Assertion: wing feather | /r/CapableOf | grow heavy
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Source sentences:
text: The presentation explains that by the time an eagle reaches the age of 30 or so, its physical condition has deteriorated to the point that survival is difficult: its talons lose their flexibility and cannot properly grip prey, its beak becomes dull and bent, and its wing feathers grow thick and heavy, sticking to its chest and impairing its flight.