Among ornithologists there is no general agreement on the number of avian orders. In the following list, the three most relevant sources on avian orders are compared with the avian orders recognised on the bird-phylogeny.de homepage.  

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]. 

Futuristic avian timetree in which all clades that are at least 52 million years 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.