Spur-winged Goose

The global population assessed in 2016 is considered to be increasing (BirdLife International 2016).

The different data sources and analysis demonstrate varying outcomes which is concerning and making it difficult to predict trends at a regional level. Currently this is not a concern as none of the negative trends exceed any Red List category thresholds. The analysis undertaken by Nagy et al. (2020) and the subsequent trend (stable population) would probably be the best prediction based on the years of experience that has been established within Wetlands International on waterbird trend analysis.

Three datasets and various analytical pathways were used to determine population trends (BIRDIE 2024, Lee 2024).

Reporting rates from SABAP2 were used as a proxy for population size, with the reasoning that higher numbers would result in higher reporting rates. Modelling the probability of reporting a species, accounting for season, BirdLasser effects, if all habitats were surveyed, sampling hours and with pentad or observer as random effect, showed with high confidence a significant increase in probability of reporting rates (Lee 2024).

Presence absence data collected per pentad through SABAP2 (grid approximately 9 km x 7 km) for the entire region were divided into two periods (2007-2014 and 2015-2023) and static models were used to predict the number of pentads species could occur in within each period. The models predicted a decrease in the AOO between the two periods of 1.4% (Lee 2024). Using the same data and available habitat per year, the number of pentads in which the species could occur were predicted annually between 2008 and 2022. Data emanating from these dynamic models estimated that the AOO declined 9.6% (BIRDIE 2024).

Using SABAP2 pentad level models, the static models (Lee 2024) indicated a slight decrease in AOO, while the dynamic models (BIRDIE 2024), predicted a decline of 9.6%. State-spaced models were applied to population counts carried out at various wetlands across the region to estimate the true number of individuals at each wetland for which adequate counts existed. Analysis of the data predicted a population decline of 12% over a 26-year period for these wetlands. More importantly, the decline was driven by the winter count estimates, as the summer counts varied considerably resulting in unpredictable trends. This is due to the species concentrating on large wetlands during moult and then dispersing to breed nearby smaller bodies of water.

Analyses of CWAC counts indicated significant declines across various modelled pathways, the magnitudes of which depended on whether historic very large counts were included (Lee 2024). This species was recorded at 539 of 719 CWAC sites. Mean winter count was 37.7, median was 0, and max was 3462. Mean summer count was 12.8, median was 0 and max was 2080. The maximum of 3462 was recorded at Krugersdrift Dam (Free State Province) on 5 August 2003. Over the 35 counts at that dam, mean count was 564, with standard deviation 703. Estimated percent change over three generations, from Year = 1997.75 to 2023 was -32% (-48%, -9.7%). However, some sites with large counts were no longer surveyed as of 2015, so there is low confidence in this trend.

CAR surveys are done twice a year (January and July) along set routes according to a set methodology, targeting amongst others specific large terrestrial birds. Data collected from 1993-2020 indicated a definite increase (Figure 1) in the average number of birds per/km from c. 0.28 birds/km in 1993 to c. 0.44 birds/km in 2020 (Lee 2024).

Data from the state space models predicted a decline in population numbers but only for 147 wetlands for which there was sufficient data. Considering the widespread distribution of the species within the region (Figure 2) it could be argued that these wetlands are not a representative sample size especially considering that only about 15 of these sites held more than 100 birds at any given time. The decline was, however, determined using the winter counts as this species congregates on large impoundments during this period to moult. Summer counts were extremely variable and nearly four times lower year for year than the winter counts. Also, there were very few sites contained in the entire data set showing high (>100 individuals) numbers of birds that were excluded in the state-spaced modelling exercise. Colahan (1997) does, however, state that numbers at moult sites in the Free State Province may vary greatly from year to year and in addition utilise new impoundments for moult sites. It is probable that between year fluctuations, plus the ability to utilise new impoundments as moult sites, would impact winter counts and subsequently trends.

Wetlands International (2024) indicate with reasonable confidence that the Southern African population is likely stable, based on longer-term survey data analysed by Nagy and Langendoen (2020), where they also determined that the trend over three generations to be stable. It must be noted that Nagy and Langendoen (2020) used data from the same data set that was used in the state-space models above. They were, however, less strict with the site selection criteria, and their data set included sites from Botswana, Eswatini and Zimbabwe as well, but the data contributing to the totals for this species is almost exclusively from South Africa.

The increase shown by the data emanating from the road counts (Figure 1) is to be expected as the routes cover large areas of agricultural land. The species is known to have relatively recently adapted to the croplands and associated artificial wetlands (Underhill et al. 1999, Harebottle and Harrison 2003, Colahan 2005). Increases in the Western Cape, (Overberg and Swartland), KwaZulu-Natal and Eastern Karoo areas (Figure 1) where significant numbers of birds are found on croplands (Harebottle and Harrison 2003) is driving the increase in the trend. It is difficult to determine with the available data whether this is an increase in population size or displacement from natural wetlands to more favourable cropland habitat.