The system of latinate scientific names originated with Carl Linnaeus in the 18th century. Linnaeus also established a hierarchy of ranked groups for use in taxonomy. The number of ranked groups has expanded considerably since his time.
The fundamental component of his system is a latinate name for every species. Every species name has two parts, a genus name and a specific epithet. The genus, which is capitalized, denotes a group of similar birds, the specific epithet is not capitalized and indicates which of those birds is meant. You can think of the genus as being similar to a surname, and the specific epithet as acting like a first name. Thus the name Piranga rubra denotes the Summer Tanager. On the TiF list, it is one of 12 tanagers in the genus Piranga.
We group similar genera into families, families into orders, and orders into classes. The class Aves includes all birds, and nothing but birds. The names of the various ranked groups are usually constructed from the generic names with their level distinguished by a suffix. The ranks commonly used in bird classification are, from highest to lowest, superorder (-imorphae), order (-iformes), suborder (-i), infraorder (-ides), parvorder (-ida), superfamily (-oidea), family (-idae), subfamily (-inae), and tribe (-ini).
How do we decide on which names to use? Over the years, ornithologists have described a number of individual birds, a process that continues to this day. The rules governing the scientific names of birds and bird families are currently codified by the International Commission on Zoological Nomenclature (ICZN). When a bird is described, it is given a two or three part scientific name (the third part is used for subspecies). In fact, the rules are set up so that the name actually designates the individual bird described (these days, this is usually a specimen which is placed in a museum or other repository). Although we think of the name as referring to a species or subspecies (or at least trying to), this is not really correct. The name refers to a single individual, the one named as type.
To decide the name of a species, we first have to decide a scientific question. What are the species limits? Which birds fit into the species, and which do not. Once we've set the limits, we then round up all the named individuals that are part of that species. Finally, we apply the oldest name (the principle of priority) to determine the specific epithet for the species. For Linnaeus it was simpler as priority starts with the names he used in 1758 (some borrowed from other naturalists). Others soon adopted the Linnaean system and chaos ensued. The same bird species sometimes had many names. The principle of priority is used to sort this chaos out, and has been a feature of all subsequent naming conventions. Sometimes the same name was used in different senses, and when necessary, we attach a reference, or at least the author name(s) and year to indicate which one we mean.
In the case of the Summer Tanager mentioned above, there are many available names. I'm not sure this list is complete: aestiva, coccinea (Boddaert 1783) cooperi, (Ridgway 1869), hepatica (Coues 1866), hueyi (van Rossem 1938), misisippica (Hermann 1783), missisippiensis (Gmelin 1788) ochracea (Phillips 1966), variegata (Latham 1790), virginica (Gmelin 1788), and rubra, named by Linnaeus himself in 1758. The name hepatica of Coues cannot be applied to the Summer Tanager as long as hepatica named by Swainson in 1827 (Northern Hepatic-Tanager) is in the same genus (such a name is called preoccupied). Swainson got there first, and his name has priority. The other names have no such problems, and we then apply the principle of priority to determine the specific epithet. Since rubra was used first, it becomes the name of the species. The Summer Tanager is currently thought to consist of two subspecies. One, the nominate subspecies, includes rubra and is called Piranga rubra rubra. The individual cooperi is the oldest named in the other subspecies, and that subspecies is then called Piranga rubra cooperi. Note that the code takes no position on what the species limits are, it only determines the name once those limits are set.
The application of genus names works the same way. Each genus name has a type species (which really refers to an individual bird). Unlike species names, names of animal genera must be unique. Once we've decided which birds belong together in a genus, we use the oldest genus name belonging to the birds in that genus. The genus name carries a gender, and the specific epithet may have to be modified to match, depending on what type of word it is.
Continuing the Summer Tanager example, it seems to have four attached genus names.
- Piranga Vieillot 1807, with type rubra (which he called Muscicapa rubra)
- Pyranga Vieillot 1816, emending the previous name, so it also has type rubra
- Phoenisoma Swainson 1837, also with type rubra
- Phoenicosoma Cabanis 1850, type rubra.
In a previous era, Pyranga was in regular use by some, but we are now stricter about priority and allow many fewer emendations, even by the original author. We now use the oldest form, Piranga. Since Piranga is of feminine gender, we use the feminine form of rubra rather than the masculine ruber.
The ICZN also applies to family-level names (superfamily, family, subfamily, tribe, or any other rank between genus and superfamily). A similar naming system applies. The system is unified across all family-level names. It doesn't matter what rank was initially applied, we can just change the ending to change the level. Each family-group name has a type. We use priority to determine which name applies. Then we attach the appropriate ending to the genus name to determine the family-group name (if this creates a naming conflict, we may have to modify the name). As the system now works, we continue to use the old genus name even if the type is reclassified into a different genus (this was handled differently before 1961, and updated names that were in use before 1961 carry the priority of the original form).
An example of a type-genus that has been recently changed is Parula (type species americana, the Northern Parula). The AOU recently moved this to an enlarged genus Setophaga. The name Parulini is still used for the tribe containing Parula americana, while Parulidae (the wood warblers) is the family containing Parula americana. It doesn't matter that the AOU now places Parula americana in Setophaga (which has priority as a genus). The name Parulini still has priority at the tribal level, as does Parulidae at the family level.
Some of the existing avian families have long-established names that predate these conventions. The code itself endorses certain changes occurring before 1961. In those cases the old name continues to be used. One such example is the Cathartidae, the American Vultures. The family name dates back to Lafresnaye in 1839. However, in 1811 Illiger had already based the family name on Vultur. Although he called the family Vulturini, we would now use the term Vulturidae. In fact, a type species for Vultur was not named until 1907 when J.A. Allen picked Vultur gryphus, the Andean Condor. At the time Illiger wrote, the genus Vultur included both Old and New World vultures. Perhaps this lack of clarity is why Cathartidae came into use. In any event, Cathartidae has been in use since at least 1840, and long use can trump temporal priority. In fact, Vulturidae has been very rarely applied to living birds. A google scholar search gave about 2840 for Cathartidae, and only 306 hits for Vulturidae, mostly concerning fossils. There are a few other avian families where long use trumps priority.
Names above the superfamily level (e.g., orders) are not covered by the ICZN Code. As is common practice in ornithology, I use an ICZN-like system to handle them. However, it is harder to get proper information on priority. Further, the standard ranks are insufficient to organize the complexity of the tree of life, and various un-ranked groups will also be used. In some case, the are no available names, and I've resorted to various informal names. Informal names that are unique to these pages or used in a non-standard way are so indicated, sometimes by using the name in quotes. Some of the informal names are standard names that do not properly apply, but which have been applied in the past, and area still used by some.
There is also a new system of nomenclature under construction, the Phylocode. The Phylocode applies to clade names, and some higher-level groupings have been named under its principles. The Phylocode does not use types, nor do the names carry explict ranks. The Phylocode considers these to be features rather than bugs. Some of these names are used on the TiF pages, often at ordinal rank and above.
The forgoing description of how names work captures the basics, but is rather oversimplified. In some cases, the ICZN has over-ridden priority. In other cases, names have been declared abandoned. There are other complexities that have not been discussed. The complete rules governing animal names can be found at the ICZN website. Once there, you can follow the link to “the code online” or preruse the other information about the code.
Other useful resources include Alan Peterson's Zoonomen, which has been particularly helpful for sorting out the complexities of scientific names. Although still under construction, John Penhallurick's database of avian nomenclature at World Bird Info is also quite useful. It will eventually include all of various scientific names that have applied to the various extant bird species. There are also a number of printed materials, some of which can be found online. The Peters Checklist and the Catalogue of Birds in the Britsh Museum (Sharpe et al.) are especially helpful.
Why do we need these ranked groups? The relationships between the birds is best described by a phylogenetic network. When hybridization is not too big an issue (and it usually isn't), this network reduces to a tree. The root of the tree is the common ancestor of all living birds. I use a standard procedure to translate this tree into a linearly ordered checklist.
The problem with the linear order is that much of the information from the tree is lost in translation. The ranked groups restore some of that information. The genera, families, orders, and other ranked groups help show which birds are more closely related to each other. The family tree shows the underlying phylogenetic tree that I have used in this list, down to family level. In many cases the actual order given is based on a genus or species level tree. The notes and references in the annotated list show where to find further information on this.
I construct the linear checklist order by finding the oldest division in the tree, and then placing one group first and the other last. Then I do the same for each of the two branches. This procedure is repeated for every branch of the tree. It is completely arbitrary which group goes first at any point. This means that very many linear orderings can represent the same tree. A convention is needed here, and I use the rule of putting the smallest group first, followed by more speciose groups. In multiway splits, I sometimes make a guess as to how the split would look if fully resolved. If two groups have the same number of species, an additional rule is needed. I use the rule that the groups whose breeding range extends farthest to the northwest goes first, using the prime meridian as the base. Note that these rules have not been fully implemented, especially in families where I do not have a proper genus-level tree yet.
You shouldn't infer that families listed sooner are somehow older than families listed later. An equal amount of time has passed for both since their common ancestor. Which is actually genetically closer to their common ancestors depends on details of their history, details which we do not know.