habitats.
And so, we have a rare case of agriculture benefitting a protected species. This is demonstrated by the sheer number of blue cranes you can spot driving through the region. But despite the success of this population, the species is still listed as Vulnerable by the IUCN.
It isn’t hard to understand why. The population stronghold is now effectively reliant on human practices in the region, a tenuous position for any species to be in. Current agricultural practices seem to benefit the Overberg blue cranes, but how sustainable is this relationship in the long term?
This idea has been investigated before, from various angles. Of particular interest to us was the approach of a conservation biology master’s student at UCT: Mark Bidwell. In 2004, Bidwell investigated the relationship between nest site selection and hatching success across 63 nest sites. He found that pasture-nesters had the greatest hatching success, and that breeding cranes largely avoided non-cereal crop fields. This highlighted the importance of the mosaic of pastures and cereal crops that characterized the region at the time. It also indicated that any large shifts away from cereal crop cultivation could reduce the breeding success of the Overberg blue crane population.
Seventeen years later, we decided to follow up on Bidwell’s study. The idea was to investigate how the agricultural landscape and consequent nesting site choices of the cranes might have changed in the interim. Hatching success in relation to these variables could give us an idea of what the future holds for this population.
Of course, it is impossible to predict the future success of a population without considering the influence of climate change. The Overberg, like many areas, has been getting hotter and drier, and this trend is predicted to continue. Extreme high temperatures would be especially bad news for the local cranes. This is because they nest out in the open, which means no escape from the sun. What’s more, they like to nest on bare ground, which radiates a lot of heat as it is warmed by the sun. Should heat levels become unbearable, incubating cranes might abandon their eggs in search of shade or water. This would leave the eggs exposed, and effectively fry the developing embryos.
Taking all of this into consideration, we decided on two aims for our study. Firstly, we wanted to understand how the landscape influences where the cranes choose to nest and how this choice affects hatching success. Secondly, we wanted to investigate how incubating adults are behaving in the heat, and how any behavioural changes might affect hatching success.
To understand these links, the principal investigator of the team, Michelle Bouwer (Conservation Biology MSc student, UCT) set out in search of breeding cranes from September 2021 to December 2021. Once an incubating crane was spotted, the nest location was pinpointed, and the measurements began. We measured the weight, length, and width of each egg. We also set up camera traps at each nest site to allow us to monitor incubating behaviour of the adults.
Additionally, we placed temperature loggers (known as “ibuttons”) at some of the nest sites. One logger was placed in the nest, under the egg, and one was placed immediately next to the nest. This allowed us to estimate how much an incubating crane shelters the eggs from the heat.
Once the eggs had hatched, we could start the habitat measurements without disrupting incubation. We measured the land type and the slope of the land as well as the vegetation height and plant species diversity surrounding the nest. We also
Additionally, there was the issue of the weather. Climate change was definitely influencing the local climate, but not as we predicted. The Overberg was unseasonably cool and wet. This delayed the harvest until late November-early December and meant the cranes usually nesting in the harvested cereal fields had a delayed start to their reproductive season. Transportation and accommodation constraints and the need for Michelle to start her coursework in January meant we missed the tail end of the delayed breeding season. This means that our data are skewed to pasture nests and we lack complete information on the fate of nests in wheatfields. A second field season at the end of 2022 is therefore imperative for a comprehensive dataset.
Despite these limitations, preliminary results highlight some interesting and concerning trends. Firstly, only 17 nests were located during the 2021 breeding season. We were searching for nests on the same farms as Bidwell back in 2004, across the same time window. This is an approximate 73% reduction in the number of nests across the same area and stretch of time.
There could be various explanations for this trend. It is possible that crop type changes on the farms in question have led to fewer suitable nesting sites. Indeed, driving around the area you will see a lot more orchards than you would have in the past. It is also possible that Bidwell had more widespread access to the farms than we did, due to increased security concerns among the farmers. Indeed, we were mostly active along the periphery of the farms, with most nest spotting occurring from open-access, public roads. And of course, we might have discovered more nests in harvested wheatfields had harvest not been delayed by the unusual weather.
The other, more concerning possibility is that the breeding density of blue cranes in the region is declining. If this is the case, it is important that we continue to work to determine the cause of this decline.
Of the 17 nests we studied, all contained two eggs. Nine successfully hatched both eggs (~53% clutch success). Two hatched only one of the two eggs (“clutch reduction” ~12%). Finally, six nests failed to produce any hatchlings (~35% clutch failure).
Of the six clutch failures, three were due to egg predation by pied crows. While the other failures were also likely due to some sort of predator, our camera traps unfortunately missed the predation events. Pied crows have proliferated across South Africa, largely due to an interaction between climate change and human disturbances. The influence of a thriving pied crow population on blue crane reproductive success in the Overberg is therefore important to study further, and we will be monitoring this carefully in our next field season.
A massive thanks to the Tygerberg Bird Club for sponsoring project costs. We are working on further analysing our camera trap and ibutton data and look forward to sharing more results with you in the future!
-Michelle Bouwer
measured how far the nest was from a water source, a natural renosterveld patch and various human construction like roads, buildings, powerlines and fences.
There were some snags during the data collection period, as is so often the case with fieldwork. Unfortunately, our old, donated camera traps didn’t consistently trigger, so not all incubating behaviours were caught. Fortunately, we were able to source new camera traps towards the end of the field season, so we have a complete camera trap dataset for the last several nests.