What is the specific structural difference aiding experts in categorizing long-distance migrants like the Swainson’s Thrush versus shorter-distance migrants like the Hermit Thrush?
Answer
Longer primary projections
While exact age determination often requires banding, ornithologists can sometimes gain subtle clues about a bird's migratory class by examining physical characteristics related to flight efficiency. The text points out that long-distance migrants, exemplified by the Swainson's Thrush, exhibit a characteristic known as longer primary projections when compared to shorter-distance migrants such as the Hermit Thrush. These primary projections relate to the structure of the primary flight feathers. This subtle structural difference is indicative of the aerodynamic needs required to sustain multi-continental flights versus shorter, less arduous journeys, allowing experts to categorize the bird's likely migratory habit.
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