Divergent trends in distribution and abundance of landbirds in low and high elevation habitats of the Kaʻū Rainforest

Judge, Seth; Brinck, Kevin; Genz, Ayesha; Berry, Lainie; Gaudioso, Jacqueline; Wang, Alexander; Camp, Richard

doi:10.36967/2303786

Federal, state, and non-governmental partners resurveyed landbirds in the Kaʻū Rainforest on the Island of Hawai‘i in 2019. Point-transect distance sampling was conducted in Hawaiʻi Volcanoes National Park, Kaʻū Forest Reserve, Kapāpala Forest Reserve, and Kapāpala Cooperative Game Management Area. This is the first comprehensive survey of the region since 2008. Through a collaborative effort, monitoring transects were consolidated to provide a consistent, repeatable sampling frame for future surveys. We estimated landbird distribution, density, and abundance for eight surveys since 1976. Changes in species-specific densities were assessed using a log-linear regression to determine trends of the six most recent surveys since 2002. Trends were assessed in three strata: low elevation (<1,500 m), high elevation (≥1,500 m), and total survey area. We detected 23 species and there were sufficient detections of 11 species to estimate density and abundance. We provided indices of relative occurrence for the remaining 12 species detected. Native birds vulnerable to avian malaria (Plasmodium relictum) were absent or trended downward in the low elevation stratum. The federally endangered ʻAkiapōlāʻau (Hemignathus wilsoni), ʻAlawī (Hawaiʻi Creeper, Loxops mana) and Hawaiʻi ʻĀkepa (Loxops coccineus) were restricted to areas above 1,500 m elevation, likely due to the low prevalence of the cold-intolerant vector of avian malaria, the southern house mosquito (Culex quinquefasciatus), in this high elevation stratum. However, downward trends of the federally threatened ‘I‘iwi (Drepanis coccinea) above 1,500 m elevation may indicate an expansion in range of mosquitoes due to warming trends. Thought to be more resilient to avian malaria, the ʻApapane (Himatione sanguinea) and Hawaiʻi ‘Amakihi (Chlorodrepanis virens virens) trended upward in all three strata. The Hawaiʻi ʻElepaio (Chasiempis sandwichensis) trended downward in each stratum. The ‘Ōmaʻo (Myadestes obscurus) trended upward in the low stratum and downward in high. Trends were inconclusive for Hawaiʻi ʻĀkepa, but favorable for ʻAlawī, which trended upward in its restricted range. There were insufficient detections of ʻAkiapōlāʻau in 2019 to estimate density, but the 2008 abundance estimate of 691 ± 410 (SE) birds was the lowest since 1976. Trends were mixed for the non-native Warbling White-eye (Zosterops japonicus), Red-billed Leiothrix (Leiothrix lutea), and Northern Cardinal (Cardinalis cardinalis). Overall, upward trends of ʻApapane and Hawaiʻi ‘Amakihi throughout the survey area, ‘Ōmaʻo in the low stratum, and ʻAlawī in the high stratum, suggests management actions to protect native forest bird habitat are having a positive benefit. Nevertheless, avian malaria is predicted to have an increasing impact on vulnerable species as mosquitoes expand into higher elevations.

Divergent trends in distribution and abundance of landbirds in low and high elevation habitats of the Kaʻū Rainforest

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