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As human populations have increased globally, so too has the transformation of natural landscapes into more urban areas. Within Africa, population growth rates and urbanization rates are amongst the highest in the world, but the impacts of these processes on Africa's wildlife are largely un-explored. In this study, we focus on a recently established population of black sparrowhawks Accipiter melanoleucus in Cape Town, South Africa. Using long-term data (16 years), we investigate the relationship between the timing of breeding and breeding performance along an urban-rural habitat gradient. We found no evidence for a direct effect of urbanization on either timing of breeding or breeding performance. However, we did find some evidence for a differential seasonal decline in breeding performance dependent on the levels of urbanization. Thus, higher productivity was found in more urbanized habitats earlier in the season, but towards the end of the breeding season, birds in less urbanized habitats performed better. Our study represents one of the first to examine avian productivity in relation to urbanization in Africa, and also represents one of the first studies to suggest that seasonal declines in productivity may differ according to levels of urbanization. These results have considerable implications for potential changes in phenology or productivity for the regions' avifauna as African urbanization continues.

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Table S1: Summary of available biometric data for 674 bird species in the SAFRING database, extracted between July-November 2018. ADU number = number assigned by the Animal Demography Unit at the University of Cape Town, South Africa; N = total number of ringing and retrap records in the database per species; n = number remaining after data cleaning and validation, lq = lower quartile, uq = upper quartile. Biometrics included were: mass (g), wing length (mm), tarsus length (mm), culmen length (mm), tail length (mm) and head length (mm). Sex-specific size differences were noted as 'Yes' or 'Unknown' and are further summarised in Table S3.