Monday, December 13, 2010

10000 birds? Part 2

In Part 1, I looked at the history of bird species discovery, and found that new bird species have been described at a fairly steady rate of an average 4.9 species per year since the 1940s. Here, I'll review what species have been discovered since the 1940s, where they are found, and why it took so long to find them.

From 1942 until 2008, 330 species encompassing 76 families and 213 genera were described:

(note: I accidently included 1 2009 datapoint, but 2009 is incomplete and more than 1 were described that year)

Among the top families, five (Tyrannidae - tyrant flycatchers, Thamnophilidae -antbirds, Furnariidae - ovenbirds, Rhinocryptidae - tapaculos, Formicariidae - anthrushs) represent the diverse suboscine lineages of South America. Two other top families (Sylviidae - old world warbles and Timaliidae - babblers) are similarly diverse lineages from the old world (I don't think Birdlife carves up these families into smaller groups as other checklists do in the last few years). My gut impression before seeing this data was that these two broad groups of birds would be the leaders for multiple reasons - they compose many of the species I could remember seeing described in the last few years, they contain many cryptic groups and are receiving active taxonomic treatment with new input from DNA and song, and they are found in the tropical areas of the world still receiving new exploration and documentation of fauna. However, the winner in the family category by a large margin was a surprise for me: Strigidae (owls). It makes sense that cryptic nocturnal birds would contain a lot of previously undocumented diversity, but I was surprised at the scale. The top genera track the top families as expected (Glaucidium - Pygmy-Owls, Otus - Scops-Owls, Scytalopus - Tapaculos, Grallaria - Antpittas, Phylloscartes - Tyrannulets, etc).

There are a myriad of historical and unquantifiable reasons that bird species persist until the present day undiscovered or unrecognized by science. However, I made several simple, straightforward predictions about recently discovered species that can be quantified and tested:

1) New species are likely to be found in underexplored tropical countries
2) New species are likely to be single-country endemics
3) New species are likely to have small ranges
4) New species are likely to have small populations and thus more likely to be endangered

Using Birdlife International's online database, I was able to extract relevant bits of information regarding these new species - where they are found, whether they are country endemics, their estimated range size, and their Birdlife conservation status. I tested my predictions by comparing the species described from 1942-2008 with random samples of species described earlier in history: 100 species described by Linnaeus in 1758 and 200 described in the 1840's-1860's.

1) New species are likely to be found in underexplored tropical countries

Here is a summary of where the new species (1942-2008) are found. The first column is overall country listings, which sums to well over 330 because of species ranging across many countries. The second column of results is for single-country endemic species.

As expected, the South American and Southeast Asian tropics top both lists, with Peru and Brazil leading both categories by a good margin.

2) New species are likely to be single-country endemics

I didn't tally the country-by-country data for my 1758 and 1800's sample groups, because so many of them span a huge range of countries. This difference is apparent when you compare the proportion of single-country endemics for each category:

6% of species described in 1758 are found in a single country
24% of species described in the 1840s-1860s are found in a single country
72% of species describe 1942-2008 are found within a single country

This confirms that recently described species are more likely to be restricted to within a single country, and are thus easier to overlook, than those species described earlier in history.

3) New species are likely to have small ranges

I binned the Birdlife range size data by order of magnitude for each of the three time periods and calculated the proportion of the sample in each:

(click to enlarge)

The species described by Linnaeus in 1758 (blue bars) are overwhelmingly species with huge ranges in the millions of square kilometers. Species described in the mid-1800's still have ranges mostly in the hundreds of thousands to millions of square kilometers, but there is a wider distribution of species among the size classes.

For new species described 1942-2008, the distribution among size classes is much wider than in the other time periods. Less than 4% of these species have ranges in the millions of square kilometers - these include species like Cryptic Forest Falcon (Micrastur mintoni) and Amazonian Pygmy-Owl (Glaucidium hardyi) which have large Amazonian ranges. The largest proportion of new species have ranges sizes that are fairly small - 1000s to tens of thousands of square kilometers.

The most stunning category is for range sizes of less than 100 square kilometers - an area only 5 times bigger than my hometown of North Tonawanda, NY. 12.5% of new species (32 total) fall into this category, with some absurdly small known ranges. Four species have a known range of less than ten square kilometers - Munchique Wood-Wren (Henicorhina negreti, 8 sq. km), Poo-uli (Melamprosops phaeosoma, 3 sq. km), Calayan Rail (Gallirallus calayanensis, 2 sq. km), and Bugun Liocichla (Liocichla bugunorum, 2 sq. km). Words fail me in trying to describe how mind-boggling, fascinating yet even heart-breaking it is to see species with ranges like this. The only species with ranges this small in the other categories are a fruit-dove and a honeyeater described in the mid 1800s from tiny South Pacific islands with range sizes around 25-50 sq. km.

4) New species are likely to have small populations and thus are more likely to be endangered

I wasn't able to easily pull data on population sizes for a large range of species from the Birdlife database, so instead I used a proxy - Birdlife conservation status. I calculated the proportion of species from each time period that fall into the six Birdlife categories:

DD = Data Deficient
LC = Least Concern
NT = Near Threatened
VU = Vulnerable
EN = Endangered
CR = Critically Endangered

(click to enlarge)

Just as the 1758 and mid-1800s samples were overwhelming skewed towards species with large range sizes, these categories are both over 80% composed of species ranked Least Concern. Only one of the 1758 species (Bald Ibis, Geronticus eremita) and one of the 1800s species (Cuban Kite, Chondrohierax wilsonii) are recognized as Critically Endangered. A plurality of the new species are Least Concern, but there is a very wide distribution of rankings, with 8.5% (28 species) being Critically Endangered.

Well, that about covers it. If you weren't keeping a running tally, all of my predictions were validated by the data! While I love playing with this data and learning more about the patterns of discovery of new species, I realized in the end that these findings are rather disturbing. A depressingly large proportion of bird species that have remained unknown to science until recently have very tiny ranges and are endangered. In this era of exceptional environmental destruction, how many more species like these have slipped away into extinction unnoticed? That is a number we can never know.


  1. Good post but aren't some missing from the list? F.e. Formicivora grantsaui in 2007.

  2. Formicivora grantsaui is not recognized by the Birdlife checklist version I used, but is on the 2010 under review species list.