- A global meta-study investigating the effects of climate change on bird reproduction has found an overall decline in annual offspring production over 50 years.
- Offspring production in migratory and large birds decreased, whereas it increased in smaller-bodied and sedentary species.
- Migratory species may find it difficult to adapt to a rapidly changing climate in their breeding and wintering areas. Large birds may have problems adapting to changing climate conditions because they tend to live a ‘slower pace of life’ and have offspring only once a year.
- Curbing our carbon dioxide emissions will halt rising temperatures impacting bird species, say the study’s authors. People can also help boost bird reproduction by creating a cooler environment for bird nests in yards and parks through dense vegetation during heat waves.
Climate change has been shown to affect the timing of reproduction in birds. Studies investigating the effects of higher temperatures have found that many bird species have started breeding earlier than in the past.
However, little is known about the impact of climate change on bird offspring production.
Now, a new study has examined changes in annual offspring production in 104 bird species around the world over a period of 50 years — from 1970 to 2019 during which global temperatures have risen by about 1 degree Celsius. Published in May in the Proceedings of the National Academy of Sciences, the study assessed the findings of previous studies in a powerful statistical tool known as a meta-analysis.
The findings, based on 201 wild bird populations, revealed that the overall reproductive output of birds had declined. However, small-bodied and sedentary species experienced an increase in offspring production, indicating they may benefit from a warming climate, whereas large-bodied and migratory species showed reduced offspring production, suggesting they may suffer due to rising temperatures. These are general trends and the team cautions that there were many exceptions.
Climate warming affects offspring production in birds differently depending on the life histories and ecological traits of species such as their body sizes, migratory habits and the numbers of broods raised in the breeding season, say the authors of the study.
Some of the large-bodied species analyzed by the researchers that showed a general decline in offspring production included Adélie penguins (Pygoscelis adeliae), Montagu’s harriers (Circus pygargus) and bearded vultures (Gypaetus barbatus). The largest decreases in offspring production were observed in two populations of white storks (Ciconia ciconia) and one population of Montagu’s harrier and common eider (Somateria mollissima).
Small-bodied species that experienced an increase in offspring production included Eurasian reed warblers (Acrocephalus scirpaceus), red-winged fairywrens (Malurus elegans) and pied flycatchers (Ficedula hypoleuca). The bright yellow prothonotary warblers (Protonotaria citrea) and Eurasian reed warblers showed large increases in offspring production.
The team found 57% of bird populations showed a declining trend in offspring production over time whereas 43% of populations produced more offspring.
“We were surprised by a huge variation in response of various species and populations to climate change,” says lead author of the study Lucyna Hałupka, associate professor at the University of Wrocław, Poland. “We did not expect such a huge variation.”
As the rate of warming increases further, Hałupka warns that the proportion of losers could grow.
“The manuscript describes an overall slight in decline in offspring production over the last 50 years, although there is a lot of variability,” says Morgan Tingley, associate professor of ecology and evolutionary biology at the University of California Los Angeles, who was not connected to the research. “The study provides some potential evidence that warming climates are the cause of these declines in offspring production, particularly based on temperatures during the nesting period. The study seems to raise more questions than it answers.”
Scott Robinson, Ordway Professor of Ecosystem Conservation at the Florida Museum of the University of Florida, who was not involved in the meta-analysis, says, “This study is a great illustration of the powers of collaborative science and powerful new tools available to address large-scale processes.”
According to Robinson, “the loss of these migratory bird species, which seem disproportionately affected by climate change, will have cascading effects on ecosystems as we diminish the roles of these birds in controlling insect populations, pollinating and dispersing fruits.”
Further, he points out, “The species that benefit from climate change, many of which are resident species, will gradually replace the migratory species,” which would have unknown impacts on the ecosystem.
Large, migratory species versus small, sedentary birds
Proper timing is crucial for successful breeding, Hałupka stresses. “It is not good to start nesting too early but also not too late.” Migratory birds may find it difficult to adapt to changing climatic conditions as the rates of warming may be different in breeding and wintering areas, explains Hałupka. “Sedentary species spend the winter at or near their breeding sites and may adjust laying dates to local meteorological conditions.”
Regarding body size, Hałupka points out two important factors that may explain why large-bodied birds are vulnerable to warming temperatures. First, she says that “all large-bodied species are single-brooded,” which means they have offspring once a year; multi-brooded species have offspring more than twice a year.
“In single-brooded species, synchronizing their breeding cycle with the peak of food resources is often crucial for successful breeding. Even a small mistiming or lack of synchronization may have serious consequences for single-brooded species: The young obtain less food and some of them die,” explains Hałupka. In contrast, multi-brooded species rely on multiple types of foods, Hałupka notes, adding that they do not need such a strong synchronization.
“Second, large-bodied species are characterized by a so-called ‘slow pace of life’: They live long, mature slowly and hence their generation turnover is much slower. Therefore, large-bodied species may have problems with adaptation to rapidly changing environmental or climatic conditions.”
Scientists studying past extinctions of animals have found that body size is an important predictor of extinction, she adds.
Boosting bird reproduction
Bird populations around the world have been shrinking — even in protected areas.
“There is no single immediate cause for these declines, although it is clear that humanity’s transformation of the biosphere is causing multiple, interacting negative impacts on birds simultaneously,” notes Tingley of UCLA. In the face of a warming climate, how can we help birds reproduce? According to Hałupka, we should undertake measures to reduce our emissions of carbon dioxide.
“We should also care for habitat quality and try to stop habitat degradation,” she adds, noting that “the main reason for a current decline in bird population sizes is habitat loss and degradation.” With habitat degradation, populations may produce fewer young because of fewer suitable sites for nesting and a low number of breeding individuals, Hałupka explains.
In 2017, Tingley and his colleagues reported that nesting success among North American birds was highly susceptible to heat waves, particularly in the warm parts of species ranges that are closer to the equator. Consequently, he emphasizes the importance of providing thermal protection to birds and their nests during abnormally hot periods and heat waves, which, he says, may help boost reproduction.
“For birds that use nest boxes or other human-made containers, we may be able to experiment with different materials and designs that keep nests cooler during hot days. Even nest placement might help, such as attaching a nest box in a shady area, rather than in an area that receives full sun,” suggests Tingley. “For birds that build their own nests in trees or shrubs, even the landscaping we choose can help the microclimate of our yards, which can keep birds cooler. Creating natural habitats in yards or parks with dense vegetation, which stays cool and shady during the middle of the day, can also help provide areas for baby birds that are buffered from the extremes of a heat wave.”
While the meta-analysis was based on bird populations worldwide, some regions are well-studied while others tend to lack data. Hałupka says that her team spent several months gathering suitable data and contacted all the researchers who should have long-term data. Unfortunately, she says, there was a dearth of long-term breeding data from Asia, Africa and South America. “Furthermore, scientists from Asia I wrote to did not reply to my emails,” she reveals.
The study authors also cautioned that their data set was biased because the tropics and central parts of the continents were data-deficient. As a result, few studies from these regions were analyzed. Tropical regions host a high diversity of birds, and inland areas of continents experience stronger climatic changes than coastal areas.
Banner image: A boreal owl, a single-brooded species. Image by Markéta Zárybnická.
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Hällfors, M. H., Antão, L. H., Itter, M., Lehikoinen, A., Lindholm, T., Roslin, T., & Saastamoinen, M. (2020). Shifts in timing and duration of breeding for 73 boreal bird species over four decades. Proceedings of the National Academy of Sciences, 117(31), 18557-18565. doi:10.1073/pnas.1913579117
Halupka, L., Arlt, D., Tolvanen, J., & Halupka, K. (2023). The effect of climate change on avian offspring production: A global meta-analysis. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.2208389120
Socolar, J. B., Epanchin, P. N., Beissinger, S. R., & Tingley, M. W. (2017). Phenological shifts conserve thermal niches in North American birds and reshape expectations for climate-driven range shifts. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1705897114