Complex sentence: The parents brood the chicks for up to three weeks after hatching, feeding them intensively for the first few weeks, then gradually less frequently until they reach independence at nine or ten months old.
Split into simpler sentences: The parents brood the chicks for up to 3 weeks after hatching. They feed the young intensively for the first few weeks, and with decreasing frequency until they reach independence at 9 or 10 months old.