Population ecology of Woolly-necked Storks (Ciconia microscelis) in a South African mosaic landscape.

Abstract

As large, aquatic birds, storks (family Ciconiidae) and their population dynamics can provide useful information about how wildlife respond to changing land use. There are 20 extant stork species with varying conservation statuses, yet few species have received much research and conservation attention. Although storks can act as indicators for their usual wetland habitats, there is increasing evidence that some species are ecologically flexible and can persist in human-altered landscapes. This study aimed to use the African Woolly-necked Stork (Ciconia microscelis; hereafter, ‘woollyneck’) in South Africa as a case study of urban wildlife colonisation and behavioural plasticity. Previously a rare and secretive species in South Africa, the woollyneck has expanded its range southward in KwaZulu-Natal over the past few decades, especially colonising urban and mixed use landscapes. We investigated how two important components of population dynamics–reproduction and nata dispersal–could have facilitated rapid population expansion in KwaZulu-Natal, and how spatially resident woollynecks use the urban environment. Firstly, we studied the urban breeding ecology of woollynecks by monitoring 112 nesting attempts during two nesting seasons in KwaZulu-Natal. We described previously unknown aspects of the natural history of woollynecks, including clutch size, breeding phenology, breeding metrics, and nest site fidelity. Fecundity in the two study seasons was very different (1.4 nd 0.7 fledglings per pair), suggesting that in some years, woollynecks produce a high number of chicks that may compensate for years of poorer reproductive output. The probability of fledging at least one nestling was significantly related to rainfall and egg-laying date, with higher success probability earlier in a breeding season and when rainfall was higher. Secondly, we investigated the dispersal of immature woollynecks using high-resolution telemetry and low-resolution colour-ring mark-resight. Immature woollynecks had large areas of spatial use, and these areas mostly decreased as they got older. From telemetry, we found the farthest distance moved from a natal nest site was 220 km. Colour-ringed woollynecks were resighted an average of 26.5 km from their natal site, with some as far as 98 km. Finally, we used telemetry to understand how resident adult woollynecks moved in an urban landscape. Adults had very small home ranges (x̄ = 4.37 km2) and travelled very short distances on average (0.82 km) from the nest while breeding. Apparently, the abundance of nesting trees and food in the form of amphibians in residential gardens and supplemental anthropogenic food has allowed adult woollynecks to become highly sedentary following the colonisation of urban mosaic landscapes in KwaZulu-Natal. The results of this study provide novel insight into (1) how KwaZulu-Natal’s woollyneck population could have expanded so rapidly (i.e., high breeding success and natal dispersal), and (2) how the established urban population of woollynecks move in an urban mosaic landscape. Given the abundance of resources for woollynecks in such landscapes, it is possible the recruitment of a high number of young storks into the population and their far dispersal movements will facilitate the continued expansion of the population southward into Eastern Cape Province and inland in KwaZulu-Natal. In contrast to their historical description as shy and secretive, woollynecks in KwaZulu-Natal have demonstrated unexpected behavioural plasticity in their adjustment to an urban mosaic environment, which suggests other species may have similar capacity that has yet to be observed.