Complex sentence: Food is caught with the serrated beak from between the riverbed gravel; often the birds dive for prey, repeatedly submerging for a quarter-to half-minute with only a few seconds' pause between dives.
Split into simpler sentences: Food is caught with the serrated beak from among the riverbed gravel. Often the birds dive for prey, repeatedly submerging for a quarter-to half-minute with only a few seconds pause between dives.