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SEXUAL SELECTION AND PLUMAGE ORNAMENTATION IN WIDOWBIRDS
One group of birds ideally suited for illustrating sexual selection, sexual dimorphism and the evolution of elaborate plumage ornaments are the highly polygynous African widowbirds ( Euplectes spp). The ornamental tails of widowbirds have provided some of the most fascinating examples of sexual signalling, including the classic sexual selection experiment by Malte Andersson on long-tailed widowbirds. Along with elongated tail plumes, most widowbird species also produce conspicuous yellow to red colour patches on their wings or body. This colouration is based on carotenoid pigmentation, which because of its dependence on diet and health has become a prime example of honest advertisements. |
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Natural history
The widowbirds (formerly Coliuspasser) together with the bishops form the genus Euplectes, a group of 17 small (13-45 g), Afrotropical weaverbirds (subfamily Ploceinae). The genus seems to have radiated at least as recently as within the Pleistocene, and all species are closely related as judged from hybridisation in captivity and similarities in general ecology and behaviour. Habitat preferences range from reed beds and moist, open grassland through to sparsely bushed savanna. Probably all are polygnous although data is insufficient for many species. Except in the lek-breeding Jackson's widowbirds (E. jacksoni), males defend nesting territories and participate to some extent in nest building, but not (or very rarely) in incubation or feeding.
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Most of my PhD research (supervised by Staffan Andersson at Göteborg University and Mike Lawes at the University of KwaZulu-Natal) focused on investigating the adaptive significance and evolutionarily coexistence of elaborate male plumage ornamentation and colouration in the bishop and widowbirds. To address these ideas I worked predominantly on three sympatric, but diverse, species of southern Africa: the red-collared ( E. ardens) and red-shouldered ( E. axillaris) widowbirds, and the red bishop ( E. orix). All species are strikingly seasonally and sexually dimorphic in nuptial plumage. Breeding males have a black body plumage with various amounts and combinations of white, yellow and red pigmentation. The male bishops are the most brilliantly coloured (either red or yellow on a black body) and have a short tail, while male widowbirds replace the tail feathers during pre- and post-breeding moult, and grow graduated tails of various lengths (65 mm to 0.5 m). Females are in most cases noticeably smaller and streaky brown or buff with short tails.
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Why do males display long tails?
The exaggerated tail plumes of birds have become a classic example of extravagant sexually selected male ornaments, and the extreme tail elongation among the widowbirds is no exception. The striking tails of breeding males vary from almost no elongation in the red-shouldered widowbird (7 cm) to the half a metre plumes of the male long-tailed widowbird (E. progne), one of the most extreme avian ornaments. Red-collared widowbirds are highly polygynous, with most males not reproducing but some males attracting as many as eighteen females to their territories.
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Although males possess a number of potentially sexually selected traits, such as carotenoid-based colouration, elaborate courtship behaviours and nesting territories, work with the red-collared widowbirds showed that females strongly targeted only tail length (18-32 cm long) during mate choice. Together with previous studies and the strong female preference for long and 'supernormal' tails in the short-tailed red-shouldered widowbird, directional (and open-ended) female choice appears to be the main selective force for tail elongation, potentially even pre-dating the evolution of long tails in the group.
Why are males so colourful?
Carotenoid-based colouration is a major component of conspicuous sexual dimorphism in birds. Owing to the nutritional and health constraints on the expression of carotenoid pigmentation, such colouration is commonly proposed as a sexually selected advertisement of quality, primarily in the context of female mate choice. However, contrary to recent avian studies, the sexually dimorphic and variable carotenoid colour patches of widowbirds are ignored in mate choice, and instead function as status signals in male contests. |
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| Among these birds, territorial competition is intense, with as little as 30% of the males in nuptial plumage establishing territories. As a result, a large population of non-territorial floater males exists that commonly intrude onto occupied territories and rapidly fill vacant territories. Only males with larger and redder plumage patches are able to acquire territories, effectively defend their territories from intruders, as well as dominate other males with smaller and less intensely coloured badges. |
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How do the two quality signals coexist?
A currently contentious issue in signal evolution is how several costly signals can be maintained rather than converging on the single most efficient (honest) trait. Discussions of multiple ornaments have centred on selection pressures from a single receiver, the choosy female. However, there are possible (and probable) alternative, additional or conflicting selection pressures on senders in different signalling contexts. Among the widowbirds, the coexistence of multiple costly signals (long tails and red badges) is a result of disparate selection pressures from female mates and male rivals. With separate receivers targeting different signals, these traits do not compete for the same receiver's attention and can therefore be selected for efficiency (honesty) within their own context. However, due to the costs of producing and maintaining both these signals they do compete in terms of sender investment, i.e. an allocation conflict (trade-off) between the investment into tail length and carotenoid display. Therefore, a male first needs to produce a large enough carotenoid signal to establish a territory, and thereafter allocate the remaining investment into tail length to attract females.
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