Size can be a valuable bird identification characteristic, particularly between very similar species, but judging bird size can be challenging even for experienced birders. With practice, however, all birders can learn how to estimate and compare bird sizes in the field quickly, easily, and accurately.
The size of a bird can quickly help identify the species, but how is a bird's size measured?
The length of a bird is typically noted as the body length from the crown to the feet or to the tip of the tail, depending on the proportions of the bird and where the longest measurement will be when the bird is standing or perched. If a bird has unusual features, however, its recorded size may be adjusted for a more accurate measurement for the "bulk" of the bird's body. For example, birds with very long tail streamers may not have those streamers included in general measurements, especially if the bird only displays streamers during the mating season.
Wingspan is another size measurement that can be crucial for identification. This measurement is the longest natural distance between the tips of a bird's wings as they would be spread while soaring. For most birds this measurement will cross over the shoulders, but in species with long, forked wings, the wingspan is measured from tip to tip crossing lower over the body.
Making Size Comparisons
Few birders take rulers or tape measures into the field, and even fewer birds will sit still to be measured. Birders who are skilled at making bird size comparisons with other known measurements, however, can quickly judge the sizes of birds even if they can't measure each bird in inches or centimeters.
When observing a bird and trying to determine how big or small it is, make these types of comparisons:
- To Known Birds: One of the easiest ways to judge a bird's size is to compare it to birds you are familiar with. For example, note if an unusual bird is closer in size to an American robin, a red-tailed hawk, or a house sparrow, or any other birds you know well. That general comparison will give a good basic clue to the mystery bird's size. This may not be a close enough estimate for a positive identification of very similar birds, but it can be helpful for many species.
- To Visible Landmarks: Comparing a bird to visible landmarks in its habitat is an excellent way to judge its size and can lead to exact measurements if you are able to later measure those landmarks. For example, if a bird is perched on a fence, note how it compares in size to the fence post or railing, then measure that railing to get a rough estimate of the bird's size. Birders can use the same trick with backyard feeders, comparing birds against the size of feeders. Branches, leaves, flowers, and rocks are also handy measuring tools.
- To a Mixed Flock: If a mystery bird of an unknown size is part of a mixed flock, compare it with other birds you can identify in that flock, such as different types of sparrows, shorebirds, or gulls. Later you can compare known measurements for the birds you identified to determine a rough measurement of the mystery bird's size, and use that extrapolation as an identification clue.
- To Body Parts: Comparing a bird to itself is a great way to judge its size and take careful note of field marks that can be crucial for identification. For example, how does a bird's bill length compare to the length of its head (a key difference between downy woodpeckers and hairy woodpeckers)? How long is its neck? How does the head or tail measure in proportion to the bird's body? Not only will these observations give you great clues about a bird's size, but you're more likely to notice other clues and markings to help you identify the species properly.
Create a Measuring Station
Backyard birders who have trouble judging a bird's size can create a simple measuring station birds will never realize is there. A few days of observation will show where birds regularly perch, and simple steps can be taken to create a ruler near that perch.
If backyard hawks regularly perch on a fence, for example, painting a line 4-6 inches below the top of the fence creates an instant ruler – birders can then quickly note the rough length of a bird's tail while it is perched. Similarly, markings made at regular intervals along the top of a fence or tree branch can help birders estimate a bird's length.
Knowing how to judge a bird's size can be a valuable skill when making proper identifications. While size is often difficult to judge, visual comparisons and keen observation can help any birder measure up and know how big or small the birds they see really are.