We encourage all eBirders to make their best estimate of bird numbers on every checklist. Most bird counting is really easy—you just identify what you see, and add the numbers together as you go. However, every now and then you'll be fortunate to run into a big flock of birds, multiple flocks, or big groups of mixed species. This is when it can get tricky, and where we're here to help.
The most important thing to remember is that your best estimate of numbers is always more useful than putting an “X’ to indicate presence. An “X’ could be 1 or 1,000,000! By making your best estimate, you're providing valuable information on the abundance of birds around the world: not just where the species is, but also how many there are. This allows researchers to understand change in numbers over time, which is often one of the most important measures of how well a bird species is doing.
When counting birds, it's also important to keep in mind that general numbers are perfectly fine. If you estimate a flock as being 50 birds, and in reality it was 40, or 63, that's fine! Of course, if you can count exactly to 63, that'd always be better, but we understand that this isn't always possible.
Using the simple techniques outlined below, you can come up with estimates for even the most difficult bird counting situations. Soon you’ll have a good idea of how many birds are present at your local birding sites with a glance. The best part is that it gets easier the more you do it!
Snow Goose by Jan Allen/Macaulay Library (ML139354761)
Bird Counting Best Practices
Count in the moment!
The first law of bird counting is that you need to get in the habit of noting things down as you go. We recommend keeping running totals for all species observed at least every 15 min to keep things manageable. The Quick Entry feature on eBird Mobile makes this extremely smooth! Keeping multiple, shorter eBird checklists also makes counting more manageable by preventing numbers from getting too big (shorter checklists are better for science anyways!).
We promote conservative counting in eBird. The key is to aim for your best count of the birds you saw and heard in the area you sampled. The goal should not be to have the highest count of a given species in eBird, but to provide an accurate count of what is present in the area you surveyed.
If you see a male Northern Cardinal in the first five minutes of your walk, and then a female later, count two. But if you see a male Northern Cardinal in roughly the same place on your way back, we recommend leaving your count at one. If you saw a male Northern Cardinal at the beginning of your walk, and then another .5 miles away, count two! Use common sense, and try your best to come up with the most accurate, yet conservative count.
You can also recognize individual birds in different ways, often due to obvious differences in the age or sex of the bird or other characteristics. Examples include male vs. female Northern Cardinals, white-browed vs. brown-browed White-throated Sparrows, or a molting House Finch. Individual differences can stand out and help ensure you’re counting all birds present.
Beware false precision
If you are counting birds by fives, tens or hundreds, adding a more precise count for the same species results in false precision. Do not add exact counts to estimates of a single species within a checklist. In other words, always keep your reported number to the lowest level of precision used when counting a species. If you counted a large flock of Surf Scoters by tens for an estimated total of 1110 birds, and then a flock of 3 birds passes by, you should only report a total of 1110 birds, not 1113. If an additional flock of 5 birds then passes, you can bump up your reported count to the estimate of 1120.
Types of Bird Counting
Counting groups of one species
Counting birds is usually simple, like when you’re counting a single species at a fixed location. Six Common Mergansers on a log are straight-forward to count because we can see and identify all individuals.
Common Merganser by Chris S. Wood/Macaulay Library at the Cornell Lab (ML67827951)
Larger flocks might seem trickier, but counting by a small grouping and then extrapolating that out to the entire flock makes it pretty easy. The only change is really that instead of counting birds by 1s, you're counting by larger increments: 5 at a time, or 10 at a time.
White Stork by Richard Gray/Macaulay Library (ML67512001)
This small flock of White Storks in flight is straightforward to count. As they pass by, you can count them one-by-one, ending up with a count of 19. Once numbers get larger, the one-by-one is left behind. Before reading the text below the next image, how many do you get if you count that flock by 10s?
White Stork by Paul Chapman/Macaulay Library (ML42551781)
The strategy here is to count the first ten birds in the flock, get a sense for what proportion of the flock they take up, and then extrapolate by tens through the rest of the flock. How many do you get when you looked? We got ~100 birds when counting by tens. The actual count of birds in this image is 102—with that as the known number, 100 is great for a snap estimate in the field!
Counting large, stationary flocks
When large numbers of birds are blanketing an area, it can feel overwhelming to get a count. We have the tools here to help you overcome this!
As with counting groups of one species, we'll start by figuring out how many birds occupy a subset of the group and then extrapolate to come up with an estimate. People generally under-estimate numbers when they encounter large flocks, so it’s important to understand how closely the birds can be packed in and the effects of distance and depth. The three-dimensional nature of some groups can be hard to comprehend. If you have a group of birds that's 100 long, 100 high, and 100 deep contains one million individuals!
To start counting a big flock, first figure out what your 'larger-than-1' counting interval is. Does counting by 10s seem manageable? 100s? 1000s? Whatever you decide on as a starting point, count that number one-by-one, and then use your template to count across the entire group.
Redhead by Jay McGowan/Macaulay Library at the Cornell Lab (ML43492371)
Let’s try with these ducks on Cayuga Lake by Ithaca, the home base of the Cornell Lab of Ornithology!
If we start our counting by figuring out how about 30 birds appear in this flock we get the following result.
Once we have a picture of what proportion of the flock is made up of about 30 birds, we can then extrapolate across the rest of the flock.
There are six segments to this flock, but they are not all equal. The same visual proportion of the flock at the rear is nearly twice the number of birds in the front. Keep in mind is that you are looking at birds in two dimensions here, width and depth. As the birds are farther away, their depth increases as does their width, so adjust your estimates accordingly.
With a straight extrapolation we’d estimate 180 ducks in this flock, but there are actually ~340 birds. If you are estimating in blocks when depth is involved, you'll need to scale your estimates accordingly. This illustrates how large flocks of birds are often under-counted. Remember, while we strive for accuracy, it’s always better to err on the conservative side!
Counting large, moving flocks
There are two ways to count large flocks of moving birds: using a 'larger-than-1' counting interval again; or by counting rate: birds passing per unit of time. Your method depends on the situation you face and the species you’re dealing with. If it's a single flock of several thousand birds heading your way, count by increments. If you have a consistent flow of birds moving past in a clear pattern, then you may be better served by counting rates.
Sooty Shearwater by Griffin Richards/Macaulay Library at the Cornell Lab (ML71751951)
The rate technique works well when you have a consistent flight that lasts for a certain period, from several minutes to many hours. At times there are so many birds that counting individuals becomes impossible (in this case Sooty Shearwaters along the Pacific Coast), but the rate at which they are passing by the count site is relatively consistent. You count individuals for a specified unit of time (e.g., 1 minute or 5 minutes) and then extrapolate the count over a specified block of time. If you had 1300 shearwaters/5 minutes, and the flight remained consistent for 15 minutes, you could safely extrapolate a total of 3,900 shearwaters. This is the best technique for coming up with a reasonable estimate of overall numbers. It’s always important to constantly monitor the flight for changes in volume when using this technique. If you note fewer or greater numbers passing, it’s time to make a new rate estimate. This ability helps for counting several species at once, which can make counting diverse flights more manageable.
Big numbers of multiple species
Multiple species flocks are the most complex challenge, but like the other examples can become manageable. There are two main approaches to counting flocks like these.
- for each species, count the total number of individuals using the best method for each, or
- estimate the total number of individuals and the proportion of each species within the flock.
Glaucous-winged Gull by Andy Wilson/Macaulay Library (ML166815011)
Let’s say we have a large flock of gulls on the Pacific Coast. We could count within a block and extrapolate as with our earlier diving duck example, or we can try estimating proportions. If the composition is 50% Western Gulls, 30% California Gulls, 10% Glaucous-winged Gulls, and 10% Heermann’s Gulls and we estimate 10,000 individuals, we’d have 5000 Western Gulls, 3000 California Gulls, 1000 Glaucous-winged Gulls, and 1000 Heermann’s Gulls. Be aware that each species won’t necessarily evenly distributed throughout the flock. Make sure to scan all the birds before extrapolating numbers to the whole flock.
Let’s try each method with this large mixed shorebird flock from Reed’s Beach, NJ, a prime spring stopover for migratory shorebirds in the Delaware Bay. This is an especially challenging example since the Red Knots are not evenly distributed in the photo, and there are Semipalmated Sandpipers, Sanderlings, and Ruddy Turnstones mixed in throughout the entire flock but they are obscured in the crowd of knots in most of the photo.
Red Knot by Cynthia Ehlinger/Macaulay Library (ML160521301)
To start we’ll estimate the total number of birds in the flock. Counting by 10s, we estimate there are ~1,400 shorebirds in total. Wow! Next we estimate that the flock is ~60% Semis (remember they are mixed throughout), 35% knots, 3% turnstones, and 2% Sanderling. That breaks down into 840 Semipalmated Sandpipers, 490 Red Knots, 42 Ruddy Turnstones, and 28 Sanderlings!
For counting each species separately, we’ll start with the Semipalmated Sandpipers and other shorebirds because they are evenly distributed in the photo. Portions are obscured by the mob of knots, but we can use the gap without knots to estimate the peeps and extrapolate. There are ~380 Semipalmated Sandpipers (by 10s), 6 Ruddy Turnstones, and 10 Sanderlings occupying the more open patch. This is ~40% of the area occupied by the flock (remember depth!), so we estimate 950, 15, and 25, respectively, in the photo. Since the uneven distribution of Red Knots makes extrapolation difficult, we counted by tens and came up with ~680. Notice that this adds up to ~1,700 birds, a higher total estimate than before (which was an underestimate since we couldn’t click all the hidden Semipalmated Sandpipers!).
Compare the differences between the two methods: counting each species individually yielded more accurate totals for the dominant Semipalmated Sandpipers and Red Knots, while estimating by proportions produced what are probably more accurate totals for the less abundant Ruddy Turnstones (there are 38 in the photo) and Sanderlings as well.
|Species||Individual Species Counts||Estimating Overall Proportions|
|Total Flock Size||~1,700||~1,400|
Each method has its perks and drawbacks, but both of them got us safely in the ballpark of a conservative estimate for all species present in the flock: valuable data when entered on a complete checklist. Use whichever works best for the situation you’re facing. Phew! That was a tough one, but we did it! Luckily most birding situations are vastly easier to count.
Birds at feeders
Whether your feeder is nectar-filled and aswarm with frantically feeding hummingbirds or a thistle tube liberally coated with finches, it can be hard to know how many birds are truly there. We recommend reporting the highest number of individuals seen at one time during the observation period, as well as any clearly different individuals. Although there may actually be more individuals, it’s the most reliable method for these situations. Obviously if you see 6 female Northern Cardinals and 3 males, and later see 6 males together, then you have at least 12 different cardinals at your feeder, and your checklist should reflect this.
Juvenile Red-headed Woodpecker by Sue Barth/Macaulay Library at the Cornell Lab (ML64213381)
Have fun counting birds and contributing to science in eBird with your newfound skills!