While aerial surveys during the breeding season are usually targeting pairs or small groups of
waterfowl, surveys during the non-breeding season, such as during winter, molt, and on staging
areas, often require observers to estimate the number of birds in flocks. Accurately counting
the number of birds in flocks can be difficult - it's a skill that takes practice. The tendency
of most observers is to underestimate the number of birds in a flock and usually, the larger the
flock, the less accurate (and low biased) the estimates are.
One technique that is often used by observers to visually count birds in flocks is to visually
break the flock into units of 10, or 100, or 1000, and then estimate the number of "units" within
the flock. The first step is learning how to accurately estimate these smaller units. You want
to get a sense of what 10 birds looks like, and visually pick out another 10, then another, until
you've counted the entire flock. Once you've mastered the 10-count, then try to get a feel for
what 100 birds looks like, and break larger flocks into 100-bird "chunks". However, it's prudent
to initially start with 10-counts because flock densities vary and can affect your perception of
numbers of birds.
This takes considerable practice to get good at it, and it's best practiced before you try to
do it from a plane flying low-level at 100 mph. It will be important to practice using the features
included in this Guide and the testing features on this web site until you're
able to make the visual estimates fairly quickly while retaining accuracy.
Some duck species are sexually dimorphic (in other words, males are brightly colored and females
are mostly brown). In those flocks, males are far more obvious and there's a tendency to "miss"
the darker females, thus underestimating the true number of birds. For example, here are two photos
of flocks - the left photo is of snow geese where males and females have similar plumage. The right
photo is of spectacled eiders with whitish males and brownish females. The number of birds in each
flock is equal, although it seems like there are more snow geese.
When we look closer at the eiders, you can see that there are in fact many brown females in the
flock, but the white plumage of the males swamps the initial visual image so you don't see the
females as readily, and likely underestimate the total number of birds.
The magnitude of this bias is dependent on the sex ratio within the flock - for example, flocks
composed of predominantly males will have relatively less bias than a flock with a more even, or
female-skewed, mix of white males and brown females.
In addition to plumage factors, the perception of number of birds can also be influenced by the
density of birds in a flock, contrast with the background environment (land vs sea, sea state, glare),
the distributional pattern of the flock (clumped versus linear), and the observer's perspective
(near-vertical vs varying degrees of obliqueness).
When you encounter large flocks (many hundreds or thousands), it's often better to fly at higher
altitude and estimate from a height where you have more time to see the entire flock, and less
likelihood of causing birds to flush or dive in response to the aircraft. For flocks with birds
numbering in the thousands or tens of thousands, aerial photography should be considered as a more
efficient and accurate way of estimating numbers of birds.
In the Test
Your Counting Skills section, you will have an opportunity to test and hone your ability to
accurately and consistently estimate numbers of birds in flocks.
Hodges, J. I. 2014. Wildlife Counts: A computerized simulation for learning the skill of wildlife
count estimation. www.wildlifecounts.com.