Evaluating and enhancing nest survival of an insular passerine threatened by an invasive snake

Predatory Boiga irregularis (brown treesnakes) have decimated Guam’s avifauna since their introduction to the island. Aplonis opaca (Såli, Micronesian Starling) is one of two native forest bird species that have survived, though it remains poorly understood how its population has persisted. We compared nest survival and causes of nest failure between snake-resistant nestboxes and natural nesting locations and tested hypotheses to identify drivers of variation in nest survival as well as the efficacy of snake-resistant nestboxes. We monitored 340 nest attempts by 40 A. opaca pairs nesting in nestboxes and 53 attempts by 32 pairs nesting in natural nests (outside of nestboxes). Overall nest survival in nestboxes (62%) was double that in natural nest sites (31%). Nest failure was equally divided between the egg and nestling stage in natural nests, whereas 82% of failures occurred at the egg stage for attempts in nestboxes. Hatch rates per nesting attempt in natural and nestbox nests were similar and generally low (overall mean ± SE = 71.2 ± 2.1%), possibly due to inbreeding depression; hatching failure was the leading cause of failure during the egg stage. We found no evidence of nest predation in nestboxes, while several nest failures during the nestling stage in natural nests were consistent with B. irregularis predation. Nest survival was positively correlated with use of nestboxes, clutch size, and distance from forest edge. Our nestbox placement strategy was highly effective at reducing predation of A. opaca nests, demonstrating its importance as a conservation tool for cavity-nesting birds on Guam. Protected nestboxes combined with a yearly mean of 4.3 successful nests per breeding pair produced a high mean annual fecundity (4.01 ± 0.34 female offspring per female), suggesting nestboxes could help offset low hatch rates and contribute to A. opaca population persistence and expansion on Guam.