Why do pelicans migrate?

According to the Cornell Laboratory of Ornithology ... Brown pelicans are resident to long-distance migrants. The seasonal movements of Brown Pelicans vary all across their range. Many Atlantic populations disperse northward in the summer after breeding and return southward in autumn, probably to follow concentrations of fish. Some Atlantic and Gulf coast populations migrate further south along the coast during the coldest months of the year. On the Pacific coast, pelicans leave the Gulf of California after breeding, cross the Baja peninsula, and migrate as far north as British Columbia, returning south to breeding areas by the next winter.