Complex sentence: Insects are the main food in the breeding season, but Blackcaps survive mainly on small fruit for the rest of the year, and garden birds will also eat bread, fat and peanuts in winter.
Split into simpler sentences: Insects are the main food in the breeding season, but, for the rest of the year, Blackcaps survive primarily on small fruit. Garden birds will also eat bread, fat and peanuts in winter.