This is the 12737th most frequent Polish word.
nieregularnej
"Of irregular" or "irregular" (genitive singular feminine form).
Here, 'nieregularnej' is used as an adjective modifying the noun 'formy', describing the irregular nature of the shape.
Kształt kamienia był bardzo nieregularnej formy.
The shape of the stone was very irregular.
In this sentence, 'nieregularnej' is attributing the characteristic of irregularity to the noun 'migracji', explaining the unpredictable movement of birds.
Zauważyliśmy zjawisko nieregularnej migracji ptaków.
We observed the phenomenon of irregular bird migration.
The word 'nieregularnej' is describing the noun 'strukturze', explaining the uneven or inconsistent nature of the area's structure.