Interactive effects of invasive rats and forest fragmentation on nest survival and behavior of native Hawaiian birds

Background/Question/Methods

Endemic island species are particularly vulnerable to habitat size reduction and biological invasion by non-native predators. Yet, how these two factors interact to jointly influence species persistence is not well understood. We used a unique system of replicated native forest fragments of varying size naturally created by recent lava flows on the Big Island of Hawai‘i to examine the effects of ubiquitous invasive rats (Rattus rattus) and forest fragment size on four species of native Hawaiian birds: ‘Oma‘o (Myadestes obscurus), Hawai‘i ‘Amakihi (Hemignathus virens), ‘Apapane (Himatione sanguinea), and ‘I‘iwi (Vestiaria coccinea). During the first year of a 5-year project, we began continuous operation of snap-trap grids within 16 study fragments to remove rats. Another 18 fragments served as controls with no rat removal. We then mist-netted and color-banded over 1400 birds, found and monitored more than 200 nests, and conducted quarterly color-band re-sighting surveys in each of the 34 study fragments over a two year period. 

Results/Conclusions

Data from years one and two suggest that nest survival of some species was related to the interaction between fragment size and rat presence. Overall site fidelity between years one and two was high for all species, but ‘Apapane showed the lowest site fidelity and ‘Oma‘o the highest. Most species showed lower rates of turnover in rat removed fragements than in control fragments. Further, the proportion of young birds caught in rat removed fragments was higher than in control fragments. These results suggest that invasive rats negatively impact the reproductive success of native Hawaiian birds and may also alter their behavior. The long-term conservation of the highly imperiled Hawaiian forest bird fauna may in part depend on understanding the combined effects of habitat fragmentation and introduced predators.