Thermally-Buffered Habitats Mitigate The Effects Of Temperature Spikes On Breeding Songbirds

Abstract

As climate change and agricultural landscapes continue to intensify concurrently, it is important to understand how we can conserve biodiversity in human-dominated landscapes now and in the future. There is some evidence that the presence of thermally buffered habitats such as patches of trees and other natural habitats can help organisms cope with increasingly common heat waves. It is therefore important to understand how temperature extremes can affect the fitness of organisms in agriculture, where the majority of natural habitat is often removed. In this study, we sought to examine how thermally buffered habitats can help mitigate the impacts of heat waves on songbirds in vineyards across California’s Napa Valley. We captured 55 breeding Western Bluebirds (Sialia mexicana) and Tree Swallows (Tachycineta bicolor), tracked their movements with GPS, and then examined how temperature and land cover within their home ranges influenced their body condition. We found that both species lost weight as temperatures increased, but the effects of temperature were mediated by the amount of vineyard and thermally-buffered habitat within their home ranges. Specifically, decreases in weight in response to temperature were diminished in areas with greater proportions of thermally-buffered habitat and lower proportions of vineyards. Together, our work suggests that maintaining and/or restoring natural habitats in agricultural landscapes could help buffer the effects of climate change on wildlife in human-dominated landscapes.

Introduction

Temperature spikes are becoming increasingly common with climate change, but informed conservation efforts can help organisms cope. For example, maintaining or restoring patches of trees or other natural vegetation can provide organisms with refuge from heat waves [1], which are becoming increasingly common with climate change. Efforts to conserve thermal refuges can be especially important in agricultural systems, where large amounts of shade-providing habitat are often removed, and temperatures are regularly >10°C warmer than nearby forests [2]. However, it is unclear how thermal refuges can mitigate the effects of heat waves on organisms that currently persist in farmlands. As agriculture continues to expand and intensify, understanding how thermal refuges can support biodiversity in human-dominated landscapes will become more important.

Birds are especially vulnerable to heat waves associated with the changing climate. North American bird populations have declined by over 3 billion individuals since 1970 [3]. Climate change has been at least partially responsible for these declines: in North America, rising temperatures spurred the collapse of entire bird communities [4]. Farmland bird populations are especially at risk, with 74% of species having decreased from 1966-2013 [5]. It is therefore critical to understand how rising temperatures could affect birds during key stages of their life history, such as reproductive periods. 

During breeding periods, many birds forage for both themselves and their offspring, making them a particularly energy-intensive time of year. As such, intense heat exposure that inhibits foraging during the breeding season can lead to major fitness consequences. A lack of thermal refuge during heat waves was previously demonstrated to force birds to seek shade far away from their typical foraging areas [6]. Rising temperatures can therefore restrict birds to distant and small patches of natural habitats in agricultural landscapes, making it crucial to understand the fitness consequences for birds in simplified landscapes, and how diversified landscapes could mitigate these negative impacts.

We sought to understand how thermal refuges can mitigate the impacts of heat waves on birds that utilize artificial nest boxes within vineyards across California’s Napa Valley. This region has undergone considerable amounts of natural habitat conversion to farmland and regularly exceeds 35°C during the breeding season [7]. We hypothesized that birds in simplified landscapes would lose more weight during their nesting period than birds in more diversified landscapes.

Methods

We examined how temperature and land cover influence the body condition of 55 individual breeding vineyard songbirds in California’s Napa Valley. 43 of these birds were Western Bluebirds (Sialia mexicana) and 25 were Tree Swallows (Tachycineta bicolor). We conducted this study across 10 vineyards with over 218 nest boxes (Figure 1) across the breeding season (March-July) [8]. These vineyards were selected to span a gradient from intensive to diversified vineyards, primarily assessed as a function of the amount of surrounding non-crop habitat (i.e., grassland, riparian forest, and oak woodland).

To assess how land cover could buffer the effects of temperature on breeding birds, we quantified the amount of vineyard and thermally-buffered habitat (i.e., habitat with trees) within the birds’ home ranges. Ranges were mapped using geospatial data collected by a small (1 g) GPS logger attached to each bird’s back upon capture. To minimize impacts on individual birds, only individuals weighing >20 g were selected. The loggers were set to record geospatial positions every 30 minutes from 06:30 to 19:00 PST (dawn until dusk) over a period of 5 days, allowing us to recapture the bird and recover the tag data before their nesting attempt is completed. We additionally weighed each bird at the time of the first capture and recapture to assess changes in body condition during the breeding season, and placed temperature loggers outside of each nest box to record temperatures across the observation period. The GPS positions were then used to generate each bird’s home range as a 95% kernel utilization distribution [9] using the R package “adehabitatHR” [10]. We then quantified land cover within these ranges using a land cover map created by Huysman and Johnson [11]. 

To determine how land cover could modulate the effect of temperature on both species, we fitted two multiple linear regression models using the base R function “lm()”. The formulae for the models are as such:

(1) weight change ~ species + mean max. temperature * thermally-buffered habitat percentage

(2) weight change ~ species + mean max. temperature * vineyard percentage

Where the first model (1) assessed the interactive effects of temperature and vineyard cover Huysman and Johnson [11] and the second (2) assessed the interactive effect of temperature and thermally-buffered habitat. We considered riparian areas, oak savannas, and forests as thermally-buffered habitats. Separate models were necessary to avoid autocorrelation, as we found occurrences of vineyard and thermally-buffered habitat to be highly correlated in our system (Pearson’s r = 0.88) using the base R function “cor()”. In both models, temperature was defined as the average maximum daily temperature recorded by each nest box temperature logger over the 5-day GPS tracking period. We additionally included a covariate to control for species-specific effects in our models and visually inspected residual plots to assess model fit. 

Results 

Songbirds lost weight in response to temperature, but this effect was mediated by the amount of vineyard and thermally-buffered habitat present. In our model testing the interactive effect of temperature and thermally-buffered habitat, it was predicted that, in response to temperature alone, birds lose 0.06 ± 0.02 grams/day for each one °C increase in mean maximum daily temperature (p = 0.001). However, each 10% increase in thermally-buffered habitat reduced the effect of temperature by 0.02 ± 0.009 grams/day (p = 0.02; Figure 2); weight loss was most prominent in areas with the lowest proportion of thermally-buffered habitat. 

In our model testing the effect of temperature and vineyard habitat, it was predicted that, in response to temperature alone, birds lose 0.09 ± 0.02 gram/day for each one °C increase in mean maximum daily temperature (p = 0.01). However, each 10% increase in vineyard habitat increased the effect of temperature by 0.06 ± 0.007 gram/day (p = 0.04; Figure 3); weight loss was most prominent in areas with the highest proportion of vineyards. 

Discussion

We found that thermally-buffered habitats in vineyards can mitigate the effects of temperature-driven deterioration in avian body condition (fat deposit, weight, etc). While hot temperatures drove increased weight loss in breeding songbirds overall, birds nesting in areas with more thermally buffered habitat and less vineyards experienced smaller decreases in weight. 

Especially during the breeding season, which often corresponds with summer high temperature events, many birds are provisioning for both themselves & dependent young, making this time of year particularly energy intensive. When heat waves occur, it becomes more energetically costly for birds to forage [12]. Our results show that birds lose weight in response to rising temperatures: this aligns with empirical evidence that heat waves limit foraging behavior and activity in South African birds [11]. As such, our results suggest that breeding songbirds in intensive agricultural landscapes may sacrifice their own metabolic needs to ensure the survival of their young. 

Importantly, we also demonstrate that decreases in songbird body weight occur in intensive agricultural landscapes but not in diversified landscapes with more thermally buffered habitats present. Hiron et al. [13,14] demonstrated in Sweden that conservation and restoration of natural habitats within agriculture increases the species richness of farmland bird communities. In addition, previous work has suggested that extreme hot temperatures reduce nesting success in simplified but complex landscapes [15].  Diversified landscapes can help enhance beneficial species richness [16] while having a more positive impact on simplified landscapes (i.e., areas with significant agricultural conversion) or areas farther away from natural and semi-natural habitats that have a lower proportion of natural habitat [16]. Overall, vineyards increased the effect of temperature, meaning, the more vineyards present the more likely the birds were to lose weight. Our results support and provide a potential mechanistic explanation for these patterns, as populations of birds in diversified agriculture are more likely to thrive when they have thermally-buffered habitats to enable them to successfully feed themselves as well as their offspring despite extreme conditions.  

While we demonstrated significant effects of hot temperatures on songbird body condition, we experienced relatively few extreme temperature events during our data collection period. Only 5 out of 55 individuals we observed were nesting when the mean daily maximum temperature was over 35°C. Observations of when and where birds were nesting occurred once captured., with specific locations of where the bird is and where it went were stored in through GPS tracking. Despite this relatively cool year, we were still able to provide evidence that heat waves could affect songbirds in terms of weight. Given computational constraints associated with our sample size, we were also not able to control for site-specific effects in our models (e.g., by including a random effect for site). While sampling effort and management practices across our focal vineyards were similar, future research can provide even stronger evidence about interactive effects between temperature and land use by controlling for confounding factors associated with site differences.

Conclusion 

As climate change continues to progress, it will become more critical to maintain and restore natural habitats in agricultural landscapes. In addition to protecting the biodiversity that currently persists alongside us, conservation efforts in agriculture can help increase landowner knowledge about species in the area, landowner satisfaction, and inspire community-led management of ecosystems [17]. Crucial to these efforts will be building lasting relationships between researchers and communities, which are necessary to the monitoring and continuation of conservation actions that benefit the people and organisms who rely on agricultural landscapes [17]. Agriculture will continue to be necessary to meet growing food demand, and creating agroecosystems with climate adaptation measures in mind can support the intertwined needs of nature and people for generations to come.

About the Author: Kristalyn Mendoza

Kristalyn Mendoza is a class of 2024 Alumni. She graduated this past summer and majored in Animal Biology. She chose this major because she is interested in Veterinary medicine and hopes to go to Vet School to pursue a specialization in Avian medicine or general practice. Kristalyn chose to write this paper because part of the Animal Biology major is students doing research with an animal of their choice. Kristalyn really loves birds and wanted to focus her research on any avian species. She collaborated with a P.hD. candidate Cody Pham on this project, she looked to see how effects of temperature spikes in agricultural systems and how these spikes impact the foraging behavior and physiological effects of adult songbird populations compared to adult songbirds in buffered microclimates. She wants readers to look at the general implications of the findings and how we can relate them to the overall conservation of not only birds but other species and habitats in agricultural environments in regards to the rising climate.

Author's Note

This piece was conducted as part of my undergraduate research project as an Animal Biology major. I’ve always been interested in avian biology and ecology and wanted to focus my research entirely on this subject. The effects climate change has on birds in particular has always been important to me as they are crucial keystone species, especially in local habitats as well as in agricultural areas as they serve as a natural form of pest control. I found myself reaching out to Dr. Daniel Karp and he navigated me to work with his Ph.D candidate, Cody Pham, who I’ve had the privilege of collaborating on this project with. I was inspired by his work so I decided to focus my project on a similar aspect. I wanted to focus specifically on the effects of temperature spikes in agricultural systems and how these spikes impact the foraging behavior and physiological effects of adult songbird populations compared to adult songbirds in buffered microclimates. While reading this paper, I would like the reader to understand the broader implications of the findings and how they pertain to species and habitat conservation in agricultural systems in regard to climate change.

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