Bird Counting 101

By Team eBird 23 Feb 2012
Sooty Shearwater Ardenna grisea

At eBird we encourage you to make your best estimate of bird numbers on every checklist. Your best estimate of numbers is always more useful than putting an “X’ to indicate presence. An “X’ could be one or it could be a thousand! We can always simplify numbers back to presence/absence for analysis. Your estimates of numbers help us judge relative abundance, and are an important part of checklist data. So what do you do if you’re having trouble counting birds? Large flocks of birds are always challenging, and in this article we’ll talk about some techniques for estimating numbers so that you can practice and become more proficient at counting birds. This is the first of two articles discussing techniques for counting birds.


This is Bird Counting 101, the first of two articles intended to help you learn how to count birds. This introduction to bird counting deals with basic concepts, why estimating numbers is important, and illustrates some examples of techniques for counting single-species flocks.

If you do not want to count birds, don;t have the time to, or are not able to, you can always enter an ‘X’ to indicate that the bird was present. Please do not enter a ‘1’ unless you saw one and exactly one bird. We get regular eBird submissions that include 20+ species with counts of ‘1’, and surely in these cases ‘X’ should have been used. If you mean to indicate presence, but no specific count, please use an ‘X’.

Better yet, read the two bird counting stories and try to estimate the number of birds present.

The Importance of Counting Birds

At eBird, we receive regular emails from users who say that they’d like to count birds, but fear that their estimates will be way off. Instead they simply report presence/absence on their checklists. Your best estimate of numbers is always more useful than putting an “X’ to indicate presence. An “X’ could be one or it could be a thousand! We can always simplify numbers back to presence/absence for analysis. Your numeric estimates help us judge relative abundance, and are an important part of checklist data.

“What if my numbers are way off the mark,” you say? Think of it this way: what do counts of birds mean biologically? Biologically, there is a huge difference between zero and one. The species was either not present, or only one was detected. Either way, this tells us that the species in question is not particularly “common” in the area you sampled at the time of your observation, or at least you didn’t detect it commonly. There is an almost equally significant difference between one and two. A record of one Painted Bunting in New Jersey could well be chalked up to a vagrant outlier, but two would be an event! But between three and ten biologically, we are essentially learning the same thing–that the species in question was found in relatively low numbers. Between 50 and 100 a species can be considered common, and between 1000 and 5000, there is not much biological significance; the bird in question was found in abundance! Essentially, by estimating numbers you’re telling us there was one, two, quite a few, a lot, or tons!!!

We’ve prepared this introductory document that we hope will help inspire you to start reporting numbers–and do it confidently! Scientists have been trying to figure out how to count birds accurately for years, and much debate exists over the best scientific methods for doing so. As birders we can use the simple techniques outlined here to do our best at counting, and ultimately provide another valuable metric for scientists wishing to analyze our data.

Bird Counting Basics

Okay, so you get why it’s important to count birds, but how do you go about doing it? Here are some bird counting basics that we hope will inspire you to start keeping numbers of all the species you’ve seen and heard on every checklist you submit!

Write it down!

The first law of bird counting is that you need to get in the habit of writing things down. Too often birders go out birding for several hours without a notebook and then try to recreate what they’ve seen after returning home. Typically you find yourself asking things like, “Did I actually see Mourning Dove on my walk today? I know I usually see them, but I can’t recall for sure today.” The psychology of birding is an odd thing. We tend to “tune-out” the common birds because our minds are working so hard to find that different shape, sound or silhouette. Searching for the unusual species in a flock tends to make us lose the “forest for the trees,” and we often come away from large flocks of common birds with no rarity, and no real estimate of the common birds either! This type of data loss is far too common, and can be avoided by keeping a simple checklist of birds you see in the field while you’re out birding. Field cards are blank bird checklists that can be taken into the field everyday. These are available for a variety of states and regions from the American Birding Association. These cards are ordered taxonomically, so that when you get to eBird it makes data entry that much easier. If you must build your own field card, just pick up a notebook that you’ll keep for your bird records. “Rite in the Rain” notebooks work well, as do the Moleskin Notebooks. First record the date, time and location, and then begin your list with species’ names on the left followed by hash marks or numbers. You can use hash marks for relatively uncommon species (fewer than 25/day), but you’ll keep a running list of numbers of common birds every 15 minutes or so, which results in something that looks like:

Brown Pelican: 14, 21, 32, 5, 17, 3 = 92

Having a line for each species in your notebook helps you keep accurate counts throughout your birding event, and will result in an accurate total for each species and your eBird checklist.

Be Conservative

Depending on the individual, birders have a tendency to either over-estimate or under-estimate numbers. Some people consistently average higher counts than others, and in the end the two balance each other out. That said, at eBird we promote conservative counting. The idea is for you to give us your best count of the birds you saw and heard in the area you sampled. So, if you see a male Northern Cardinal in the first five minutes of your walk, and then see a female later, your count would be two. But, if you see a male Northern Cardinal in roughly the same place on your way back we’d recommend leaving your count at one. While it is possible that two male Northern Cardinals were present, it’s best to be conservative. If you saw a male Northern Cardinal at the beginning of your walk, and then another .5 miles away, you should safely count two! Use common sense, and try your best to come up with the most accurate, yet conservative count. 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.

A good example of over-counting comes with the common Carolina Chickadee. Birders typically report “flocks of 25” Carolina Chickadees from a single location, but this species’ social system is nearly invariably 2-4 birds per mixed-species flock (Van Remsen pers. comm.), so a max of 8 might be tallied if two adjacent groups come together. This represents one of many examples of over-counting of small, active birds that are very obvious to observers. Knowing a bit about the birds can help too!

Bird Counting Overview

Counting birds can be simple, such as when you’re counting a single species at a fixed location. For example, counting eighteen Eared Grebes on a small lake shouldn’t present much difficulty. You can feel reasonably sure that you’ve got the right total with just a simple straight-forward count, especially when the number of birds is few. Naturally, your margin of error goes up as the number of birds is greater, but the techniques outlined below are useful in minimizing this error.

On the other hand, counting can be very challenging! When birders encounter very large numbers of birds in mixed species flocks, things can quickly become complicated. We’ll leave that for our second installment in this series. For now, let’s focus on bird counting tips, and techniques for counting single-species flocks.


Counting this single-species flock of Red-necked Phalaropes is straight-forward, as we can see and identify individuals.

Stationary Counting – A favorite thing among many birders is to perform stationary counts at hawkwatches and seawatches around the country. Over the course of a few hours, you will see many different species, and typically these birds are migrating, so running totals will need to be kept. Birds will pass by both singly and in flocks, and at times this can become nearly overwhelming. We recommend writing down totals for all species observed at least every 15 min. Despite how sharp your mind might be, it’s very difficult to keep track of running totals for 40 species over a few hours! It’s best to again keep these notes in your field notebook as described above, adding numbers as you go. If you are the kind of birder who performs repeated counts at the same location, you might consider making your own data sheets to take with you in the field. Data sheets help you organize your records, and keep your numbers straight, and you can easily make a data sheet in Microsoft Excel and print it out on your home computer (see below). Make sure to add a few blank lines for species you might not encounter on a daily basis!


Single-species Flocks – Birders typically visit places with large concentrations of birds. No surprise there! But counting these large flocks can often be very difficult. Generally, you want to count by the smallest grouping possible. For example, if a medium-sized flock of Broad-winged Hawks is streaming over your head in upstate New York, you might be able to count them singly as they pass. A larger flock might require you to count by fives or tens to keep up. The largest flocks passing through Veracruz, Mexico require that observers count by hundreds! The idea is to count every bird if you can, but when that becomes impossible try larger and larger groupings until counting again becomes feasible. If only we all had this problem–too many birds to count!!!

Take a look at this flock of Crested Auklets in flight.


These birds are relatively straight-forward to count. As a flock this small passes by, you can count them individually, ending up with a count of 14 for this flock.

Sometimes though even single species flocks present a challenge. When there are large numbers of birds on the move it becomes impractical to count them individually, and then you must move on to counting by increments.


In the flock above, we have to count by tens because there are too many birds to count individually. The technique for doing this 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. Give it a try.


It’s not unlike when you were a kid and you used your fingers to measure out a distance on a map using the legend, but this time you’re doing it with your eyes! We get roughly 140 birds when extrapolating by tens for this flock. The actual count of birds in this image is 118, again, not perfect, but close enough to be within the acceptable limits of error. Remember biological significance!

Write it down, and then move on to the next flock! If things are fast and furious, take totals every 15 minutes.

In other situations, such as with waterfowl or blackbirds, single-species flocks might present a different kind of challenge–how do I make a count when these birds are simply blanketing an entire area? The answer is to figure out how many birds occupy a certain part of the area in question, just like we’ve done above, and then extrapolate your count to come up with an estimate. People generally under estimate numbers when they encounter large flocks, and it’s important to understand how closely the birds can be packed in, and also how the effects of distance and depth come into play. Humans don’t do very well when counting in more than one dimension of space. Counts of linear flocks aren’t too bad, but as soon as one adds a depth dimension, under-counting becomes rampant. And it gets much worse for flocks in 3-D…it’s tough for humans to comprehend that a blackbird or swallow flock that is 100 x 100 x 100 has 1 million birds in it!

The best technique to use when encountering large flocks of birds is to carefully count a sample, or subset, of the flock. In some cases you can count part of the flock, in others you can count how many birds there are per area. For example, if the impoundment before you is filled completely with Snow Geese, there are two ways to go about counting them. The best thing to do is to count off 100 birds, see how much of the impoundment they cover and then extrapolate. If that is inadequate move up to 500 or 1000–you get the idea! The other option is to count how many geese cover roughly 10% of the impoundment and then multiply by ten! Both techniques will work for rough estimates. Let’s try it with these gulls.


If we start our counting by figuring out how 25 birds appear in this flock we get the following result.


I know what you’re thinking, “I don’t see 25 birds in that circle!” Well, some were blotted out by the super-wide white line, so take our word for it, this is roughly 25 birds! Once we have a picture of what proportion of the flock is made up of 25 birds, we can then extrapolate across the rest of the flock.


One important thing to keep in mind is that you are looking at birds in two dimensions here, both in width and depth. As the birds are farther away, their depth increases as does their width, so it’s important to adjust your estimates accordingly. With a straight extrapolation we’d estimate 150 gulls in this flock, but in reality there are 206 birds. Here’s how it breaks down.


In our mind’s eye 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. That’s because you’re actually looking at a larger area as distance from the observer increases. Even without this correction our estimate of 150 birds was not bad, especially considering the actual total was 206. This is simply meant to illustrate how large flocks of birds are often under-counted. Remember, while it’s always best to be as accurate as possible, it’s always better to err on the conservative side when counting!


Using these simple techniques, you should be able to come up with conservative estimates for single-species flock bird counting situations. If you feel like you’re having trouble, take heart, it only gets easier the more you do it. After a while you’ll have a good idea of how many birds are present at your local birding sites with just a quick glance. Estimating numbers promotes better record-keeping and in turn results in better and more rewarding birding!

Check out — Bird Counting 201!

The next installment will help you learn how to count mixed-species flocks! Look for it in a few weeks.

Go count some birds!

–Team eBird