Evolution in island birds and the ‘insularity’ syndrome | FitzPatrick Institute of African Ornithology

Islands are important centres of endemism and key natural laboratories for the study of ecology and evolution, but also are hotspots of human-driven extinctions. This programme investigates how island communities are assembled (island biogeography), how new species arise (speciation), how populations adapt to island environments, and assesses threats to their persistence. Our research focuses on birds from the Gulf of Guinea, Cape Verde and Tristan islands.

Organisms on islands often exhibit convergent evolution on a wide suite of traits, which are together termed the “island syndrome”. These common evolutionary outcomes are linked to the isolation, small size and stable climate associated with oceanic islands. Isolation reduces species richness relative to mainland areas, limiting inter-specific competition and allowing for ecological release. Low species richness also results in fewer predators and parasites. During the last decade, we have investigated several island syndrome traits (e.g. size, color, song, life-history) and their explanatory factors (e.g. competition, parasites, immune system). Currently we are exploring how reduced interspecific competition and low resource availability (due to small island areas) combine to cause island species to evolve more generalist dietary niches.

The São Tomé storm-petrel mystery
After the discovery of a new species of scops-owl on Príncipe, one might think that São Tomé and Príncipe had no more secrets regarding birds. However, Martim Melo has been trying to unravel the mystery of the São Tomé Storm-Petrel since his first visit to the islands in 1996. Robert Flood recently led a team that has solved this mystery. A combination of observations in the southern forests of São Tomé and from boats offshore, together with genetic sequences from six birds attracted to the lights of a palm oil factory, has resolved whether these storm petrels are a population of Hydrobates castro, a population of the Cape Verde Storm-Petrel H. jabejabe, or a new species. The results are reported in a paper currently under review.

Cape Verde sparrows
The Iago Sparrow Passer iagoensis, endemic to Cape Verde, is an excellent model to study climate adaptation as it occupies a wide aridity gradient on 13 islands. It is also exhibits a range of commensalism, as humans only colonised the archipelago 500 years ago, and some islands remain uninhabited. Martim Melo and Rauri Bowie are collaborating with Ângela Ribeiro, Mark Ravinet and José Cerca on a project that combines fieldwork and genomics to understand the sparrow’s evolutionary history. The species succeeds by being a ‘super-generalist’ in a harsh environment characterised by scarce, unpredictable food resources. Surprisingly, sparrows from the Rombos, two small islets, differ phenotypically and genetically from all other populations. The genes determining bill size and shape are the same as those causing bill diversification in Darwin’s finches, but the mutations likely evolved after the islands were colonised.

Tristan’s landbirds
The Tristan archipelago of three islands in the central South Atlantic is home to the most remote human community. The presence of people and their commensal animals on the main island of Tristan since the early 19th Century has seen the extinction of the endemic moorhen and finch, leaving the Tristan Thrush Turdus eremita as the only surviving native species. Fortunately, the other two islands lack introduced predators, and still support several endemic landbirds. Peter Ryan works with the Tristan Conservation Department and the RSPB to conserve these vulnerable species.

Activities in 2024

The identity of the storm-petrel population breeding on São Tomé was resolved.
The arts and sciences project ‘Obô’ (the São Tomé name for primary forest) is growing steadily. An exhibition, including art, live music and poetry readings, took place in Montpellier, France.
A paper on the evolutionary history of the Iago Sparrow was submitted.
During two trips to Tristan in 2024, Peter Ryan and Coleen Moloney re-assessed the population of Wilkins’s Finch Nesospiza wilkinsi on Nightingale Island after extensive storm damage to island trees. They also studied the introduced population of Gough Moorhens Nesiotis comeri on Tristan. Finally, they collected blood samples from Tristan Thrushes from all three islands to determine the degree of genetic differentiation among populations, and attempted to estimate the numbers of thrushes surviving on the main island of Tristan.

Highlights:

Two reviews on evolution on islands were published, and two papers on the conservation status of Tristan landbirds were accepted for publication.
Intriguing observations of Oceanites storm petrels off São Tomé were described in a paper in Marine Ornithology.
Martim Melo wrote an article on the importance of the Gulf of Guinea islands for birds in BirdLife International’s publication Birdlife: The Magazine.

Key co-supporters
FCT - Portuguese Science and Technology Foundation; CNRS-France (PEPS); Marie Sklodowska-Curie Grants; Royal Society for the Protection of Birds; Tristan Conservation.

Research team 2024
Team leaders and collaborators:
Dr Martim Melo (FIAO, UCT/CIBIO, U. Porto)
Dr Claire Doutrelant (CNRS/FIAO, UCT)
Dr Rita Covas (FIAO, UCT/CIBIO, U. Porto)
Dr Robert Flood (FIAO, UCT)
Dr. Claire Loiseau (University of Montpellier)
Dr Ângela Ribeiro (IPVC, Portugal)
Dr Mark Ravinet (U. Oslo)
Dr José Cerca (U. Oslo)
Dr Martin Stervander (National Museum, Edinburgh)
Dr Luis Valente (Naturalis Biodiversity Center, Leiden)
Dr Ricardo Lima (cE3c, U. Lisbon)
Prof. Bengt Hansson (Lund U.)
Emer. Prof. Peter Ryan (FIAO, UCT)
Dr Raquel Ponti (CIBIO, U. Porto)
Dr Ana Leitão (CIBIO, U. Porto)
Prof. Rauri Bowie (U. California Berkeley)

The origins of the storm-petrel population that breeds within the rainforests of São Tomé Island have long remained unsolved. A concerted effort launched in 2023 by Dr Robert Flood, led to data collection from six new birds (attracted by lights) which were combined with available museum specimens, allowing for an integrative taxonomic study, the results of which will reveal all in early 2025 (Photo: Michael Mills).

The scarce nominate race of Tristan Thrush, confined to the main island of Tristan, is smaller and paler than the birds on Inaccessible and Nightingale Islands (Photo: Peter Ryan).

The Iago Sparrow of the Cape Verde Islands is providing fascinating insights into micro-adaptation (Photo: Alexandre Vas).

A Newton’s Sunbird Anabathmis newtonii, endemic to São Tomé Island, gives its song. Birds on islands have more variable songs because the acoustic space is less saturated with songs and calls from other vocal organisms, such as birds and insects, As a result, the frequency bandwidth of island species’ songs are broader than those on the mainland (Photo: Lars Petersson).

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