From the 13th edition
An further essay on
'Defining' a Bird Family [March 2014]

As is sometimes said, defining the limits of a bird Family is “more art than science.” Ornithologists do not yet have a formal definition of this ordinal level. In 2012, in an essay on this site that was an effort to define Family level status in passerines, I wrote that “I have evolved a preference for Family level taxa to be (a) distinctive and diagnosable and (b) to have been evolving on their own evolutionary path since at least the early Miocene (i.e., 16-23 mya). I don't advocate drawing any hard lines as to how old a clade must be for Family status — indeed, many lineages of birds appear to be diversifying at different rates — but younger monophyletic lineages may be better addressed as subfamilies. Families should be ancient and distinctive.”

New research (Burns et al. 2014), a review of prior papers (e.g., Graur & Martin 2004, Pereira & Baker 2006), and commentary on BirdForum and elsewhere have compelled a modification of my prior summary. The evidence for differences in the rate of diversification among bird lineages continues to grow. Barker et al. (2013) explain that:

A detailed study of the Thraupidae found that within that lineage, the diversification rates of several “finch” lineages – and especially Darwin’s finches – were exceptional, “even when compared to the overall rapid rate of diversification within tanager.” If diversification rates are different among various evolutionary lineages, then it makes rational sense to develop the construct of a “Family” in light of those differences.

Sibley & Ahlquist (1990) once attempted to define ordinal levels of bird diversity within specific limits of “melting curves” when molecular relationships were viewed from DNA-DNA hybridization studies. This proved essentially unworkable. Now, with direct sequencing of genetic evidence available, we are closer to understanding the timing of evolutionary changes – especially when molecular signals are constrained against the background of the fossil and geologic evidence – and yet no single approach has gained wide acceptance.

In the prior essay, I expressed a preference that Family level taxa in the passerines be those clades that had been “evolving on their own evolutionary path since at least the early Miocene (i.e., 16-23 mya).” This definition does not work well if applied to, say, waterfowl or parrots or other non-passerines. Most if not all of the currently accepted Families in non-passerines are much older. My passerine definition would, if applied to parrots, produce at least 11 families of parrots, and probably more (based on evidence in Rheindt et al. 2014, which did not include all the parrot lineages). In the other direction, my definition, if applied to the nine-primaried passerines, would create one huge Emberizidae, with tanagers, New World warblers, icterids, cardinals/grosbeaks all reduced to subfamilies. [Indeed, for a short time the AOU adopted that concept, following Sibley & Ahlquist (1990).]

A modification of the concept is necessary. Again, it is reiterated that I do not propose any certain time limit should be used to standardize Family level status, even within an Order. Rather, bird diversity can be expressed at the Family level by preferring that such Family level taxa be

Applying this expanded preference to new evidence (e.g., Barker et al. 2013, Burns et al. 2014, Alström et al. 2014), I have added five new passerine families in my 13th ed. list. One is Asia and five are from the New World. Alström et al. (2014) discovered that Spotted “Wren-Babbler" Elachura formosa of the eastern Himalayas is an early offshoot of the lineage leading to the Bombyciloidea (waxwings and allies). They proposed Family rank for this lineage, the monotypic Elachuridae. As the lineage is ancient; their proposal has been widely accepted.

The New World situation is more complex. Barker et al. (2013) produced a multilocus phylogenetic analysis of the nine-primaried passerine radiation in the New World, and proposed it be divided into 16 families. Some of those are long-familiar families [e.g., New World Warblers, Tanagers, Icterids] and some only recently accepted [Longspurs & Snow Bunting] but ten are brand-new Family proposals. I agree with Barker et al. (2013) that a conservative approach is to “minimize changes to higher level avian classification and to continue to rank the lineages within [the nine-primaried passerines] as Families.”

To retain the old familiar families, such as Parulidae and Icteridae, means accepting as a Family some lineages that are only 10-11 million years old. Doing so recognizes the rapid rate of diversification in these birds, compared to all other bird groups, and helps to structure the phylogeny of this species-rich radiation. I think that at this moment in time, the same diversity can be shown with the addition of just four New World families, instead of the ten proposed by Barker et al. (2013). One of them is a monotypic family [Rhodinocichlidae] for Rosy Thrush-Tanager Rhodinocichla rosea, and it is moderately ancient at about 14 mya. The remaining splits are less old (10-13 mya) but single-species families of younger age can be avoided without violating monophyly, at least on the current evidence. The splits that must occur to avoid polyphyletic families are these three: (a) to split the Old World Buntings [Emberizidae] from New World Sparrows [Passerellidae]; (b) to create a new family [Mitrospingidae] from 3 genera of “tanagers” (Mitrospingus, Lamprospiza, Orthogonys) that are neither Tanagers nor Cardinals (I call them the "Mitrospingid tanagers"); and (c) to create a new family [Phaenicophilidae] from the lineage of “tanager-like” birds on Caribbean islands. Barker et al. (2013) had proposed these as four families, 3 of which were single-species families. There is little doubt from their evidence that genera Phaenicophilus, Xenoligea, Microligea, Nesospingus, and Spindalis are all more closely related to each other than to other species, and can easily form a Family grouping. Genus Calyptophilus [two chat-tanagers from Hispaniola] is less certain, but its placement in their phylogeny is only “weakly supported” and, if the “species tree” in their Figure 8 is used, fits closely with the other Caribbean lineages. For the moment it seems best to tentatively place the chat-tanagers with the other Caribbean species in a single Family.

This leaves unresolved the position of Zeledonia [wren-thrush], Teretistris [two Cuban warblers], and Icteria [Yellow-breasted Chat]. Currently these are assigned to the Parulidae (New World Warblers) but some (especially the chat) may be closer to Icterids. The situation is not yet fully resolved. For the moment, the conservative approach is to leave these three intriguing genera in Parulidae, pending further research. However, the wise world birder might consider planning trips to Costa Rica (for Zeledonia) or Cuba (for Teretistris).