General Comments:
The authors investigated how maximal activity of 10 key metabolic enzymes in locomotory muscles of 11 species of diving ducks and 5 species of dabbling (non-diving) ducks reveal possible metabolic specializations related to diving capabilities across species. This is a classic comparative study in the sense that muscle tissue is sampled from individuals of each species and assumed to be representative of the species at large, and an appropriate phylogentic context is incorporated into the statistical analyses. The authors find significant differences between the three comparison groups (sea ducks, pochards, dabblers) in the activity of several of the measured enzymes, and a relatively weak relationship between average dive time (derived from the literature) and enzyme activities (only significant in one enzyme, citrate synthesis, associated with the citric acid cycle). It is the comparative differences in the activity of key metabolic enzymes that most readers will find of interest because such comparisons provide insights into key metabolic specializations associated with feeding underwater for air-breathing animals such as ducks.

A major strength of this study is the construction of a phylogeny that is then used to compare metabolic enzyme activities within an appropriate phylogenetic framework. In addition, the selection of metabolic enzymes that are representative of multiple metabolic pathways (e.g., glycolysis, TCA cycle, beta-oxidation, electron transport system) provides a broad context for assessing how cross-species differences in diving capabilities and phylogeny relate to metabolic specializations. I have pointed out in the Specific Comments a few ways to improve the description and presentation of the work. Most importantly, for studies such as this one that measure key metabolic enzymes that can change considerably and quickly in response to activity, such inter-specific comparisons require careful consideration of possible phenotypic flexibility when sampling individuals. More discussion of this topic would provide for a more helpful context for interpreting these inter-specific differences.

Specific Comments
Fig. 1: consider color coding the lines (or the genus species names) to clearly show which species are w/in the Mergini, Aythyini, and Anatini. All other tables/figures present the data for these three Tribes, yet the delineation of which species are in each Tribe is only provided in the text (and would be easy to add to this very helpful figure).

Fig. 2: very informative figure that places much of the enzyme work into the appropriate perspective. Might help to add to the legend or figure itself what parts are the mitochondria and the cell membrane.

Lines 100-102: please add a bit more information to the text about the collection of these birds. Specifically, when were they collected (mostly May-June, a few species in March; it’s in the Suppl Table 1 but helpful to add to text), at what point in their annual cycle were these birds collected (in migration? On breeding areas but prior to breeding?), and why were they collected at this point in their annual cycle? This information is necessary because many of the 10 enzymes measured in this study are known to change over the annual cycle and especially during migration.

Line 164: not sure of the RSPB policy about whether authors can cite an ‘in review’ paper for description of the detailed methods. Doesn’t help my ability to assess these methods here.

Lines 199: some readers may get confused by the many names for the three comparison groups: the three Tribe names (Mergini, Aythyini, Anatini) vs. sea ducks, pochards, dabblers vs. non-diving ducks to refer at times to dabblers and ‘divers’ referring to both sea ducks and pochards. Most importantly, be consistent in the naming of the three (or fewer) groups and avoid going back-and-forth in the naming when it is not really necessary.

Line 200: this is the only mention in the methods about the calculation of ratios of enzyme activity. More information needs to be provided about why calculating these ratios might be helpful and how you chose the particular selected ratios reported in the Results.

Line 281 and 345 and 352: what does ‘stronger diving sea ducks’ and ‘strongest diver(s)’ mean? If referring to dive time and depth, then better to say this again because ‘stronger’ is ambiguous in this context.

Lines 300-303: a too-complicated sentence that needs simplifying.

Line 344: Not sure why the Suppl Table 2 was cited here since there is no information about dive time.

Lit Cited: carefully check the citations for punctuation and the authors full names – I did not carefully check them all, but did notice errors.