The conservation of birds on oceanic islands is particularly challenging. These species may have inherently small population sizes, inhabit limited ranges, and be vulnerable to changes in the predator community. As explorers and settlers inevitably arrived, they altered habitats and introduced new predatory species. These changes led to drastic reductions, and occasional extinctions, in island bird populations. For those species that survived, understanding the extent of these remaining threats is integral to their recovery. One species that experienced these challenges, but is now trending toward recovery, is the San Clemente Bell’s Sparrow.
This subspecies of Bell’s Sparrow occurs only on San Clemente Island off the California coast. San Clemente Island is somewhat unique among islands because it contains native mammalian predators: the San Clemente Island fox. However, the arrival of non-indigenous settlers to the island brought changes to habitat and predator communities. Ranching reduced native shrublands and introduced non-native rats and feral cats. These changes contributed to the decline of the Bell’s Sparrow and ultimately led to its listing as federally threatened. Reduced nest success due to the destruction of nesting habitat and increased presence of nest predators may have contributed to the Bell’s sparrow decline. Facilitating the recovery of this species therefore requires an understanding of nest survival and causes of nest failure.
To collect these valuable data, we used video cameras to monitor 110 Bell’s sparrow nests across seven breeding seasons. These cameras allowed us to observe nests remotely to accurately determine nest status, timing of nest failure, and cause of nest failure. We were also able to explore whether nest survival and predation patterns were related to environmental factors such as the vegetation type nests were built in.
Overall, nest success rates were relatively high compared to mainland Bell’s sparrow populations. Evolving with island foxes may have helped Bell’s sparrows avoid strong predation pressure from invasive mammalian predators. Surprisingly, rats and cats, invasive predators that decimate bird populations on other oceanic islands, were not the most important nest predators for Bell’s sparrows. Island foxes depredated more than twice as many nests as black rats, and we did not document any nest predation by feral cats. However, non-native predators still add increased predation pressure, and may affect survival of fledglings and adult sparrows.
The ongoing habitat restoration activities on SCI may also benefit Bell’s sparrow nest success. Much of the recent habitat recovery has been expansion of sagebrush and other moderately tall shrub species. Nests built in sagebrush, a relatively tall shrub, had higher survival rates than nests built in boxthorn, a low shrub. Notably, no nests built in sagebrush were depredated by rats, suggesting that this habitat recovery may protect Bell’s sparrows from invasive predators in particular.
By using nest cameras, we identified the most common threats to Bell’s sparrow nests and identified potential avenues to mitigate these threats via habitat recovery. These findings, along with ongoing population monitoring, indicate the subspecies is well on its way to recovery. We hope that these results help managers in achieving recovery goals and mitigating future threats.
Department of Wildland Resources
Utah State University
The results of this study were published in the Journal of Field Ornithology:
Parsons, M. A., S. T. Meiman, S. A. Munoz, A. S. Bridges. 2022. Nest survival and cause-specific nest mortality in the San Clemente Bell’s Sparrow 93(4):2. https://doi.org/10.5751/JFO-00177-930402.