Questions & Answers about Одна чайка летела низко над водой.
Because одна agrees with чайка, and чайка is a feminine singular noun.
Russian один changes for gender:
- один — masculine
- одна — feminine
- одно — neuter
So with чайка (a seagull), you get одна чайка.
In this sentence, одна most naturally means one / a single seagull.
It can suggest one seagull or a single seagull, depending on context.
Russian has no articles (a / the), so чайка by itself could often mean a seagull or the seagull depending on context. Adding одна makes the idea of one single seagull more explicit.
Sometimes одна can also mean alone, but in this sentence the most natural reading is simply one seagull.