Bird Counting 201

By Team eBird February 22, 2012
Semipalmated Sandpiper Calidris pusilla

In this second installment of our series on counting birds we’ll discuss some approaches to counting more problematic groups of birds including large numbers, moving flocks, and mixed species flocks. How do the hawk counters do it? How should you count that constant stream of Cormorants passing your local headland? How do you go about picking out different species in large flocks of birds. “Birds of a feather flock together!” Well, sometimes, but more often than not we’re presented with heterogeneous flocks of birds comprised of many different species. Learning how to look at species ratios within flocks is important, and developing an eye for picking out what is different will help you find that proverbial needle in the haystack that we’re all hoping for–a rarity. Armed with the tips in this column we hope you’ll improve your ability and begin to enjoy counting birds.

Gulls flock on the shoreline

Gulls feeding on sardines, Monterey Bay, CA, 8 January 2007.


Bird counting can be simple, as we’ve seen in first installment on counting single-species flocks. On the other hand, counting can be very challenging! For example, on 8 January 2007, a boating accident in Monterey Bay caused tons of sardines and anchovies to spill overboard into the water. Over the next 24 hours, thousands of gulls gathered to feast on the free meal along a relatively small section of coastline near Lovers Point in Pacific Grove. For a birder, this concentration of birds represented a significant challenge, both in terms of finding a rare gull mixed in with the masses, but also in estimating the total number of birds present. In this “worst case scenario” for counting birds, you’re looking at a massive number of birds both in the air and on the water, made up of a variety of species. In this case, one must use sampling techniques instead of straight-forward counting, as the number of birds is just too great to lend itself to accurate counting. Using sampling techniques can provide acceptable results, and doing so yielded over 30,000 gulls on a single eBird checklist that day! Find out how it was done below.

Counting Big Numbers

Many birds concentrate in large flocks during part of the year. Waterfowl, raptors, gulls, shorebirds, and blackbirds are just a few examples. We typically encounter large flocks of these species as migrants passing by a lookout, like a hawkwatch or seawatch, and at favored migratory stop-overs and wintering areas. It’s easy to become overwhelmed by the numbers when faced with counting a flock of 300,000 Snow Geese rising into the air, or a seemingly endless stream of blackbirds or grackles going to their evening roosts. That said, there are techniques to help estimate these numbers, and it’s important to realize the biological significance of your estimates. Remember that your best estimate is always better than simply putting an “X” to indicate presence, because your estimate gives us a measure of relative abundance. For more on this read “Bird Counting 101” by clicking the link on the upper right hand side of this page.

When faced with huge numbers of birds that are relatively static, like ducks in an impoundment or blackbirds in a field, it’s best to use to sampling method described for single species flocks in “Bird Counting 101.” Although this technique becomes increasingly messy as bird numbers increase, it’s still the best way to go about counting a flock of birds of any size. The real challenge comes when birds are moving, typically flying by a stationary position from which you are counting. In this case it is not uncommon to see virtual rivers of birds, and it is then that one needs to use a different technique.

Big numbers of Moving Birds

Their are two ways to count large flocks of moving birds: either by blocking off a group of individuals, counting them, and then extrapolating to the whole of the flock; or by counting birds per unit of time. Which method you use depends on the situation you face, and the species you’re dealing with. For example, if you see a coherent flock of several thousand Blackbirds heading your way and you’d like to count them, the logical thing to do is to use the blocking method. In this scenario you count by the lowest common denominator, often 10 or 50. Once you figure out how much of the flock comprises 10 individuals, you can use the shape of that “block” of birds to extrapolate the numbers to the remainder of the flock. This is the method typically used when counting large, coherent flocks of birds such as raptors or shorebirds. In Veracruz, Mexico, where it is not uncommon to see hundreds of thousands of hawks in a day, counters use this technique to count flocks of Broad-winged and Swainson’s Hawks.

The second option is counting birds per unit of time. In cases where counting streaming birds individually becomes too onerous or impossible, using birds/unit time is the best option. This technique works well when you have a consistent flight that lasts for a certain period, from several minutes to many hours. A good example of this is counting Sooty Shearwaters along the California coast. At times there are so many shearwaters that counting each individual becomes impossible, but the rate at which they are passing by the count site is relatively consistent. In this case a counter will 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. Thus, if the counter had 1300 shearwaters/5 minutes, and the flight remained consistent for 15 minutes, you could safely extrapolate a total of 3,900 shearwaters. It’s always important when using this technique to constantly monitor the flight for changes in volume. Numbers often fluctuate, and it’s important to get a new estimate as soon as things begin to change. If you note fewer, or greater numbers passing, it’s time to make a new 5 minute count. Flights like this can last for hours, and this is the best technique for coming up with a reasonable estimate of overall numbers. Another benefit of this technique is the ability to count several species at once, which makes counting diverse flights a bit more manageable!

Mixed-species Flocks Counting multiple species flocks adds several layers of complexity to the technique described above for large single-species flocks. The first thing you should do is figure out “flock composition.’


Counting this mixed alcid flock at Gambell, Alaska, is quite straight-forward, as we can see and identify individuals. This flock contains 9 Common Murres, 5 Thick-billed Murres (look for their blacker backs), and 3 Horned Puffins. The counting technique differs for larger flocks as described below.

Counting this mixed alcid flock at Gambell, Alaska, is quite straight-forward, as we can see and identify individuals. This flock contains 9 Common Murres, 5 Thick-billed Murres (look for their blacker backs), and 3 Horned Puffins. The counting technique differs for larger flocks as described below.

This just means, doing a rough scan of the flock through binoculars, and determining what proportion of each species is present in relation to the whole. It often helps to determine the overall proportion of the most numerous species in the flock, as it is then easier to fill in the remaining proportions with the less numerous species. Is it 10% Glaucous-winged Gulls, 10% Heermann’s Gulls, 30% California Gulls, and 50% Western Gulls? If so, then a flock of gulls that you estimate to contain 10,000 individuals would have 1000 Glaucous-winged Gulls, 1000 Heermann’s Gulls, 3000 California Gulls and 5000 Western Gulls.

Another thing to keep in mind is that for certain groups of birds, like gulls, social hierarchy might determine where in the flock a certain species sits or flies, so each species is not necessarily evenly distributed throughout the flock. Bear this in mind when making estimates, and make sure to take a look at all the birds before extrapolating numbers to the whole flock. A few Glaucous Gulls might be mixed in with the large gulls, but perhaps sitting together in the same place, as they are large, powerful and dominant gulls that will likely take the most desirable perches!

Let’s give it a try with this mixed-species shorebird flock in flight!

Large flock of shorebirds

I know what you’re thinking–ugh!!! But before you throw up yours hands, using our sample of 100 birds let’s try to figure out what species comprise this flock. Starting with the rarest birds first, we see 1 Long-billed Curlew, 7 Long-billed Dowitchers, 11 Willets. The remainder, which is 81, is made up of Marbled Godwits.

Large flock of shorebirds with a circle around some of them

Now we can extrapolate these figures across the rest of the flock.

Large flock of shorebirds divided into 6 sections

If we extrapolate our estimate of 100 birds across the entire flock we come up with 600 individuals. The breakdown then becomes 6 Long-billed Curlews, 42 Long-billed Dowitchers, 66 Willets and 486 Marbled Godwits.

The real totals in this flock (yes, we counted them!) are 6 Long-billed Curlews, 73 Long-billed Dowitchers, 71 Willets, and 538 Marbled Godwits. Not too bad! So we were a little off when it comes to the Dowitchers but that is to be expected when estimating large flocks of birds.

Beware False Precision

If you are counting birds by fives, tens or hundreds, it is important to understand that adding a precise count to these estimates within the same checklist results in false precision. For example, if you counted a large flock of Surf Scoters by tens, and estimated a total of 1110 birds, and then a flock of 3 birds passes by, you should not combine these efforts and report a total of 1113 birds. Rather you should keep your reported number to the lowest level of precision used when counting a species, in this case tens, resulting in the same count of 1110. If an additional flock of 5 birds then passes, you can bump up your reported count to the estimate of 1120. Adding exact counts to estimates of a single species within a checklist should be avoided, but you can mix estimates with exact counts of different species on a single checklist (e.g., 1110 Surf Scoters and 3 King Eiders). Eventually we hope to record whether or not your counts are exact or estimates!


Using these simple techniques, you should be able to come up with conservative estimates for even the most difficult 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!