Small houses, big impacts

by Zane Pickus

A male tree swallow poses for the camera (Zane Pickus).

A Home For Birds

The home is a place of safety, warmth in times of cold and cool in times of hot, a place to sleep at night and rest during the day. For cavity-nesting songbirds, home is a hole in a hollowed-out tree. These homes are becoming few and far between, yet conservation biologists (scientists who work to protect natural resources and species diversity) have an opportunity to help.

Humans have an impact on their local ecosystems. One example of this is the removal of old, often deceased trees, which eliminates natural tree-cavities. To counteract this, some conservation biologists are building birdhouses — small wooden boxes hung on trees or fence posts that replicate a natural tree cavity — to provide birds with a place to build their nests and raise their young. Humans have built birdhouses for hundreds of years, perhaps since as early as the 15th Century (Nesting Cavities and the History of the Birdhouse). In conservation biology, birdhouses are referred to as “nestboxes,” and they provide opportunities to both help imperiled species and study the birds that use them. Researchers at the University of California, Davis, have done exactly this, studying how changing climate and habitats are impacting cavity-nesting songbirds (Riggio et. al, 2023).

A male western bluebird, who was born the year prior in a nestbox at the UC Davis Arboretum. The color bands on his leg allow researchers to identify him (Zane Pickus).

Little Boxes With Big Conservation Implications

In California’s Central Valley, songbirds face a variety of challenges. These include, but are not limited to: extreme summer heat, long and unpredictable periods of cold in winter, very little remaining natural habitat, and a significant decline in the availability of natural nest cavities. Locally, Putah Creek flows out of the coast range and into the valley’s flatland, right by UC Davis. Today, its flows are very carefully controlled, its banks are highly channelized, and it borders miles of agricultural land. Yet the ribbon of trees alongside the creek provides vital habitat for a massive variety of species, including cavity-nesting songbirds. UC Davis maintains and monitors more than 200 nestboxes along the creek’s banks, collectively known as the “Putah Creek Nestbox Highway.” The boxes in this project have produced more than 13,000 baby birds over its 24 year existence, primarily from four species — the western bluebird (Sialia mexicana), tree swallow (Tachycineta bicolor), ash-throated flycatcher (Myiarchus cinerascens), and house wren (Troglodytes aedon) (Putah Creek Nestbox Highway). The nestboxes are critical to the local populations of these species.

A recent publication drew from data in the Putah Creek Nestbox Highway to investigate what factors influence a successful nesting season for these species. The study used eleven year’s worth of data, with over 2,000 documented nest attempts, to consider how habitat choice and weather variability relate to reproductive success.

A Look Into the Lives of Local Birds

One aspect of this study explored what habitat choices these birds make when selecting a nest site. The study found that ash-throated flycatchers, for example, prefer their nest cavities to be further from the creek itself, and they almost always avoid orchards. This means that preserving open areas along the edge of the creek channel is important to their conservation. House wrens like to be right by the creek, in areas with dense cover. Therefore, protecting creekside habitat is crucial in sustaining house wrens.

Conversely, western bluebirds showed a preference for nestboxes in orchards, and generally avoided the creekside forest. This could bode well for bluebird conservation, as orchards are increasing in Central Valley agriculture. Finally, tree swallows were found to be the “generalists” of the group. They showed no preference for any given nest box location, using boxes close to the creek, on the edge of the creekside forest, and in orchards in the same amounts.

A tree swallow pokes his head out of a nestbox along Putah Creek. Tree swallows build their nests out of feathers and small grasses, like the one he is holding in the image (Zane Pickus).

Climate Change Takes Its Toll

The effects of climate change vary greatly from one place to another, but in the Central Valley it is characterized by increasing summer temperatures and spring rainfall. These changes, researchers found, are already impacting cavity-nesting bird species.

Increasing summer temperatures are having a negative effect on bird nesting. All four species in the study showed significant declines in nesting success when there was increased summer temperatures. Baby birds tended to be smaller in periods of extreme heat, and were less likely to “fledge” (leave the nest). This could be explained by multiple factors, including fewer insects available during periods of extreme heat. Additionally, the amount of energy required to stay cool when it is hot out is costly, both for the parents who may choose to avoid looking for food for their babies, and for the babies who struggle to regulate their body temperature.

Another aspect of a changing Central Valley climate is increased spring rainfall. In recent decades, spring storms have become more frequent, which has hurt the nesting success of both western bluebirds and tree swallows. Similar to extreme heat, spring rains likely decrease the availability of insect prey, and the cold temperatures associated with rain require the birds to eat more food to stay warm. This leaves baby birds without the energy they need to survive and fledge.

The Home Must Change with Time

This research provides important insight into the lives of four cavity-nesting bird species in California’s Central Valley, and has direct implications for the conservation of the species. It adds to a growing understanding of what makes a home for these birds — not just the physical nest box, but also the surrounding habitat and climate. The study recognizes that climate change is already having an impact on our local birds. Perhaps in the future changes will be made to keep nest boxes in places with cover, which could help birds manage extreme temperatures and stay dry in spring rains. A home is a place to live, and when providing a home for birds, it is critical that the home be safe and allow birds the best chance to survive and raise their young.

Recently hatched western bluebirds, next to two eggs (Zane Pickus).

References

“Nesting Cavities and the History of the Birdhouse.” Cranmer Earth Design Information, www.earthdesign.ca/bihi.html#:~:text=Birdhouses%20have%20been%20part%20of,around%20the%2015th%20%2D%2016th%20century. Accessed 31 Jan. 2024.

“Putah Creek Nestbox Highway.” Museum of Wildlife and Fish Biology, University of California, Davis, 14 Apr. 2021, mwfb.ucdavis.edu/research/california/nestboxes/putah-creek.

Riggio, Jason, et al. “Long-term monitoring reveals the impact of changing climate and habitat on the fitness of cavity-nesting songbirds.” Biological Conservation, vol. 278, https://doi.org/https://doi.org/10.1016/j.biocon.2022.109885.