Indeed, birds may be careerists (7)
I believe the answer is:
yuppies
'careerists' is the definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps there's an association between them I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for yuppies that I've seen before include "Well-paid middle-class professionals with a lifestyle to match" , "Ambitious types" , "Young city professionals" , "Young urban professionals" , "aspirational" .)