This is the 12652nd most frequent Polish word.
niebieskich
The Polish word "niebieskich" means "blue" (genitive or accusative plural form).
In this sentence, 'niebieskich' is the plural genitive masculine or animate use, qualifying 'ptaków' (birds).
Widzę niebieskich ptaków na drzewie.
I see blue birds on the tree.
Here, 'niebieskich' describes 'płatkach' (petals), using the plural locative masculine or neuter form.
Kupuję kwiaty o niebieskich płatkach.
I am buying flowers with blue petals.
In this case, 'niebieskich' stands alone as an adjectival noun referring back to a previously mentioned group.