'op is a noun, so it can participate in a noun-noun construction. The first noun modifies the second noun. Birds. What birds? Some birds. All other noun-noun constructions work like this. tlhIngan Hol A language. What language? The Klingon language.
'op is not a verb, which could be used after the noun to modify it. chuS be noisy is a verb. bo'Deghmey chuS noisy birds.
(And yes, these can be combined. 'op bo'Deghmey chuS some noisy birds.)
Do not be fooled by the fact that English can use nouns like adjectives and then just treats them like all other adjectives. Klingon treats verbs used adjectivally (which go after the noun they modify) differently from nouns used adjectivally (which go before the noun they modify).