The detrimental effects of a warming climate on global bird populations have been revealed in a study conducted by researchers from the University of California Los Angeles (UCLA) and Michigan State University. The warming climate is causing spring to arrive approximately 25 days earlier by the end of the 21st century, with birds adjusting their breeding season only about 6.75 days earlier. This mismatch between the early onset of spring and the readiness of birds to breed is projected to exacerbate further as the world continues to warm.
The research, published in the Proceedings of the National Academy of Sciences (PNAS), highlights how advancing spring-like weather due to climate change is disrupting birds' breeding patterns and leading to a decline in their reproductive success.
The researchers found that for the average songbird species, breeding productivity is expected to decrease by approximately 12 percent. Birds that start breeding either too early or too late into the season tend to produce fewer offspring, putting their populations at risk. The timing of breeding is critical for their survival, as harsh weather during breeding can harm eggs and newborns.
One significant consequence of the changing climate is the impact on food availability. With the shift in spring timing, birds may struggle to find food sources when they need them the most, leading to insufficient resources to sustain their young.
To conduct the study, the scientists analyzed data from 41 migratory and resident bird species at 179 sites near forested areas across North America between 2001 and 2018. Satellite imaging was used to determine the emergence of vegetation around each site, indicating the onset of spring.
The results revealed that, on average, every four days that leaves appeared on trees, indicating an earlier spring onset, caused birds to breed approximately one day earlier. While some non-migratory species were found to counter the trend, most couldn't adapt to the changing conditions.
Migratory species face an even greater challenge, as earlier springs lead to shorter breeding timeframes. After arriving at their breeding sites, these birds require time to establish territories and physiologically prepare for egg-laying and offspring-rearing before they can begin breeding.
Morgan Tingley, the study's senior author and a UCLA associate professor of ecology and evolutionary biology, expressed concern over the severe decline in bird populations in North America, where nearly a third of bird species have been lost since the 1970s. Tingley emphasised the urgent need for conservation strategies that address how bird species respond to climate-driven shifts to mitigate further losses.