Evolution in island birds and the ‘insularity’ syndrome
The shared characteristics of island environments, such as impoverished biotas and less-seasonal climates, are thought to trigger specific adaptations by organisms. Birds are good island colonisers, and are among the best studied groups. However, few broad-scale systematic analyses have examined patterns of evolution in island birds and as a result our understanding of which traits are affected by the ‘insularity syndrome’ remains weak. We have been studying patterns of adaptation in reproductive life histories, morphology and sexual ornaments in island birds worldwide. Our previous results show that insularity favours a slower pace of life in birds, with reduced fecundity, extended developmental periods, and an increase in the frequency of cooperative breeding.
A poorly understood topic was whether insularity affected other important traits such as animal signals. We tested whether birds’ plumage colouration and songs are part of the island syndrome. For coloration, we used museum specimens and analysed, with spectrophotometry, the colouration of 116 species endemic to islands and their 116 closest mainland relatives. We found a pattern of reduced brightness and colour intensity for both sexes on islands. In addition, we found a decrease in the number of colour patches on island bids that, in males, was associated with a decrease in the number of same-family sympatric species. These results, which were published in Ecology Letters, demonstrate a worldwide pattern of parallel colour changes on islands and suggest that a relaxation of selection on species recognition may be one of the mechanisms involved.
However, other mechanisms may influence the reduced colouration of island birds. For example, it has been suggested that sexual selection would be higher in species facing higher parasite pressure and would thus select more for elaborate ornaments that signal their resistance to parasites. Therefore, on islands, lower parasite pressures are expected to lead to a reduced pressure to maintain elaborate ornaments. To investigate this hypothesis, we conducted detailed field-based work on avian malaria immunity and plumage colouration in the Gulf of Guinea islands and the nearby mainland (Gabon and Cameroon). As expected, we found an overall decrease in parasite pressure on islands (manuscript in review). However, this was not followed by reduced immune capacity. Instead the results, published in Ecology and Evolution, showed that different immune parameters responded differently to the island environment. On the other hand, reduced parasite pressure appears to influence the decrease in coloration found on islands and a manuscript is currently in preparation.
Additionally, in October 2016 we initiated a field-based project to test the role of reduced interspecific competition in the evolution of bird song on islands. We used SM3 passive recorders to record the acoustic space of island and mainland bird communities. Simultaneously, we focused on seven pairs of island-mainland species, quantified the number of species co-singing with these focal species, and measured the acoustic niche occupied by them. The analyses are currently underway.
Activities in 2016
- Elisa Lobato concluded a Post-doc at CIBIO, U. Porto, investigating parasite pressure, immunity and the links with bird colouration on the islands of São Tomé and Principe compared with Gabon. Elisa worked under the supervision of Rita Covas and Claire Doutrelant. Two manuscripts were published in 2016, one is in review and a fourth one is in preparation.
- Martim Melo and Claire Loiseau (CIBIO) finalised a manuscript on the prevalence and diversity of avian malaria in birds from the four Gulf of Guinea islands and nearby mainland (Gabon and Cameroon), which is currently in review in Journal of Biogeography.
- Claire Loiseau initiated a global-scale comparative study in collaboration with Rita Covas and Claire Doutrelant on whether there are consistent differences in parasite prevalence and diversity on islands versus the mainland and whether avian malaria parasites follow the island biogeography rules described for macrofauna.
- Claire Doutrelant, Martim Melo, Thierry Lengagne and PhD student Alois Robert conducted one month of fieldwork in São Tomé and Cameroon. They recorded the songs of 14 species of birds, as well as the dawn chorus of the bird communities in three different locations each morning using SM3 passive recorders
- A paper in Ecology Letters described how island birds worldwide generally have reduced plumage brightness and colour intensity.
- Two additional papers comparing immunity parameters and gut bacteria in island and mainland populations were published in Ecology and Evolution and Symbiosis, respectively.
- A book chapter on 'Life-history evolution on islands' by Rita Covas was published in the Encyclopedia of Evolutionary Biology (Springer).
- Rita Covas and Claire Doutrelant convened the Symposium 'Patterns of adaptation on islands and the insularity syndrome' at the II International Conference of Island Evolution, Ecology and Conservation, Angra do Heroísmo, Azores, Portugal.
- Martim Melo convened a Symposium on 'Biodiversity in the Gulf of Guinea Islands' at the same conference. This will form the basis for a reference book on this system.
- A presentation at the 16th congress of the International Society for Behavioural Ecology by Rita Covas reported that island birds are more likely to live in family groups and breed co-operatively than their mainland relatives. She identified the life-history and climatic factors associated with these traits.
Impact of the project
This project is uncovering novel patterns of adaptation in island birds and investigating for the first time some of the mechanisms underlying these adaptations. The results are making a significant contribution for ourunderstanding of the ecology and evolution of island environments. Given the large number of species endemic to islands worldwide and the role of islands as ‘natural laboratories’, our results make a significant contribution to understanding and conserving island environments.
FCT (Portugal); National Geographic; British Ecological Society; CNRS (PEPS); DST-NRF CoE grant; University of Montpellier (PhD grant to Alois Robert).
Dr Rita Covas (FIAO, UCT and CIBIO, U. Porto)
Dr Claire Doutrelant (CNRS and FIAO, UCT)
Dr Martim Melo (FIAO, UCT and CIBIO, U. Porto)
Dr Elisa Lobato (CIBIO, U. Porto)
Dr Claire Loiseau (CIBIO, U. Porto)
Dr Thierry Lengagne (CNRS)
Students: Aloïs Robert (PhD, U. Montpellier)