Lumps and Splits: What’s It All About?

Written by Andy McCormick

Andy McCormick, Volunteer and Former Board President of Eastside Audubon

Each year at this time, the North American Classification Committee (NACC) of the American Ornithological Society (AOS) reviews proposals for changes in bird taxonomy, that is, the classification of bird species. The committee evaluates and codifies the latest scientific developments in bird nomenclature and produces the official Checklist of North American Birds and publishes the results of its deliberations in Ornithology, a journal of the AOS. The committee decides on the efficacy of proposals to change the status of bird species, and they have options to decide to lump some species together or split a species into two or more new species. These lumps and splits in the reorganization of birds helps ornithological scientists understand the relationship of one bird species to another and also benefits birders in our understanding of the birds we see every day.

To understand this process, we begin with the levels of taxonomic organization of birds. At the most inclusive level all birds are included in the Class AVES. This class is subdivided into 42 Orders, such as Accipitriformes for hawks and eagles, Piciformes for woodpeckers, and Passeriformes for passerines – the perching birds. Passeriformes is the largest order of birds and is in turn divided into 142 families of birds, such as sparrows or warblers. Within families there are 1,161 genera (plural of genus) and over 6,500 species of birds, a total which includes more than half of all birds in the world. This article has a focus on the genus and species of birds.

LUMPS AND SPLITS

A lump occurs as a result of a NACC decision to bring what had been thought of as two or more bird species together into a single species. If a scientific report shows that birds which had been thought to be different but have a significant history of interbreeding, the two species can be considered the same species and lumped together, even if there are some differences in appearance. For example, the committee recently lumped the redpolls. Until two years ago, the redpolls were treated as three separate species, Common Redpoll (Acanthis flammea), Hoary Redpoll ( A. hornemanni), and Lesser Redpoll (A. cabaret). Researchers found that the genetic differences among them are very small and related only to aspects of their appearance. After reviewing the paper which presented this information and the comments about it, the committee decided in 2024 to lump the three former species together as Redpoll (Acanthis flammea). They also recognized that there are differences in the populations of redpolls and listed five subspecies within the species.

A split follows the opposite procedure. Birds which were thought to be the same are later determined to be different. The birds may look similar but the fact that they do not interbreed is often enough to support a split of the species into two or more separate species. When a lack of interbreeding is combined with differences in behavior and songs, the argument for splitting is stronger.

A split like this occurred with the Eastern Meadowlark (Sturnella magna) which is in the genus Sturnella. A form of this meadowlark in the Chihuahuan Desert region was considered a subspecies and was known as “Lilian’s Meadowlark” (Sturnella magna lilianae). There was enough difference between then for Lilian’s to be considered a subspecies of Eastern Meadowlark, but not enough evidence to decide it should be its own species. However, recent DNA analysis added another layer of information to the investigation. The new data that the structure of the DNA of Lilian’s Meadowlark was different from Eastern Meadowlark, combined with the already known differences in vocalizations and appearance, satisfied the criteria for Lilian’s Meadowlark to be raised to full species status. The bird is now known by the English language name Chihuahuan Meadowlark and the scientific name Sturnella lilianae.

SCIENTIFIC ORGANIZATION OF BIRDS

Biologists organize organisms from bacteria to the largest mammals into a scientific model first developed by Swedish biologist Carl Linnaeus (1707-1778). He developed the model of binomial nomenclature which is the modern system of naming organisms using a genus and species. The first part of the name is the genus to which the species is assigned, and the second part is the species name also known as the species epithet. For this contribution to science Linneaus is known as the father of modern taxonomy.

All known birds have been assigned to a genus and given a species name. For example, The Western Meadowlark has the scientific name of Sturnella neglecta. The genus and species are usually written in italics to indicate that they are in Latin. In this case Sturnella is from the Latin sturnus, the word for starling, with the addition of -ella for the diminutive form. The species epithet neglecta is also from Latin for neglected. The Western Meadowlark looks so much like the Eastern Meadowlark that the differences in appearance and song were not studied until about 100 years after the Eastern Meadowlark was catalogued. The person who decided on the scientific name wanted to echo the history of delay in understanding the differences between these two bird species and thought of it as having been neglected by science.

BIOLOGICAL SPECIES CONCEPT

Even with current science and the use of DNA analysis, deciding to lump or split bird species can be controversial. There are different ways to approach the decision of what constitutes a bird species. Among bird scholars there are two main criteria for figuring out if a group or population of birds can be considered a species. One is based on appearance; the other is based on bird biology.

The phenetic species concept relies on the bird’s appearance, that is, if birds are phenotypically similar in appearance, and they also look different from other birds, they can be considered the same species. Unless studied closely, Eastern and Chihuahuan Meadowlarks look very much alike. This similarity may have been unnoticed by early observers, and in that case would likely consider all these birds to be the same species. It makes sense from that perspective.

The phenetic approach is a simpler method of deciding how to group birds. If the birds look alike in shape, coloration, and some behaviors and vocalizations, they could be grouped together. In this configuration, some similarly looking birds which have different songs or breeding behaviors could be considered subspecies of an overall species. This was true of the Eastern Meadowlark when Lilian’s Meadowlark was considered a subspecies.

The biological species concept takes a different approach and is now the dominant method for determining which birds belong to a species. This approach is based on bird biology, specifically, the actual or probable ability of the birds to interbreed and produce fertile offspring. Many hybrid birds are unable to reproduce. Our three species of meadowlarks, Eastern, Western, and Chihuahuan do not interbreed even when they inhabit the same territory. There may be rare occasions when they do interbreed, but in those cases the intergrade meadowlarks are not successful breeders. The intergrades die off without leaving offspring and therefore do not pass on their genetic information. Without interbreeding the species stay separated over time.

Ornithologists today make these determinations by using the results of DNA studies by geneticists who examine the genetic code of closely related birds. These studies provide information about the evolutionary history of birds and can reveal evidence of historical interbreeding. When none is found, it provides support for a decision that the two subspecies are really separate species. The Chihuahuan Meadowlark was split from the Eastern Meadowlark using the results of this type of study.

Photo credit: Western Meadowlark by Mick Thompson