The Willow Flycatcher Is a Bird Species with a Summer

The willow flycatcher is a bird species with a summer range throughout much of the United States. Which of the following would directly decrease the size of a willow flycatcher population in a given year?

A) the disappearance of a species that preys on willow flycatchers
B) the arrival of migrating willow flycatchers from populations in other areas
C) the emigration of male willow flycatchers that did not secure territories
D) the hatching of a larger percentage of eggs in the willow flycatcher population