"50.1. [...] If a work is by more than one person but it is clear from the contents that only one of these is responsible for the name or act, then that person is the author".
According to the introducing text (pp. 159-161, in English, signed by Hartert alone), Tring purchased a collection of birds from Argentina from Venturi, which came with a manuscript (a French translation of an originally Spanish text) that they agreed to publish. Hartert found many issues in the manuscript, thus reworked it extensively; and he also added information covering additional specimens from Argentina that Tring had received from other collectors (mostly, as per pp. 159-160: P Newman, CB Brittain, EW White, GA Baer, L Dinelli, J Steinbach, J Koslowsky).
If you go through the paper, you will see that most (but not all) accounts include a text explicitly quoted from Venturi's manuscript (in double quotes, attributed to "(S. V.)"). No such text is present in the Leptasthenura aegithaloides berlepschi subsp. nov. account, and the specimens from Argentina that were referred to this subspecies were from Dinelli (3), Steinbach (1) and Baer (1) -- not from Venturi. It follows that this particular account must have been added in its entirety by Hartert in the process described above; thus Hartert was the only person responsible for it, and is the author of the name.
Note also "Mon ami le Comte de Berlepsch" in this account: this implies the writer saw himself as a single author.
