Jim Mountjoy
Active member
I have been wondering recently if we can now come up with a better answer to the question of how many orders of birds should be recognized. Of course, an order (or any other higher taxonomic category) is to some degree arbitrary, but I agree with the view that at least within a class it would be desirable to have orders that reflect roughly equivalent levels of divergence from other branches. The number of orders recognized in various world lists varies from 27 in the Peters Check-list to 23 in Sibley and Monroe’s book, 40 in both the I.O.C. and Clements/eBird classifications (but not the same orders exactly), and up to 46 in John Boyd’s ‘Taxonomy in Flux’. The recent broad studies of avian classification, especially Jarvis et al. 2014 and Prum et al. 2015, seem to give us useful information to attempt a more objective standard for comparison. (Holt and Jønsson (2014) attempted a time-based delineation of avian orders, but being based on the composite phylogeny of Jetz et al. (2012) their results are very different from what would be indicated by the recent studies.)
It would be great to be able to simply say that ‘if taxa A & B diverged at least X millions of years ago, then they should be considered distinct orders’. Unfortunately, even though Jarvis et al. and Prum et al. both present time-calibrated phylogenies (and the topologies of their trees are quite similar), there seems to be fairly large differences in their estimated divergence dates. For example, the divergence of ostriches and tinamous is dated to barely more than 50 Ma by Prum et al., but is estimated to have occurred about 84 Ma by Jarvis et al.
Sibley and Ahlquist used the order Passeriformes as a reference point indicating roughly when a distinct, but well-supported, order should have diverged. According to Jarvis et al. the Passeriformes shared a common ancestor about 40 million years ago, but Prum et al. estimate this divergence as occurring about 50 million years ago. I tend to think that the Passeriformes might still be a reasonable initial guideline for what constitutes an avian order, but this marker might be hard to relate to a precise number of years.
I feel that Prum et al.’s phylogeny, as the most recent study, and with much better taxon sampling than Jarvis et al., should be the starting point for this discussion. If we use a criterion of ~52 Ma for their data (to comfortably include the starting point of the passerine radiation) then we can see which taxa meet, or fail to meet, this guideline.
To begin, the 52 million years criterion suggests that all of the palaeognaths fit within a single order (although a slightly less generous guideline of, say, 50 million years might lead to recognition of ostriches as a separate order from the remainder of the ratites and tinamous). This does contrast with the phylogeny in Jarvis et al., as they date the split between ostriches and tinamous to about 84 million years, but this estimate has a very wide credibility range, and the fact that the other ratites were not sampled by them makes their data difficult to compare.
Another possible ‘lumping’ of orders would be combining the Phoenicopteriformes and the Podicipediformes, as Prum et al. estimate that this split occurred only about 40 Ma. Again, Jarvis et al. provide an older estimate for this split (about 56 Ma), but this date has a notably large credibility range that includes 40 Ma.
One additional lump would be the Galbuliformes, as they diverged about 36 Ma ago in the Prum et al. tree, but this order seems to only be included in the Clements list.
On the other side of the ledger, there appear to be a number of taxa that deserve ordinal status that have not been generally recognized. To begin with, the traditional Caprimulgiformes appears to be a prime target for splitting. The ‘Taxonomy in Flux’ list has already included Steatornithiformes, Nyctibiiformes and Podargiformes, but other world lists have not. Even Boyd’s ‘TIF’ list has not included Aegotheliformes, although the A.O.U.’s North American Checklist Committee did include a mention of this order in their 57th supplement. According to the Prum et al. tree, all of these taxa were distinct lineages before 52 Ma, and so qualify as orders. (Jarvis et al. did not sample most of these groups.)
Taking this a step further, Prum et al. date the divergence between swifts and hummingbirds to about 54 Ma, and even Jarvis et al.’s estimate of about 43 Ma predates their estimate for the passerine divergence. So, I would suggest recognizing the Trochiliformes as well as the Apodiformes.
One group that has not received much attention in the taxonomic discussions is the Fregatidae, but Prum et al. have the frigatebirds diverging from the booby/darter/cormorant branch at about 53 Ma, suggesting that yet another order should be carved from the traditional Pelecaniformes. (They were not included in Jarvis et al.)
Finally, it has been evident since molecular studies of birds took off that there are some old branches among the traditional Coraciiformes and Piciformes, but not much consensus on which taxa deserve ordinal status. As mentioned earlier, Prum et al.’s results do not (quite) seem to support a separate Galbuliformes, and the Upupiformes of Sibley and Monroe’s classifications falls similarly short. However, there do seem to be three ancient branches amongst the old Coraciiformes that could deserve recognition – the rollers + ground-rollers, the todies, motmots and kingfishers, and then a separate order for the bee-eaters.
OK then, if I had a vote, here is what I feel (based largely on Prum et al. (2015)) should be the 46 avian orders. (I haven’t tried to sort out all of the nomenclature…)
1 Struthioniformes
2 Anseriformes
3 Galliformes
4 Caprimulgiformes
5 Steatornithiformes
6 Nyctibiiformes
7 Podargiformes
8 Aegotheliformes
9 Apodiformes
10 Trochiliformes
11 Musophagiformes
12 Otidiformes
13 Cuculiformes
14 Mesitornithiformes
15 Pterocliformes
16 Columbiformes
17 Gruiformes
18 flamingos and grebes
19 Charadriiformes
20 Eurypygiformes
21 Phaethontiformes
22 Gaviiformes
23 Sphenisciformes
24 Procellariiformes
25 Ciconiiformes
26 frigatebirds ('Fregatiformes'?)
27 Suliformes
28 Plataleiformes
29 Pelecaniformes
30 Ardeiformes
31 Opisthocomiformes
32 Cathartiformes
33 Accipitriformes
34 Strigiformes
35 Coliiformes
36 Leptosomiformes
37 Trogoniformes
38 Bucerotiformes
39 bee-eaters ('Meropiformes'?)
40 Coraciiformes
41 todies, motmots, and kngfishers
42 Piciformes
43 Cariamiformes
44 Falconiformes
45 Psittaciformes
46 Passeriformes
Comments and criticisms would be welcome.
It would be great to be able to simply say that ‘if taxa A & B diverged at least X millions of years ago, then they should be considered distinct orders’. Unfortunately, even though Jarvis et al. and Prum et al. both present time-calibrated phylogenies (and the topologies of their trees are quite similar), there seems to be fairly large differences in their estimated divergence dates. For example, the divergence of ostriches and tinamous is dated to barely more than 50 Ma by Prum et al., but is estimated to have occurred about 84 Ma by Jarvis et al.
Sibley and Ahlquist used the order Passeriformes as a reference point indicating roughly when a distinct, but well-supported, order should have diverged. According to Jarvis et al. the Passeriformes shared a common ancestor about 40 million years ago, but Prum et al. estimate this divergence as occurring about 50 million years ago. I tend to think that the Passeriformes might still be a reasonable initial guideline for what constitutes an avian order, but this marker might be hard to relate to a precise number of years.
I feel that Prum et al.’s phylogeny, as the most recent study, and with much better taxon sampling than Jarvis et al., should be the starting point for this discussion. If we use a criterion of ~52 Ma for their data (to comfortably include the starting point of the passerine radiation) then we can see which taxa meet, or fail to meet, this guideline.
To begin, the 52 million years criterion suggests that all of the palaeognaths fit within a single order (although a slightly less generous guideline of, say, 50 million years might lead to recognition of ostriches as a separate order from the remainder of the ratites and tinamous). This does contrast with the phylogeny in Jarvis et al., as they date the split between ostriches and tinamous to about 84 million years, but this estimate has a very wide credibility range, and the fact that the other ratites were not sampled by them makes their data difficult to compare.
Another possible ‘lumping’ of orders would be combining the Phoenicopteriformes and the Podicipediformes, as Prum et al. estimate that this split occurred only about 40 Ma. Again, Jarvis et al. provide an older estimate for this split (about 56 Ma), but this date has a notably large credibility range that includes 40 Ma.
One additional lump would be the Galbuliformes, as they diverged about 36 Ma ago in the Prum et al. tree, but this order seems to only be included in the Clements list.
On the other side of the ledger, there appear to be a number of taxa that deserve ordinal status that have not been generally recognized. To begin with, the traditional Caprimulgiformes appears to be a prime target for splitting. The ‘Taxonomy in Flux’ list has already included Steatornithiformes, Nyctibiiformes and Podargiformes, but other world lists have not. Even Boyd’s ‘TIF’ list has not included Aegotheliformes, although the A.O.U.’s North American Checklist Committee did include a mention of this order in their 57th supplement. According to the Prum et al. tree, all of these taxa were distinct lineages before 52 Ma, and so qualify as orders. (Jarvis et al. did not sample most of these groups.)
Taking this a step further, Prum et al. date the divergence between swifts and hummingbirds to about 54 Ma, and even Jarvis et al.’s estimate of about 43 Ma predates their estimate for the passerine divergence. So, I would suggest recognizing the Trochiliformes as well as the Apodiformes.
One group that has not received much attention in the taxonomic discussions is the Fregatidae, but Prum et al. have the frigatebirds diverging from the booby/darter/cormorant branch at about 53 Ma, suggesting that yet another order should be carved from the traditional Pelecaniformes. (They were not included in Jarvis et al.)
Finally, it has been evident since molecular studies of birds took off that there are some old branches among the traditional Coraciiformes and Piciformes, but not much consensus on which taxa deserve ordinal status. As mentioned earlier, Prum et al.’s results do not (quite) seem to support a separate Galbuliformes, and the Upupiformes of Sibley and Monroe’s classifications falls similarly short. However, there do seem to be three ancient branches amongst the old Coraciiformes that could deserve recognition – the rollers + ground-rollers, the todies, motmots and kingfishers, and then a separate order for the bee-eaters.
OK then, if I had a vote, here is what I feel (based largely on Prum et al. (2015)) should be the 46 avian orders. (I haven’t tried to sort out all of the nomenclature…)
1 Struthioniformes
2 Anseriformes
3 Galliformes
4 Caprimulgiformes
5 Steatornithiformes
6 Nyctibiiformes
7 Podargiformes
8 Aegotheliformes
9 Apodiformes
10 Trochiliformes
11 Musophagiformes
12 Otidiformes
13 Cuculiformes
14 Mesitornithiformes
15 Pterocliformes
16 Columbiformes
17 Gruiformes
18 flamingos and grebes
19 Charadriiformes
20 Eurypygiformes
21 Phaethontiformes
22 Gaviiformes
23 Sphenisciformes
24 Procellariiformes
25 Ciconiiformes
26 frigatebirds ('Fregatiformes'?)
27 Suliformes
28 Plataleiformes
29 Pelecaniformes
30 Ardeiformes
31 Opisthocomiformes
32 Cathartiformes
33 Accipitriformes
34 Strigiformes
35 Coliiformes
36 Leptosomiformes
37 Trogoniformes
38 Bucerotiformes
39 bee-eaters ('Meropiformes'?)
40 Coraciiformes
41 todies, motmots, and kngfishers
42 Piciformes
43 Cariamiformes
44 Falconiformes
45 Psittaciformes
46 Passeriformes
Comments and criticisms would be welcome.
