syntactic analysis - Identifying the subject of a complicated sentence

This sentence took me a lot of time to parse:

For migrant birds, which habitats are suitable during the non-breeding
season influences habitat availability, population resilience to
habitat loss, and ultimately survival.

_{(from Blackburn, E. and Cresswell, W. (2015), “Fine-scale habitat use during the non-breeding season suggests that winter habitat does not limit breeding populations of a declining long-distance Palearctic migrant”. Journal of Avian Biology, 46: 622–633. doi: 10.1111/jav.00738)}

Finally I figured out that the predicate is influences and the subject is hidden somewhere behind the "which. But is this correct English? What is the subject of this sentence then?