Shocking for a bird when taking temperature (9)

I believe the answer is:

startling

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'shocking' is the definition.
(startling is a kind of shocking)

I don't understand how the rest of the clue works.

(Other definitions for startling that I've seen before include "Alarming due to rattlings" , "Very surprising, astonishing" , "Remarkable" , "Rattlings of alarming kind" , "Causing shock" .)

I've seen this clue in The Guardian.

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