This is an important addition to the literature on the evolution of avian neuroanatomy, filling in some of the gaps in the fossil record and providing a thorough discussion of various neuroanatomical traits that relate to the new endocast presented by the authors. I enjoyed reading it and recommend it for publication pending a few very minor edits.

60: hypotheses

85: olfactory bulb

85-86: I would add a citation to the Early et al. 2020 paper you cite elsewhere as it investigated the optic lobes of the same L. plebius specimen published in Zelenitsky et al. 2011.

90: Check this institution code, in some cases USNM is preferred over NMNH

121: olfactory bulbs

123: Please include a caveat about the brain surface area and volume of Lithornis plebius, as it is a composite endocast rather than a complete. You note some damage to it in 332-333, but this doesn’t account for the fact that the front and the back were two different pieces, with the middle inferred and rendered with 3D modeling by an anatomical expert. See figure 3B in Early et al. 2020 for reconstruction details on this endocast.

191-198: It is unclear to me why you are doing a rather roundabout way of calculating the endocranial volume for L. vulturinus when you have a nearly complete endocast. The endocast is complete enough for you to publish the surface area of the endocast, so why not publish the volume of the endocast based on the endocast model instead of, or at least in addition to, the volume of the endocast based on body size? I think both approaches are fine for calculating endocranial volume, but I don’t understand why you wouldn’t also include the more direct way of measuring this trait.

226: If you are comparing the relative size of the optic lobes of L. vulturinus with L. plebius, it would be worth including the previously-measured surface area of the optic lobes of L. plebius published in Early et al. 2020.

331-332: In line 194, you say “The endocranial volume of L. plebius is 2.51 cm3 (Ksepka et al., 2020)” but here, you say “The volume of the L. plebius endocast (NMNH 336534) (Zelenitsky et al., 2011) is approximately 25 cm3”. I thought both publications were based on the same endocast, so which is the correct value? I also wasn’t able to find endocranial volumes in Zelenitsky et al. 2011, so even though it may have been the first publication of an image of this L. plebius endocast, it would be more appropriate to cite the first publication of its endocranial volume, likely the Ksepka et al. 2020 paper (unless I’m mistaken and Zelenitsky et al. did publish this value in 2011).

331-338: You seem to have reasonably accurate estimates of the body mass and brain volume of L. vulturinus in addition to L. plebius, so why not include L. vulturinus in this part of the discussion?

360: I think the axes are either proximal-distal or rostral-caudal, and in this case, I think you mean rostral-caudal.

367: “Brain architecture” can refer to internal structure of the brain, which may confuse readers as fossils lack this. I suggest using “brain morphology” here instead.

451-454: They are similarly reduced in at least one species of moa, as published in Early et al. 2020 (although that paper did not make such specific functional lifestyle hypotheses as the others you cited). I suggest including that here and adding their published values (which may be in the Early et al. 2020 supplement) to Table 2.

456: olfactory bulbs

512: olfactory bulbs

549: Please add a data availability statement indicating where the scan data of this specimen are available, if they will be shared. I also suggest adding a supplementary file listing the availability of other endocasts used in this study (ex. MorphoSource DOIs or other links), either those generated by this study or those mentioned in the acknowledgements section.

879: This seems to show a neurocranium rather than endocranium