Multiple Choice, Single Answer

Read the passage and answer the question.

Emperor penguin colonies in Antarctica depend on a delicate balance between sea ice extent and breeding timing. Adults arrive at coastal colonies in autumn, well before the sea ice reaches its most stable state, and incubate eggs through the harshest winter months. If the ice forms too late, birds cannot reach breeding grounds in time; if it breaks up too early the following spring, chicks that have not yet grown waterproof feathers can be swept into the ocean and drown. Researchers monitoring several colonies have found that years with unusually early ice breakup correlate strongly with near-total breeding failure, even when adult survival rates remain unaffected. This suggests that chick mortality, rather than adult population decline, is the more immediate threat posed by shifting ice patterns.