Demystifying the ecology and evolution of Fiji’s forest birds

The evolution and ecology of island biota have captivated scientists for centuries. Thanks to their central role in establishing the theory of natural selection as an evolutionary principle, Darwin’s finches of the Galapagos Islands are world-famous. They are an assemblage of 17 species that evolved over the past 1.5 million years with a range of beak sizes and shapes suited for different ways of feeding. The beaks and behaviours of these finches are adapted to utilize various food resources (insects, seeds, fruits etc.) in different ways.

The Fiji archipelago is an island group in the Pacific Ocean that is much older, having emerged from the ocean at least 26 million years ago, compared with the younger Galapagos Islands, which are only about 3 million years old. With 66 land bird species, of which 34 are endemic, Fiji even has a slightly higher bird diversity than the Galapagos. However, while some land birds on the Galapagos have been well studied, little remains known about the ecology and evolution of Fiji’s native and endemic forest species.

PhD student Alivereti Naikatini (Fig. 1) at the University of the South Pacific is trying to change that. He is collecting data about the distribution and behaviour of forest species and investigating what these data are telling us about the ecology and evolution of Fiji’s bird fauna. Alivereti is an experienced ornithologist, investigating the relationship between food resources and bird abundance in his Masters research project, undertaking countless bird surveys, and assisting many ornithologists from outside his home country with their studies. He is also an excellent naturalist, with extensive and deep knowledge of Fiji’s fauna and flora. His project is supervised by Gilianne Brodie, Sonia Kleindorfer, and Gunnar Keppel. Recently published papers from his PhD show the importance of secondary forest for the conservation of Fiji’s land birds and the role that niche partitioning played in their evolution.

Fig. 1: Alivereti Naikatini with a New Holland Honeyeater near Adelaide in 2019. (Photo: Sonia Kleindorfer)

More than 50% of Fiji’s forests have been converted into grasslands, agricultural land, and secondary forests but it is not known how suitable these habitats are for land birds. Using annual bird surveys in the same locations over 5 years (2016–2020), Alivereti showed that species composition and bird abundance differed between forested and grassland habitats, but not between primary, secondary, and plantation forests. Foraging guild composition also differed between forested and grassland habitats, with granivores (birds feeding on seeds) and introduced species being more prevalent in grassland. Therefore, any kind of forest cover provides valuable habitat for Fiji’s land bird species.

To investigate if the utilization of different niches to reduce interspecific competition may have facilitated the speciation and coexistence of land birds in Fiji, Alivereti explored whether interspecific foraging behaviour in Fiji’s forest birds overlapped. Looking at three species of nectarivores (birds that feed on the nectar of flowers), two species of insectivores (feed on insects), and two species of omnivores (broad diet), he documented vertical partitioning of foraging in each group, supporting niche partitioning as a result of interspecific competition. To further investigate the importance of interspecific competition, Alivereti further investigated the nectarivores by comparing the heights at which the Orange-breasted Myzomela (Myzomela jugularis, Fig. 2) honeyeater on Viti Levu and Leluvia islands. Viti Levu is Fiji’s largest islands and has three honeyeater species, while on the coral atoll of Leleuvia the Orange-breasted Myzomela has no competitors. As predicted, the Orange-breasted Myzomela occupied a broader vertical foraging niche on Leluvia Island in the absence of the competitor honeyeater species. The findings support the importance of interspecific competition in creating niche divergence that can lead to ecological speciation in Fiji and on islands in general.