Rising temperatures are posing challenges for bird breeding, according to a recent study published in the journal 'Proceedings of the National Academy of Science.' Scientists from UCLA and Michigan State University have found that increasing global temperatures are making it harder for birds to accurately determine the onset of spring and the appropriate time for breeding.
The study reveals that birds experience a decline in their reproductive success if they start breeding either too early or too late in the season. Unfortunately, birds are struggling to keep pace with climate change, which has led to earlier spring-like conditions. This mismatch between the timing of spring and birds' readiness to reproduce is expected to worsen as global temperatures continue to rise, potentially having significant consequences for numerous bird species. Traditionally, bird breeding seasons coincide with the emergence of the first green plants and blossoms, which are occurring increasingly earlier due to rising temperatures.
Casey Youngflesh, the study's first author, who conducted the research as a postdoctoral researcher at UCLA and is currently a postdoctoral fellow at Michigan State, explains, "By the end of the 21st century, spring is likely to arrive about 25 days earlier, with birds breeding only about 6.75 days earlier. Our results suggest that breeding productivity may decrease about 12% for the average songbird species."
The authors emphasize the need for conservation strategies to address the responses of bird species to climate-driven shifts. For nearly three decades, scientists have speculated about the potential misalignment between animals and plants as springs begin earlier. While some case studies have provided insights into this phenomenon, it has remained uncertain whether advancing springs would pose a widespread problem for most species.
Timing plays a crucial role in bird breeding as it affects their ability to raise their offspring successfully. Breeding too early or too late can expose eggs and newborns to harsh weather conditions. Moreover, timing relative to food sources is also essential. If birds search for food before or after its natural availability, they may not have sufficient resources to support their young.
Morgan Tingley, the study's senior author and a UCLA associate professor of ecology and evolutionary biology, explains, "Critically, we found evidence for impacts on bird reproduction of both the absolute and the relative timing of birds."
To conduct the study, researchers utilized data from a comprehensive collaborative bird banding program conducted by the Institute for Bird Populations. They analyzed the timing of breeding and the number of young produced by 41 migratory and resident bird species at 179 sites near forested areas across North America from 2001 to 2018. Additionally, satellite imaging was used to determine the emergence of vegetation around each site.
The study revealed that each bird species has an optimal time for breeding, and the number of young produced decreases when spring arrives very early or when breeding occurs too early or late in relation to plant emergence.
While most bird species were negatively affected by variations in the start of spring, a few species such as the northern cardinal, Bewick's wren, and wrentit demonstrated improved breeding productivity with earlier springs. These species are primarily non-migratory and can quickly respond to the emergence of spring plants, which signals the start of the breeding season.
The study also highlights that non-migratory species, by breeding earlier and without the constraints imposed by migration, may have the ability to reproduce multiple times per season.
However, these exceptions are rare, and even most non-migratory species struggle to adapt to earlier spring arrivals. Overall, for every four days that leaves appear on trees earlier, species breed only about one day earlier.
For migratory species, this discrepancy means that the time between their arrival at breeding sites and the start of breeding itself is likely to shorten as spring-like conditions begin earlier. Birds require time to establish territories and physiologically prepare for egg-laying and rearing their young, so such changes could further disrupt their reproductive cycles.
Morgan Tingley concludes, "North America has lost nearly a third of its bird populations since the 1970s. While our study demonstrates that the most severe impacts of timing mismatch may not occur for several decades, we need to focus now on concrete strategies to boost bird populations before climate change takes its toll."
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