Scientists have observed that polar bears are moving farther inland and hunting and eating more birds in arctic regions. What is the best explanation that can be made using this information?

Climate change has pushed the polar bears south in search of nutrients.
Shrinking sea ice has forced the polar bears to seek new food sources.
With cooler temperatures in winter, the birds move into the polar bear's habitat in search of prey.
With warmer temperatures in the summer, the birds are producing more young in the Arctic.