Green-up phenology, or the period when winter ends and new vegetation starts sprouting, is now happening sooner due to climate change. The unfortunate reality is that many birds are not adjusting fast enough, particularly birds that migrate long distances. In the study “Decoupling of bird migration from the changing phenology of spring green-up”, the authors studied the migration patterns of 150 Western-Hemisphere species of birds from 2002 to 2021. Data from eBird, which many local birders have contributed to, was utilized to form insights.
What the researchers discovered is that most of these birds base their actions on deep-rooted behavior driven by, for instance, guidance from parents or seasoned migrators. This is known as climatological synchrony and is a good way to conserve energy because the next step is already known. The opposite of climatological synchrony is current synchrony. With birds that follow entrenched learnings, there is not as much flexibility to deal with changing circumstances compared to those species that observe current synchrony and are constantly looking for their next food source. It is important to note that much about migration is still waiting to be understood; however, a few different migratory cues have been identified including temperature, length of day, the stars, landforms, the magnetic field of the Earth, and what is encoded in their genetics. Further research into the study of migration can help us better identify the burden of climate change for individual species.
Photo of a Veery, one of the birds that observes climatological synchrony during migration (by Camerauthor Photos @camerauthor)
The authors first pinpointed the time-based “midpoint” of spring migration, specifically the average of all observation days for each species by year, and noted the location of where that midpoint occurred as well. To ascertain the mid-green-up day for every identified area by year, they used the Moderate Resolution Imaging Spectroradiometer (MODIS) Global Vegetation Phenology product (MCD12Q2). This second midpoint represents the day that the MCD12Q2’s Enhanced Vegetation Index’s amplitude is estimated as 50% of its maximum.
For the climatological green-up value, the researchers computed the long-term average mid-green-up day of their 20-year dataset for each location in question. Additional analyses included using different models to determine whether a species is more inclined to climatological or current synchrony and to identify green-up synchrony strengths for different species traits.
Per the research, 103 out of 150 species of birds were more prone to rely on ingrained behavior than make on-the-fly adaptations. The authors also determined that green-up as a whole was happening earlier within migration routes. Land types that showed meaningful trends in earlier green-up included Grassland, Woody Savannah, and Evergreen Needleleaf Forest. Only two types showed the opposite trend: Savannah and Open Shrubland.
Photo of a Blue Jay, a bird that has begun adjusting its behavior due to climate change (by Ruben Ortega @garigol)
An evaluation of the phenological interval, or the gap between the temporal migration midpoint and the mid-green-up day, showed that changes in this interval (signifying asynchrony in timing) are much more often driven by changes in the mid-green-up day versus the migration midpoint. However, an interesting detail is that for herbivores and shorter-distance migrants, green-up timing had a smaller effect, and migration timing had a larger effect on the phenological interval. Herbivores and shorter-distance migrants may have an easier time then dealing with changes in vegetation green-up. However, they are not immune to the effects of climate change.
Although further research needs to be done, there is already strong evidence that migrating birds, in general, are not able to keep up with changing green-up trends. Examples of such birds include Green-Winged Teals and Veeries.
The article “As Spring Shifts Earlier, Many Migrating Birds Are Struggling to Keep Up” goes into detail about why this is such a concern: resources, including plants and the insects they attract, become less abundant later in the season, and birds that come too late miss out on food they rely on. This could lead to fewer offspring or even death. There is evidence that some birds are nesting earlier, such as Yellow Warblers and Blue Jays, which means that their young can feed on more insects. However, it may not be enough as changing weather patterns demand too many behavior adjustments from birds. The onus is really on humans to do what we can to help birds in the face of climate change.
Photo of a Yellow Warbler, another bird that has begun adjusting its behavior due to climate change (by Patrice Bouchard @patriceb)
Sources:
Decoupling of bird migration from the changing phenology of spring green-up | PNAS
https://www.audubon.org/news/spring-shifts-earlier-many-migrating-birds-are-struggling-keep
https://www.hcn.org/articles/climate-change-is-happening-too-fast-for-migrating-birds/
https://blog.nature.org/2024/05/05/earlier-springs-cause-problems-for-birds/