Sunday, December 12, 2010

10,000 Birds?

"You think you know every bird in the world, and then someone shows you a Curl-crested Aracari"
David Quammen, The Song of the Dodo

An oft-cited figure for the diversity of birds in the world is around 10,000 living species, a reasonable rule of thumb although estimates vary - Birdlife International currently recognizes 10,027 while the IOC list is currently at 10,396. Duncan Wright says the number should be closer to 12,000 if recently extinct birds are included (and he also coincidentally begins with a David Quammen quote). Pinning down an exact number is an impossibility because differing opinions on species limits in species and subspecies complexes create a near-infinite number of list permutations. Pinning down a number is also impossible because the bar keeps getting raised by the new bird species discovered every year.

New species of bird often receive a small splash of press when discovered or described, because birds are regarded as an exceptionally well described group of vertebrates and we've long reached the point where new birds are few in number. Additionally, the hunt for new species has been romanticized in such good reads as A Parrot Without A Name or this more recent Birding article (pdf) by Joseph Tobias. Tobias quotes Mayr (1946) as proclaiming the general end of new bird discovery - “I doubt that in the entire world even as many as 100 new species remain to be discovered” - but states that a yearly trickle of new species have been described since then. Just how many new birds are discovered every year? How much has the trickle of new birds declined? How many new birds are left to discover?

I can't answer the last question, but I can find data for the first two. It is a logistical nightmare to track down this data by finding every new species description in the primary literature. Instead, I picked a world checklist to find what I needed. The formal way to list species in literature includes a citation for the description of the species, such as Puffinus griseus (Gmelin 1789). By using these dates, you can quickly extract the year by year data for species descriptions. This method also only counts newly described species, not splits through the elevation of previously recognized subspecies.

The only quickly downloadable checklist I found with data like this is Birdlife International's checklist, so I used their most recent version (v3, June 2010) and plotted out the data. Here is the number of new bird species described each year, beginning in 1758 with Linnaeus and ending in 2008 (the Birdlife checklist is incomplete for new species in 2009 and 2010):

(click to enlarge)

The early decades show relative inactivity in bird taxonomy punctuated by massive monographs (like the 400+ that Linnaeus described in one work). The golden age of bird taxonomy peaks in the mid-1800's, with several decades of 50+ species described every year. The number of new birds described then generally declines to the present day.

Another way to present this data - the most useful for comparison - is to create a species accumulation curve. This represents, for each year, what percentage of the total number of known species was described at that point. This allows you to examine what those small numbers of new species each year mean in the context of the total number of known bird species. The slope of the curve is a proxy for the rate of discovery of new species, and you can easily eyeball the differences in slope representing different rates of species description over time.

Here is the species accumulation curve for birds:

(click to enlarge)

This represents a typical curve for a well-described fauna - the middle range is steepest, and the curve plateaus as new species become increasing hard to find. Compare the curve for birds with that of a very different fauna, another favorite of mine, geckos:

(click to enlarge)

In stark contrast to birds, the rate of discovery of geckos has only accelerated through time, and the last few years have seen the highest yearly rates of gecko discovery ever - but that's another post. The point is that geckos are a representative of an under-described group with lots of new species to be found and taxonomic description left to be done. Birds are comparatively well-described and have relatively little left to be found.

We can estimate how little is left to be found by studying the decline in new species found. So how about that decline? Zooming in to the last 120 years, we get this:

(click to enlarge)

At about the time Mayr proclaimed the end of bird discovery in the 1940s, the overall rate of description of new birds had been trending steeply downward for decades. After the rate reached rock bottom - 1941 is the last year with more than ten bird species described - there is indeed a steady trickle of new species every year, averaging 4.9 species per year and varying from zero (in 1954 and 1978) to ten (1960, 1974, 1997) new birds yearly.

However, this trickle does not seem to show any sign of stopping, even decades after Mayr's prediction. In fact, if you take just the data from 1942-2008, look at the general trend line:

(click to enlarge)

That's right, that is a (very weakly supported) POSITIVE trend line for the discovery of new species in the last seven decades. Given the overall wide variance in discovery from year to year in these decades, I double-checked this trend my making a moving ten-year average of new birds per year:

(click to enlarge)

This is a really fascinating result! Despite the fact that the overall description rate of birds has plummeted and won't ever get back up to the rate it was 100 years ago, discovery of new birds has stabilized and shows no signs of slowing since the 1940's. That means that, while the species accumulation curve has plateaued, it is not exactly drifting towards zero and the total number of remaining bird species can't be extrapolated. Who knows what is left to find?

In part 2 of this post, I'll outline what new species have been discovered in the last few decades, where they are being found, and some reasons for their late discovery. Stay tuned.


  1. Very cool :)

    Care to post a link to the most user-friendly version of the (bird) data you have used??

  2. As I mentioned, I simply used the Birdlife International downloadable checklist for species dates and the Birdlife Data Zone for species-by-species data.

    But, even better, I can email you the excel file with all my work :)

  3. Very interesting analysis Slybird. I like the idea of extrapolating the recent upward trend to show that there are thousands of species yet to be discovered.


  4. Tom - Who needs extinct species to reach 12,000? We just gotta follow the trends.