A major cause: starvation. The birds have flown a vast distance direct from the far northern Pacific in accord with their annual migration and their life depends on finding food on their return. It is said that their diet of seafoods from our coastal waters is in short supply. Warm currents and wild weather events are likely causes of the shortage.
Experts are figuring out whether all this is within normal bounds.
Biodiversity is crucial for all life and for the food chain to continue. Humans reap the food chain like never before. Preservation of biodiversity is a global imperative. Permit your garden to grow bugs, insects, lizards and snakes - insectacide free! Limit plastic - it fills the oceans - most of it changed to minute forms of plastic.
They say no sardines were found off California this year - if that is to the point.