This detailed study of the non-breeding distribution of the Atlantic Calonectris species complex is very neatly done. The questions addressed are relevant, the data set is impressive, the methods seem appropriate, the conclusions are (mostly) supported by the data, and the writing and presentation is excellent. My comments are thus fairly minor.

One thing that confused me a bit was the authors' use of the intermediate organisational level of 'population'. This is in general a fluffy term, and it is a good idea to define precisely what is meant in each specific instance, not least when it is the middle of three hierarchical levels as here. It is not surprising (or particularly interesting) that migratory connectivity is intermediate at an intermediate organisational level. So why are the specific populations used here interesting, and how were they defined? Are they used in management, or is there some other a priori definition? The confusion already appears in the abstract, where it is stated on l. 16: 'At population level, we provided ...' - without explaining what these populations are.

I'm not very convinced by the claim of intermediate habitat preferences of birds from the contact zone between Cory's and Scopoli's shearwater (abstract l. 16-20, conclusions l. 506-509). This is discussed at length, but I don't see where it is shown quantitatively. There may well be indications in the data of such intermediate preferences (which can be discussed), but unless there is clear evidence I would suggest not giving the subject such a prominent place in the abstract and conclusions.

Very minor comments:
- l. 44-46: perhaps mention here if there is evidence that pair formation occurs in the non-breeding areas.
- l. 58-59: what do you mean by 'these species'?
- l. 103-104: change 'below the Labrador Peninsula' to 'south of Newfoundland'.
- l. 117: out of curiosity, why are only data up to 2016 used? Did data collection stop?
- l. 300: Fig 3 is referenced before Fig 2.
- l. 481 & 484: two consecutive sentences start with 'In addition ...'.