Note "Avian orders"
Among ornithologists there is no general agreement on the exact number and composition of avian orders. In the following list, the three most relevant sources on avian orders are compared with the avian orders recognised herein:
Clements Checklist Birds of the World v2019
www.birds.cornell.edu/clementschecklist/overview-august-2019
HBW & BirdLife Int. digital checklist birds of the world v2019
www.datazone.birdlife.org/userfiles/file/Species/Taxonomy/HBW-BirdLife_Checklist_v4_Dec19.zip
bird-phylogeny.de
1.) Struthioniformes
2.) Rheiformes
3.) Apterygiformes
4.) Tinamiformes
5.) Casuariiformes
6.) Anseriformes
7.) Galliformes
8.) Podicipediformes
9.) Phoenicopteriformes
10.) Columbiformes
11.) Pterocliformes
12.) Mesitornithiformes
13.) Cuculiformes
14.) Musophagiformes
15.) Otidiformes
16.) Caprimulgiformes
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17.) Opisthocomiformes
18.) Gruiformes
19.) Charadriiformes
20.) Eurypygiformes
21.) Phaethontiformes
22.) Gaviiformes
23.) Sphenisciformes
24.) Procellariiformes
25.) Pelecaniformes
26.) Ciconiiformes
27.) Suliformes
28.) Accipitriformes
29.) Cathartiformes
30.) Strigiformes
31.) Coliiformes
32.) Leptosomiformes
33.) Trogoniformes
34.) Bucerotiformes
35.) Piciformes
36.) Galbuliformes
37.) Coraciiformes
38.) Cariamiformes
39.) Falconiformes
40.) Psittaciformes
41.) Passeriformes
1.) Struthioniformes
2.) Rheiformes
3.) Apterygiformes
4.) Tinamiformes
5.) Casuariiformes
6.) Anseriformes
7.) Galliformes
8.) Podicipediformes
9.) Phoenicopteriformes
10.) Columbiformes
11.) Pterocliformes
12.) Mesitornithiformes
13.) Cuculiformes
14.) Musophagiformes
15.) Otidiformes
16.) Caprimulgiformes
17.) Apodiformes
18.) Opisthocomiformes
19.) Gruiformes
20.) Charadriiformes
21.) Eurypygiformes
22.) Phaethontiformes
23.) Gaviiformes
24.) Sphenisciformes
25.) Procellariiformes
26.) Pelecaniformes
27.) Ciconiiformes
28.) Suliformes
29.) Accipitriformes
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30.) Strigiformes
31.) Coliiformes
32.) Leptosomiformes
33.) Trogoniformes
34.) Bucerotiformes
35.) Piciformes
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36.) Coraciiformes
37.) Cariamiformes
38.) Falconiformes
39.) Psittaciformes
40.) Passeriformes
1.) Struthioniformes
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2.) Anseriformes
3.) Galliformes
4.) Podicipediformes
5.) Phoenicopteriformes
6.) Columbiformes
7.) Pterocliformes
8.) Mesitornithiformes
9.) Cuculiformes
10.) Musophagiformes
11.) Otidiformes
12.) Caprimulgiformes
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13.) Opisthocomiformes
14.) Gruiformes
15.) Charadriiformes
16.) Eurypygiformes
17.) Phaethontiformes
18.) Gaviiformes
19.) Sphenisciformes
20.) Procellariiformes
21.) Pelecaniformes
22.) Ciconiiformes
23.) Suliformes
24.) Accipitriformes
25.) Cathartiformes
26.) Strigiformes
27.) Coliiformes
28.) Leptosomiformes
29.) Trogoniformes
30.) Bucerotiformes
31.) Piciformes
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32.) Coraciiformes
33.) Cariamiformes
34.) Falconiformes
35.) Psittaciformes
36.) Passeriformes
1.) Struthioniformes
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2.) Anseriformes
3.) Galliformes
4.) Podicipediformes
5.) Phoenicopteriformes
6.) Columbiformes
7.) Pterocliformes
8.) Mesitornithiformes
9.) Cuculiformes
10.) Musophagiformes
11.) Otidiformes
12.) Caprimulgiformes
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13.) Opisthocomiformes
14.) Gruiformes
15.) Charadriiformes
16.) Eurypygiformes
17.) Phaethontiformes
18.) Gaviiformes
19.) Sphenisciformes
20.) Procellariiformes
21.) Pelecaniformes
22.) Ciconiiformes
23.) Suliformes
24.) Accipitriformes
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25.) Strigiformes
26.) Coliiformes
27.) Leptosomiformes
28.) Trogoniformes
29.) Bucerotiformes
30.) Piciformes
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31.) Coraciiformes
32.) Cariamiformes
33.) Falconiformes
34.) Psittaciformes
35.) Passeriformes
A comparison of the ordinal systems outlined above shows that they differ from each other only with respect to seven orders. For this homepage, I decided to accept only those orders that are represented in each of these ordinal system, leaving a total of 35 orders. Although not explicitly mentioned, this approach has also been chosen by Kuhl et al. (2021). It should be borne in mind, however, that all currently recognized ordinal systems are based on tradition rather than science. Therefore, it might be worthwhile to envision an alternative system of avian orders that takes into account the supposed age of the orders. Although this would be an objective way to define avian orders, it would result in significant changes to the traditional taxonomy and thus is not recommended at the moment. The main argument against changing the traditional system is the fact that the estimated divergence times often differ significantly among authors, largely depending on the selection of fossils and the maximum age constraint set be the investigators. [Explanation: Divergence time estimates are based on molecular dating techniques, which in turn depend on reliable fossil calibrations (i.e. correctly dated and taxonomically correctly placed fossils, which is a very difficult task)].
Futuristic timetree in which all clades that are at least 52 million old are attributed ordinal status. Where the names of traditional orders have been retained, although they have been split into several orders, these names are represented in blue colour. The newly recognized orders are represented in green colour. The total number of the novel avian orders amounts to 50.