This subject was suggested by a reader, and it's a good one because people ask this question all the time. The two types of birds are quite similar in several important ways. Both swim underwater (for feeding, and sometimes it looks like they're just having fun). In addition, you will see both types drying their outstretched wings - unlike ducks and other waterfowl, they have no oil in their feathers to help shed the water. This helps with underwater swimming, but also means that air drying is the only method to dry off for flying. Both types are also very social.
In the area where I live, I now see mostly Anhingas. They nest in the Mangroves. As nestlings they are tan, but the necks and heads of adult females will become more of a buff color. There are typically 3-4 eggs to a nest and I counted about 8 such nests in the Mangroves by this lake.
The Anhinga has a pointed beak which is critical to hunting for food. It will spear its prey with that beak and then surface with it. It will then toss the speared prey up into the air and swallow it on the way back down. They are expert swimmers - they can dive 60 feet and stay underwater for a minute or more. They propel themselves with webbed feet (duckish) and steer by using their wings as rudders.
The adult Anhinga has a black and white pattern on its back and wings that is very visible while drying its wings.
The male Anhinga maintains a black head and neck. This one below is still a juvenile with some "wild hair".
The Cormorant is a little stockier than the Anhinga, but it swims just as well. It's beak is quite different, however. It's hooked at the end, not pointed, and the Cormorant grabs its prey using that hook. Technically, the name is a Double-Crested Cormorant, but the crests on the head are not typically visible. They also have a pattern on their backs and wings, but it's far less obvious.
In my area, I actually see far fewer Cormorants than Anhingas. The ones I have seen were congregating on a very small outcrop in the lake, and when that outcrop disappeared a few years ago I have seen almost none here in this lake.
Fun Fact: The Anhinga is also known as a "snake bird". Due to the lack of oil, both of these birds float quite low in the water and the body is not always very visible. Because of the Anhinga's skinny neck, it looks like a snake sticking up out of the water.
So, in short, the best way to distinguish these two birds is by the beak: pointy or hooked!!!
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