A new study observes that most birds - not all - adjust their migratory patterns with green vegetation that changes with the seasons.
In a first study of its kind, researchers from the Cornell Lab of Ornithology inquired across the Western Hemisphere in pursuit of the year-long life cycle of North American migratory birds whose diets vary between small insects, seeds, nectar, and vegetation. Researchers published the details of their study in the Journal of Animal Ecology.
A Phenological Synchronization Among Birds
"As you might expect, migration synchronization with vegetation greenness is strongest for birds that eat vegetation, seeds, or both, during spring and autumn migration, but especially during spring," explains Frank La Sorte, lead author of the study from Cornell. He explains that birds follow the "green wave" northward during spring and then goes in reverse in fall, following a wave that precedes winter in North America.
This green wave, a moving pattern of maturing and dying vegetation, is visualized in the animation of data from MODIS - or Moderate Resolution Imaging Spectroradiometer - installed on the Terra (originally the EOS AM-1) and Aqua (originally the EOS PM-1) satellites. It showed Central and South America seeing mostly stable vegetation, save for its east side.
On the other hand, the same pattern observed in migratory birds is not applicable for carnivorous birds, like eagles and hawks, in the West for either part of the migration cycle. Also, evidence for the same behavioral tendencies remains weak among insect-eating birds in eastern and central regions of the United States during spring. Even birds that mainly feed on nectar, like hummingbirds, were also found to have migratory patterns weakly connected to greenness. The lack of synchronization among insect-eaters, according to the study, might be explained by the presence of the Gulf of Mexico - serving as a "large ecological barrier to migration."
Following Environmental Cues
To conduct the study, researchers used two approaches in observing the spatial and temporal connection between bird migration and vegetation. The first approach involved tracking individual birds, leading to positive associations during spring migration for herbivores and vegetation. The other approach uses data gathered from citizen science programs that monitor population-level associations. Researchers noted that while the second approach lacks "individual continuity," it covers a lot of species over wide spatial extents.
Additionally, the researchers cross-referenced the vegetation data with eBird observations for 230 different species of migratory birds for twelve years - from 2006 to 2018. eBird is an online database, driven by citizen-science efforts, that provides readily available data for both professional scientists and amateur enthusiasts.
Previous studies also suggested that the vegetation phenology, as observed in the study, is a response to global climate change. Aside from the seasonal growth and maturity of plants, the subsequent migratory patterns of birds also change in response. The Cornell Lab report notes that species that breed in central and easter parts of North America tend to migrate long distances. Additionally, climate change has caused earlier springs, longer seasons, and delayed autumns, which could adversely affect bird populations.
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