Temporal threshold values (cutoffs) are inherently arbitrary and depend on conventions that taxonomists must agree upon. Thus countless alternatives are possible. Below, I propose an exemplary set of serial cutoffs that might be implemented one day for avian chronoclassification: 

Set of serial cutoffs defining categorical ranks for avian chronoclassification. Ranks and their respective endings follow the pioneering study of Sibley et al. (1988). [black: class-group names are not regulated, blue: order-group names and endings are not regulated, green: family-group names and endings are covered by the ICZN-code, red: genus-group names are covered by the ICZN-code, grey: species-group names are covered by the ICZN-code].

For comparison:

• Avise & Johns (1999) proposed the following avian cutoffs: 145 Ma for orders, 65 Ma for suborders, 56 Ma for superfamilies, 33 Ma for families, 24 Ma for subfamilies, 5 Ma for tribes, and 2 Ma for genera.
  
• Holt & Jønsson (2014), based on the timetree of Jetz et al. (2012), determined cutoffs that yielded the same number of taxa as previously recognised in the respective categorial rank: 65 Ma for avian orders, 37.5 Ma for avian families, and 11.4 Ma for avian genera.
  
• Considerably younger cutoffs are applied to passerine families. Thus, Jønsson et al. (2016) assigned family rank at 21.5 Ma, Cai et al. (2019) at 18 Ma, and Cai et al. (2021) at 15 Ma. Although probably favoured by many avian taxonomists, I dislike the idea of establishing a second, younger series of cutoffs especially for the Passeriformes. 

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