Persistent plumage polymorphism occurs in around 3.5% of bird species, although its occurrence is not distributed equally across bird families or genera. Raptors show a disproportionately high frequency of polymorphism, and among raptors it is particularly frequent among the Accipiter hawks. However, no systematic study of polymorphism in this genus exists. Using a long‐term study of the black sparrowhawk (Accipiter melanoleucus), a widespread polymorphic African Accipiter, we first demonstrate that the species shows discrete polymorphism (cf. continuous polymorphism), occurring as either dark or light morph adults, and that morph type and plumage pattern are invariant with age. We then demonstrate that adult morph type follows a typical Mendelian inheritance pattern, suggesting a one‐locus, two‐allele system within which the allele coding for the light morph is dominant. This inheritance pattern provides further support for classifying polymorphism in this species as discrete. In most of the species' range the dark morph is the rarer morph; however, in our study population where the species is a recent colonist, over 75% of birds were dark and this remained fairly constant over the 10 years of our study. This reversal in morph ratio may represent an adaptive response to different environmental conditions or could be a founder effect with colonizing individuals having been mostly dark morph birds simply by chance. The extreme differences in environment conditions (seasonality of rainfall) that occur across the species' range in South Africa provide support for an adaptive explanation, but further work is needed to test this hypothesis.

In a rapidly changing world understanding the capacity of populations to adapt to novel environments is increasingly urgent. Timing of breeding can be a highly fl exible trait and adjustments in this trait can potentially buff er populations from climate change and facilitate the colonisation of new environments. Recent range-expansions into novel climatic regimes provide a valuable opportunity to investigate the implications of plasticity in timing of breeding for population processes. Black sparrowhawks have recently colonised the Cape Peninsula of South Africa where they experience dramatically diff erent weather patterns to those in their historical range. Th ese include a total reversal in the rainfall regime, with the majority of rain falling in the winter as opposed to the summer months. We investigate the breeding phenology of black sparrowhawks in relation to both regional and local climate variation and, using a long-term dataset, explore the implications of phenological shifts for reproductive success and population growth following colonisation. In the recently colonised Cape Peninsula the breeding season began up to three months earlier than within their historical range and these early breeding attempts produced more off spring. Population models suggested that this adjustment assisted the colonisation of the Cape Peninsula, reducing the probability of extinction by 23%. Contrary to expectations, we found little support for the hypothesis that black sparrowhawks were responding to local variation in rainfall. We suggest that shifts in breeding phenology may be driven in part by other novel processes, such as interspecifi c competition for nest sites and lower temperatures during late summer. Th ese results provide insight into the processes that facilitated the colonisation of a novel climatic regime highlighting the potential role of a diverse range of factors.

SummaryBlack Harrier Circus maurus is a rare southern African endemic that may have lost over 50% of its core breeding habitat in the last century as a result of extensive land transformation by agriculture, invasive alien vegetation and urbanization in the Fynbos biome. We partially surveyed both the western (Swartland) and southern (Overberg) coastal plains of south-western South Africa, over 3 years (2000)(2001)(2002) for breeding Black Harriers, and found a distinctly polarized distribution. Nests were concentrated either along the coastal strip or inland in montane habitats, and generally absent from heavily cultivated and transformed inland plains areas. Limited evidence (direct observations, prey remains) suggests that harriers forage in cereal croplands but generally do not breed in these modified environments. We recorded breeding success at nests in coastal (Dune Thicket) and montane (Mountain Fynbos) habitats. Harriers bred successfully along the coast and nests were aggregated in loose colonies around wetlands. Harriers in montane environments bred poorly, took a wide range of prey, and were subject to high levels of nest predation. We propose that Black Harriers have been displaced from lowland Renosterveld and Fynbos habitats (characterized by better foraging and nesting opportunities), primarily by the advent and spread of cereal agriculture. The conservation and future research implications of this hypothesis are discussed.

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