A bird’s size is often one of the first things many people notice about a bird, and size is an important clue to a bird’s identity.
Field guides will normally show the size of the bird, measured from the tip of the bill to the tip of the tail. Knowing the approximate size is a good first step in trying to identify the bird.
Size can also be a source of confusion. It can be difficult to judge distance and size, especially when a bird is flying. Even birds at rest can sometimes cause confusion.
When using size, try to compare the bird to a nearby object of known size. This can be a known nearby bird (such as a robin or crow), or bird feeder or cola can. Whatever is nearby. Keep an open mind on size if you are inexperienced and there are no nearby birds or objects to provide a direct comparison.
Warning: Size is hard to judge if another species or object of known size is not also present for comparison. Distances can be difficult to judge and lighting and surroundings can cause confusion. The Northern Mockingbird (left) in the image above is 10 in. long, the Blue-gray Gnatcatcher is 4.5 in. long. The mockingbird is just further away from the camera.
While size is important, it use is often limited to eliminating certain family groups. If an unknown bird is about the size of a robin, the bird is certainly not a duck or an eagle. But there over 100 species in several family groups that are approximately the size of a robin. Additional elements, such as color, behavior, location and habitat will be required to correctly identify the species.