Arrival and Peak Abundance of Barn Swallows Hirundo rustica in Three Regions of South Africa in Relation to Climate Indices, Deduced from Bird Atlas Data

For three regions of South Africa (Greater Gauteng, Greater Cape Town and Greater Durban) with the greatest coverage by bird atlas (SABAP2) fieldwork, we related arrival timing and abundance in each region of a long-distance migrant bird, the Barn Swallow, from July 2007 to March 2024. Using monotonic regression, from the atlas data we derived (1) the “annual anomaly of arrivals” from the average multi-year pattern; (2) the “average maximum” reporting rate at the completion of arrivals, in each region. We related these measures of the Barn Swallow timing and abundance in each of the 17 seasons of arrivals (July–January) in each of three region, with the large-scale climate indices, and temperature and precipitation in the Iberian+Apennine Peninsulas, for the Balkan Peninsula, and for the Sahel, averaged for the months when Barn Swallow visits areas between where these climate indices operate, at their breeding grounds in Eurasia, and along four southwards migration routes to South Africa. We used multiple regression modes with no more than two climate indices at a time, out of 84 explanatory variables, allowed by 17 data points (seasons) for each region. Our exploratory analysis indicated that the timing and abundance of Barn Swallow arrivals were related to a selection of these climate indices. The related climate indices varied between three regions in a pattern corresponding with the proportions of Barn Swallows arriving there from different breeding areas along different flyways, derived from an earlier study based on ringing recoveries. The paper shows the potential of the SABAP2 database as an annual monitoring approach, primarily due to the strong fieldwork protocol. We recommend that the project be continued indefinitely.