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Trends in the evolution of insular land birds, exemplified by the Comoros, Seychelles and Mascarenes.
... A. f. brutus has a juvenal plumage which does not exist in any other population (Herremans et al. 2001, Herremans et al. 2011 . Compared to the nominate form from Madagascar, A. f. brutus on Mayotte is thus partially neotenic (Herremans 1990) . A relationship between neotenic evolution (reflected for example in plumages) and reduced territoriality and increased densities has been shown on the Comoros as one of the "insular syndromes" shared by several bird species (Louette et al. 1993) . ...
... This divergence in morphology has led to the suggestion that the Gulf of Guinea thrush and the morphologically similar Comoro thrush T. bewsheri – endemic to three Indian Ocean islands of the Comoros archipelago on the other side of the African continent – are representatives of an old mainland lineage now extinct (de Naurois, 1984; Keith & Urban, 1992). Alternatively, the distinctiveness of T. olivaceofuscus and T. bewsheri could represent a parallel retention of a juvenile barred plumage in the adults of insular Turdus (de Naurois, 1984; Herremans, 1990; Jones & Tye, 2006). The objectives of this study were: (1) to clarify the relationships between the Pr´ıncipePr´ıncipe and Saõ Tome´populaTome´populations of T. olivaceofuscus, with the aim of determining whether they constitute distinct species; (2) to determine the phylogenetic affinities of the two Gulf of Guinea thrush taxa. ...
The Gulf of Guinea thrush Turdus olivaceofuscus is endemic to the islands of São Tomé (nominate olivaceofuscus) and Príncipe (subspecies xanthorhynchus). Relationships between the two island taxa, originally described as two different species, are uncertain. This problem has been difficult to resolve due to the scarcity of information from Príncipe birds. A focused effort to find birds from Príncipe resulted in new observations, the first records of its song, and in the capture of four individuals, which provided new data for analyses. We obtained additional data from museum specimens. Our analyses indicate that the two populations differ substantially in size, bill shape and bill, eye and leg coloration as well as in several plumage characteristics. In addition, xanthorhynchus utters a low call of a type not previously recorded in the genus Turdus. Genetic evidence corroborates the phenotypic evidence: both taxa constitute clearly independent evolutionary lineages (2368 bp from the mitochondrial markers ND2, ND3 and cytochrome b (cyt-b) from four individuals of each population). Genetic divergence between the taxa (cyt-b: uncorrected: 6.4%; corrected: 8.8%) suggests that they may have been isolated for over 4 Myr. These results support the split of T. olivaceofuscus into two species: São Tomé thrush T. olivaceofuscus and Príncipe thrush Turdus xanthorhynchus. The latter is a very rare species, restricted to the most inaccessible parts of Príncipe Island. Phylogenetic inference favoured the African thrush Turdus pelios as the closest living relative to the Gulf of Guinea species.
In two study plots on Mayotte Island, the territorial, neotenic-endemic race brutus of the Frances's Sparrowhawk occurred at world-record breeding densities for a small raptor of 2.0-2.6 ha/pair, with average inter-nest distances of 140-159 m in the breeding season of 1997. Measured in nine transects, the abundance of birds of potential prey-size correlated negatively with sparrowhawk abundance on Mayotte, while numbers of birds larger than sparrowhawks varied in parallel with sparrowhawk abundance, both corresponding to rainfall and lushness of the vegetation. Throughout the Comoro archipelago, there is a negative relationship at island level between sparrowhawk abundance and the proportion of small birds in the community. Several species of small birds are lacking from islands with sparrowhawks compared to islands without sparrowhawks; anomalies that cannot be explained by island size, distance to source or habitat. More species are lacking from islands with high sparrowhawk abundance. We suspect that the chances of survival for colonizing birds may have been affected historically by the abundance of Frances's Sparrowhawks on some of the Comoro Islands.
M.W. Fraser, P.G. Ryan, W.R.J. Dean, D.J. Briggs, & C.L. Moloney 1994. The biology of the Tristan Thrush Nesocichla eremita. Ostrich 65:14-25.The Tristan Thrush Nesocichla eremita is a little known species restricted to the Tristan da Cunha Islands, South Atlantic Ocean. We describe several aspects of the species' biology, based primarily on observations made at Inaccessible Island. Tristan Thrushes are monomorphic, assuming adult plumage after a partial juvenile moult. There is little sex-related size dimorphism within island populations, which may reflect limited genetic variation. Tristan Thrushes are generalist foragers, using a variety of techniques to consume both animal and plant matter. Breeding occurs during the austral spring, with a single brood raised from clutches of two eggs at Inaccessible and Tristan. Three-egg clutches have only been recorded at Nightingale Island, where they comprise about half of all clutches. Adults perform a very soft, melodic warbling song, which is most frequently uttered immediately prior to the spring breeding season. Typical calls are simple, high-pitched whistles. Adults moult after the breeding season, and primary moult is completed within 80 das. Moult commences at least one week earlier at tie coast than on the plateau (elevation 250–500 m) at Inaccessible Island. We provide a revised population estimate for Inaccessible Island, and make recommendations for the species' conservation.
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