The rise of the Woolly-necked Stork in urban KZN

In the 1980s, it was estimated there were only about 30 breeding pairs of Woolly-necked Stork (Ciconia microscelis) in all of South Africa. At the time, they could be found in low densities in Kruger National Park and northern Zululand, especially in coastal wetlands. Fast forward to 2024 and the Woolly-necked Stork (hereafter, Woollyneck) is literally a household species in KwaZulu-Natal (KZN), one that many urban residents have found strolling through open doors into homes in search of food; nesting over neighbourhood streets; and hunting toads along verges. This rapid rise to urban prominence is unprecedented for large waterbirds around the world, especially for a species that had always been described as shy and secretive, avoiding areas of human activity.

Large groups of Woolly-necked Storks are not uncommon in parts of urban KZN in recent years.

It is not easy to answer questions in hindsight, such as how did the Woollyneck come to take over urban KZN? But, as is sometimes said in some biological disciplines, the present is the key to the past! This is why, for my PhD project at the University of KwaZulu-Natal in Pietermaritzburg, we set out to understand the population ecology of Woollynecks following their colonisation of urban KZN. Specifically, we sought out to learn how a population of large waterbird could expand so rapidly, both demographically and geographically. To answer our questions about this, we investigated two aspects of Woollyneck ecology: breeding and movement.

Breeding

Historically, Woollynecks were found to nest in secluded swamp forests or riparian woodlands in undisturbed habitats. However, today in KZN one can find nests in alien trees along roads, in gardens, and even occasionally on electrical pylons. It has been speculated that widespread supplemental feeding by KZN residents allowed the Woollyneck population to grow. While this super abundance of anthropogenic food certainly has helped them settle in urban areas, I am less inclined to think this was something that facilitated colonisation.

Left: Kloof in 1957 (photographer unknown). Right: Kloof in 2023 (GoogleEarth 3D imagery). Red arrows indicate the same reference locations to demonstrate how the landscape has changed.

Those who have resided in the Upper Highways Area, for example, for many decades will know that the lush, treed landscape that now characterises the area was not always so. Historically, the area, like much of inland KZN, was rolling hills of grassland interspersed with forests in low areas like gorges. In the past, if a wandering Woollyneck flew over this part of KZN, it would not find much in the way of habitat. Yet as urban areas expanded and residential neighbourhoods were developed, people planted trees, especially of the alien variety, such as pines, camphors, and gum. In the past few decades, this has resulted in an abundance of large trees throughout urban areas that would not have naturally occurred. Consequently, large birds are spoiled for choice when it comes to nesting trees, and I think this is a major factor that has allowed the establishment and growth of the urban Woollyneck population. But a wide availability of nesting sites alone does not facilitate population growth.

Population growth requires recruitment of young into a breeding population, which is why we focused on studying nest survival and breeding success of Woollynecks in urban areas. Nest survival is defined as the proportion of nests that successfully raise young that leave the nest (when young leave the nest this is called fledging). Breeding success is how many young fledge from a nest. These are essentially metrics with which we can gauge productivity of a population. In collecting these data, we can then assess how environmental conditions in different years influence breeding and what factors most affect the contribution of these population metrics in overall growth.

Despite the stigma surrounding drones, this is generally the reaction of Woolly-necked Storks to monitoring nests with a drone–that is, if they even bother to stand up at all!

For the past two Woollyneck breeding seasons (2022, 2023), I have monitored 110 nesting attempts (essentially, egg-laying events) at 74 unique nests. This effort has only been possible with the extensive contribution of members of the public, who have kindly reported nest sites to me and, in some cases, even helped with monitoring to lighten my workload. Every one to two weeks, I recorded the status of nests to determine the ultimate fate of each nest. During incubation, I used a small drone to check nest contents and count the number of eggs. Shortly after the forecasted date of hatching, I did the same to determine how many eggs successfully hatched. Subsequently, I monitored nests until chicks either died or successfully fledge.

Although I have one remaining monitoring season for my PhD, the results from the first two seasons are an interesting contrast. Breeding success was fairly high in 2022, with an average of 1.4 young stork fledged per nest. In 2023, however, breeding success was half that at 0.7 fledged young per nest. In 2022, nest survival was 58%, and in 2023 this decreased to 38%. The reason for lower breeding success and nest survival in 2023 is clear: frequent, intense, and relatively cold storms early in the season caused young nestlings to die of exposure or entire nests to be blown out of trees. With re-nesting attempts (a second clutch of eggs laid after an initial failure) relatively low at 12.7%, it is unsurprising that pairs gave up after these initial nesting failures.

Adult Woolly-necked Stork with three nestlings.

At the moment we only have two very different breeding seasons to compare, so it is not yet possible to assess what is normal for Woollynecks. However, we can say that high breeding success is possible for urban Woollynecks, which provides some insight into how the population could have grown so rapidly. This doesn’t mean every year will have large numbers of young entering the population, but with the majority of nests fledging two to three young in good seasons, that is a lot of Woollynecks going out into the world to eventually breed on their own with a few years. We think this brings us somewhat closer to understanding how, demographically, Woollynecks expanded so rapidly in KZN.

Movement

Woollynecks have never been tracked before, so no one knew if there was any seasonal movement or the extent to which they are sedentary. To answer such questions required capturing woollynecks to attach GPS tracking units, which was not an easy task and had not been done before. After much trial and error, I designed a foot-noose trap that led to success in capturing 11 storks, on which I fitted solar-powered trackers. Programmed to record locations every 30 minutes, these units have allowed me to collect tens of thousands of GPS locations for each individual in a year’s time. Additionally, to answer questions about juvenile movement without the great expense of tracking units, I have been colour-ringing woollyneck chicks in nests. These rings have unique alpha-numeric codes that act as identification numbers. As these juveniles move about, inevitably visiting gardens and public places, members of the public (and myself) can spot them and identify individuals. When ringed storks are reported to me with a location, I can then get an idea of movement between sightings–whether that be how far they are from their natal nest or from the last sighting of an individual.

Attaching a solar-powered GPS tracking unit to a Woolly-necked Stork in a backpack-type style.

A colour-ringed Woolly-necked Stork nestling at about 45 days old.

We have found that adults are highly sedentary and do not move seasonally. After all, with stable habitat in the form of gardens and public green spaces and year-round, stable food in the form of human supplemental feeding and abundances of skinks and toads in gardens, why would they need to move far! This isn’t necessarily surprising, and our findings about movement during nesting further support this thought. While breeding woollynecks spent more time away from nests during incubation (averaging 3 hours) than during chick rearing (averaging 2.3 hours), they only moved an average of 900 meters from the nest to forage during both stages of the breeding season. If such small movements can result in high breeding success (that is, in 2022), then it goes to show that the urban environment can indeed provide good quality habitat for woollynecks.

Woolly-necked Stork with a GPS tracking unit. Note the black colour ring on its leg, too.

I am still collecting movement data on juveniles so data analyses are pending. However, I can say that it is already very clear that young woollynecks move much more than adults. Adults have an average home range size of around 1 km², with some individuals even smaller than that. Juveniles, on the other hand, are regularly moving across hundreds of square kilometres, some even covering a hundred kilometres straight-path distance in a day. I have also had reports of colour-ringed juveniles as far as 100 kilometres from the nest they came from. This is also unsurprising because most young animals go through this type of dispersal period where they leave their natal area to explore the landscape before settling down and establishing their own home ranges or territories. The large movements of juveniles provide somewhat of an explanation as to how, geographically, the woollyneck population in KZN could have grown so rapidly. That is, with a lot of young storks going out into the world and moving far from their natal areas into unoccupied areas, some are bound to colonise new areas and contribute to a geographic population expansion.

Moving forward

Beyond the topics covered here, there are still other aspects of urban woollyneck ecology that deserve investigation in the future. Eventually, resightings of colour-ringed storks will allow us to estimate survival of young woollynecks. Woollynecks are relatively long-lived, so collecting data for this kind of analysis will take years. But that is the great thing about the colour rings and public participation in citizen science! Further, I will be looking into genetics of woollynecks in KZN and across the rest of their range in Africa to determine how different populations are related to one another. Within KZN, this approach can answer questions about movement and genetic exchange between urban storks in the more southern parts of KZN and those further north that still largely rely on natural wetlands.

Please keep an eye out for colour-ringed storks and report them to contribute to this study.

About the author

Jonah received his BSc in Wildlife Biology at Unity College and his MSc in Wildlife Ecology at Texas State University, both in the USA. He has studied African storks since 2018 and moved to South Africa for his PhD in 2022. Besides Woollynecks, his research more broadly focuses on waterbird population ecology and movement in Africa. He currently has field research projects in Botswana, South Africa, and Zambia on storks, spoonbills, egrets, and herons.