Title: Nest temperatures reveal reproductive consequences for a forest-nesting raptor

Program: Raptor Biology MS

Committee Chair: Jay Carlisle

Committee: Jay Carlisle, Jen Cruz, Allison Simler-Williamson

Abstract: Rapid climate change can shift regional climate conditions beyond individuals’ thermal tolerances, contributing to local population declines. However, climate risk projections are often based on coarse-scale macroclimate data, which may not represent the fine-scale microclimatic conditions experienced by organisms. In forests, vegetation structure and canopy cover can strongly buffer temperature relative to surrounding macroclimate, and canopy cover loss can accelerate local warming beyond regional climate trends. Because birds are confined to nests during reproduction, they cannot readily avoid unfavorable microclimatic conditions, potentially affecting reproductive success. Despite this potential sensitivity, which has been documented in other avian species, nest microclimate remains poorly studied for forest canopy-nesting raptors. This thesis investigates the American goshawk (Astur atricapillus), a North American forest-nesting raptor whose population in the Minidoka Ranger District of the Sawtooth National Forest has experienced declining reproductive success in recent years. We aimed to evaluate the influence of nest temperature on goshawk reproductive success in the Minidoka Ranger District across 2015–2025. To address this aim, we measured goshawk nest temperature over two breeding seasons in 2024 and 2025, developed models to hindcast past years’ nest temperatures in years 2015–2023, and isolated the effects of nest temperature on goshawk hatching success and nestling survival, using models developed with directed acyclic graphs. Our thermologger-measured nest temperatures often existed within a buffered range relative to macroclimate temperature estimates, reflecting the thermal buffering capacity of forest canopies. Hindcasting nest temperature allowed us to estimate past nest conditions using models derived from measured nest temperature data specific to our study species. Average minimum nest temperature during incubation did not appear to influence hatching success in our study sample. However, during the nestling stage, nestling survival declined with warmer maximum nest temperatures. We offer the first empirical support that warmer nest temperatures may substantially limit goshawk nestling survival. Increased efforts to preserve areas of thermal refuge within goshawk breeding habitat may support reproductive success. The approach outlined in this thesis can be applied to evaluate microclimate-reproduction relationships in other forest-nesting populations.