How many birds are there? This is a question of great importance for those concerned with the catastrophic disappearance of wildlife. If we don’t know how many there are now, we won’t know if populations are continuing to diminish in the future or if conservation remedies are succeeding.
There are some well-known bird-counting activities, such as Audubon Christmas Bird Counts and statewide Breeding Bird Atlas projects, but these are useful only as relative measures from year-to-year. They are also useful for detecting changes in the geographic range of species, such as the spread of the Northern Cardinal into New England or the disappearance of Northern Bobwhite quail from much of its range.
Another popular bird counting program is eBird, used by birdwatchers to keep track of their sightings, but at present, eBird is useful only for determining if a species is present or absent at a particular place or time. eBirders flock to the same places in response to reports of rare birds, so it is not a good method for scientifically surveying a population.
None of these methods tells us the size of the population, but armed with knowledge of the extent of a species’ range established by these methods, we would just need to know a bird species’ density — in other words, how many birds there are per acre — to be able to estimate total population sizes.
How is the density of a bird species estimated?
Watching bird feeders won’t work because they concentrate birds unnaturally in a small area. Relying on birdwatchers won’t work because they are mostly interested in how many species they see, rather than how many individuals of each species.
The method of choice for estimating bird population density is to perform “point” counts in which every bird heard at randomly selected points is recorded by trained observers. This requires experts who can recognize every song and call each species of bird in an area makes. Ageing birders need not apply, because hearing acuity is extremely important for this task.
Counters stand still at the point for 10 minutes and listen intently, trying to determine if it’s two Carolina Wrens singing, or one wren moving back and forth and singing in two places.
But knowing how many birds of each species were detected is not enough, because some birds are much easier to detect than others.
Crows, for example, frequently make a lot of noise and can be heard from far away. Worm-eating Warblers, on the other hand, are not loud or talkative. If an observer hears one warbler and one crow at every point, in reality there are a lot more warblers than crows, because many warblers went undetected.
To remedy this, the point counters must accurately estimate the distance between themselves and each bird detected, a harder task than you might imagine. Birds that are detected only nearby, and never far away, can have their numbers adjusted upward accordingly.
Many aspiring ornithologists get their first jobs as point counters and have helped to establish our knowledge of how many of our precious feathered tribe remains.
Dan Cristol teaches in the Biology Department at the College of William and Mary and can be contacted at email@example.com. To discover local birding opportunities, visit williamsburgbirdclub.org/