Abstract. Floral nectar is offered by plants to animals as a reward for pollination. While nectar is typically a clear liquid containing sugar and trace amounts of amino acids, colored nectar has evolved in several plant families. Here we explore the functional significance of the phenolic compounds that impart a dark brown color to the nectar of the South African succulent shrub Aloe vryheidensis. Flowers of this aloe are visited for their nectar by a suite of short-billed birds that are occasional nectarivores, including bulbuls, white-eyes, rock thrushes, and chats. Dark-capped Bulbuls were more likely to probe model flowers containing dark nectar than those containing clear nectar, suggesting a potential signaling function for dark nectar. However, the main effect of the phenolics appears to be to repel ''unwanted'' nectarivores that find their bitter taste unpalatable. Nectar-feeding honey bees and sunbirds are morphologically mismatched for pollinating A. vryheidensis flowers and strongly reject its nectar. However, the frugivorous and insectivorous birds that effectively pollinate this aloe are seemingly unaffected by the nectar's bitter taste. Thus the dark phenolic component of the nectar appears to function as a floral filter by attracting some animals visually and deterring others by its taste.

Previous studies that tracked the movements of single bird species within human‐modified landscapes have shown that the ability of forest birds to move across matrix habitat differs among species. Functional guild specificity as well as landscape characteristics have been shown to influence bird movements, entailing different movement behaviour of birds within a community. Studies investigating how both these factors influence the movements of entire bird assemblages across fragmented landscapes are essential but have rarely been conducted. In this study, we determined how species’ traits and different forest matrices influence bird movements among nine forest patches in a highly fragmented South African landscape. We combined 90 h of bird observations with capture–mark–recaptures (104 754 mist‐net hours) to distinguish between movements among patches (all birds that conduct long‐distance movements across the landscape) and movements within patches (all resident birds that conduct only short‐distance movements within a fragment). Overall, we detected a high bird movement activity across the fragmented landscape. Dietary specialization, habitat affinity and body mass strongly shaped the relative distribution of bird species across the nine fragments with frugivorous birds, forest specialists and large‐bodied species showing the highest movement abilities. In contrast, resident insectivores and forest generalists tended to move only within particular forest fragments. Our results suggest that remnant forest fragments may represent valuable stepping stones as well as permanent habitat for local bird assemblages. We emphasize that beside the conservation of natural forests, the maintenance of nearby, structurally rich forest fragments is pivotal in maintaining regional forest bird assemblages in human‐modified landscapes.

The morph ratio distribution in polymorphic species often varies clinally, with a gradual change in morph ratios across the distributional range of the species. In polymorphic bird populations, clinal variation is rarely quantified. We describe a cline in the morph ratios of Black Sparrowhawks across South Africa, which is principally driven by a higher ratio of dark morph birds in the newly colonized southwest of the country. Across the 1400 km of our cline, the probability of a bird being a dark morph declined from over 80% close to the Cape Peninsula to under 20% in the northeast. Higher frequencies of dark morphs were associated with a higher proportion of rainfall falling during the winter breeding months. Further investigation revealed relationships between the proportion of dark morphs and altitude, amount of rainfall during the breeding months, and an interaction between this variable and temperature. These results provide some support for the suggestion that the higher frequency of dark morphs in the southwest is an adaptive response, rather than the result of a founder effect or genetic drift. These findings also suggest that, in theory, polymorphic species may be better adapted to cope with the challenges of climate change or may be able to expand their ranges more quickly into novel climatic areas, since selection pressure can act on a pre-existing trait that may be beneficial in new conditions.

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