Some birds show distinctive male and female traits. Others are more difficult to discern. Because many avian pairs share nest sitting duties and the rearing of young, the bird with the young can be either sex. There are exceptions, however, and ducks are an example.
In the case of the common merganser duck, the males and females are completely different in coloration. The males have dark heads and white sides but are rarely seen after mid-June as they leave our lakes (and the females on the nests) and head back to the ocean. (Yes, they are dead-beat dads.) The females have chestnut heads and grey sides and this is the coloration that the ducklings develop initially. Mallard males are brightly colored while the females are demur and this is the color of their ducklings, too. Because these duck females sit on the nests (with the males not around), they need to be well hidden so these differences in coloration are appropriate.
Male and female loons are virtually identical in appearance. The males do tend to be a bit larger than the females but that is only an average so the size is not always a determinant. The patterns of their feathers are the same for both sexes. (I guess they can tell which sex is which or we won’t get baby loons.) When loons are caught for banding, one source says that it is easy to tell the sex of the bird because males are vocal when caught and the females are. If it talks loudly, it is a male.
Male and female eagles are also hard to tell apart, especially when they are by themselves. However, if two eagles are side by side, the larger bird (generally about 50% larger) is the female. The difference is size is easy to see if they are next to each other but hardly a good determinant of the sex of the bird when they are alone although the female beak and talons are also more massive than the male’s. With adults, the male does have a pure white head while the female has grayish feathers on the side of her head that has been described as “eye shadow”. These differences are not easy to see except when you are relatively close and have a good pair of binoculars. (Eagles do like their space so stay 100 yards away. At the same time, they may land closer to you but that it their decision……)
Stress is a problem for all birds and getting too close will cause stress. Good bird photographers keep their distance and use a very long lens with a high resolution camera. Then, the picture is cropped so that even taking a picture of a bird at 100 yards, a good image fills the frame.