PS 23-65 - Body Language: Sublethal Effects of Introduced Rat’s on Hawaiian Forest Birds

Abstract: Human trade and colonization have led to the global introduction of rats (Rattus spp.), resulting in negative consequences for the diversity and function of numerous ecosystems. In island ecosystems, birds have been disproportionately affected by rats, resulting in widespread declines and extinctions. Specifically, rats affect food availability, forest structure, habitat availability, competition, and predation pressure, all of which contribute to the health of native bird populations. The effects of rats on bird populations are often measured in changes that result from direct predation, such as lower nest success or monitoring predation events on artificial nests, yet indirect effects, such as physiological and sublethal impacts, are less well known. The objective of this study was to determine if rodent control can influence physiological and demographic characteristics of island forest birds. To test this objective, we obtained morphometric measurements and demographic data of four focal Hawaiian forest bird species that reside in Kauaʻi’s Alakaʻi Wilderness Preserve in relation to rat abundance. Birds were mist-netted, aged, sexed, measured, and released during the 2022 and 2023 breeding seasons. We compared birds caught within areas that have had active rodent control for the past 4–7 years to birds from areas with no rodent control. Body condition for each individual bird was calculated using the Scaled Mass Index (SMI) method, and the recruitment and capture rate of hatch year birds for each species were calculated. We modeled the effects of rat removal on bird body condition and demography using linear mixed models. Preliminary results suggest no significant effect of rodent control on the body condition of these birds. In areas with rodent control, two of the four focal species had higher capture and recruitment rates of hatch year birds. Data on the sex ratios and current or past malarial infection will also be collected and evaluated in relation to relative rat abundance and body condition, respectively. If data continues to indicate a lack of relationship between health and demographic characteristics and rodent control, other management issues such as introduced disease, habitat loss, food availability, or impacts from other predators may be better at explaining differences in body condition and demographic characteristics of bird populations. This data will contribute to a deeper understanding of rat impacts on forest bird health and have implications for conservation and restoration by helping to inform which management actions should be prioritized and determining where to direct potentially limited conservation funds.