They usually contain from two to four light blue eggs. Both adult birds incubate the eggs for about 28 days by wrapping their feet around them, and care for their young until they become fully independent, about 10 weeks after hatching. Cormorants are expert divers, adapted naturally to forage under water for fish. Fully-webbed feet propel slim, streamlined bodies on dives usually from 8 to 20 feet. Greater depths are possible.
Eye muscles are specialized to allow acute vision both above and under the water.
Feathers absorb moisture, helping cormorants to stay under water for about 30 seconds. After foraging, cormorants often dry their feathers by perching in a distinctive wing-spreading posture. These chemicals caused reproductive failure, and chicks that hatched sometimes had crossed bills, club feet, and eye and skeletal deformities. In addition to contamination, human disturbance and nest destruction contributed to the decline. In the early 1970s the Great Lakes population had plummeted, with few birds remaining or breeding successfully.