Foraging behaviour and habitat-use drives niche segregation in sibling seabird species

To mediate competition, similar sympatric species are assumed to use different resources, or the same but geographically separated resources. The two giant petrels (Macronectes spp.) are intriguing in that they are morphologically similar seabirds with overlapping diets and distributions. To better understand the mechanisms allowing their coexistence, we investigated intra- and interspecific niche segregation at Marion Island (Southern Indian Ocean), one of the few localities where they breed in sympatry. We used GPS tracks from 94 individuals and remote-sensed environmental data to quantify habitat use, combined with blood carbon and nitrogen stable isotope ratios from 90 individuals to characterize their foraging habitat and trophic ecology. Females of both species made distant at-sea foraging trips and fed at a similar trophic level. However, they used distinct pelagic habitats. By contrast, males of both species mainly foraged on or near land, resulting in significant sexual segregation, but high interspecific habitat and diet overlap. However, some males showed flexible behavioural strategies, also making distant, pelagic foraging trips. Using contemporaneous tracking, environmental and stable isotope data we provide a clear example of how sympatric sibling species can be segregated along different foraging behaviour dimensions.


Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
Overall this paper is well-written and coherent. Conclusions followed well from the evidence in the text and were appropriately drawn in the context segregation of foraging and habitat use segregation as a consequence of competition avoidance. Support from the literature was sufficient to bolster claims and references were appropriately selected. However I have some methodological question descried below.
See comments below: L31: You use characterise here and along the text characterize. Please, try to be coherent on this.
L49-57: Why do you do not show also studies on seabirds to demonstrate the segregation both in space and time for sympatric species? It would be good to see some examples on seabirds.
L127: The default conclusion of non-overlap in seabird studies is most often intra-specific competition, thereby perpetuating an assumption that has little or no support in my opinion. One must demonstrate resource limitation and competitive exclusion to confirm competition, which is rarely possible with seabird studies (especially from remote tracking). There are other equally plausible explanations to spatial and habitat segregation in seabirds that you could explore here.
L134-135: Could you explain more in detail the oceanography of the study area? Such intermediate position in relation to frontal zones should have particular oceanographic features that may or not limit or explain the activity and distribution patterns both species and sexes.
L138-140: The last censuses for these populations were in 2009? It would be good to see recent numbers. Plus, regarding populations numbers did you thought in relate the foraging patterns with the population size to analyse to what extent foraging effort change according to the population size?
L178: Please explain the unit of h factor. Degrees or meters? Moreover, why you used an h factor of 1.0? You could have followed the methodology in Lascelles BG, Taylor PR, Miller MGR, Dias MP, Oppel S, Torres L, Hedd A, Le Corre M, Phillips RA, Shaffer SA, Weimerskirch H, Small C (2016) Applying global criteria to tracking data to define important areas for marine conservation. Divers Distrib 22:422-431. https://doi.org/10.1111/ddi.12411. In this way, the mean ARS behaviour for all the foraging trips in each year could be used separately as h factor and you could also use the average ARS for each individual per year as a response variable. Perhaps females show higher ARS scale as a response to the longer and widespread oceanic trips? L177-179: Moreover, one thing that I noticed is that you used all the positions to generate utilisation distributions and do not filtered the positions where birds engage in ARS or searching behaviour. Do you considered filtering the positions based on speed thresholds or to use clustering analysis (e.g. EMBC -Expectation-Maximization Binary Clustering) or Hidden Markov Models to select the positions indicative of searching behaviour and measure the kernel overlap over these positions only? -I really think that this study would benefit from investigating the habitat use by measuring the spatial overlap only in the foraging locations where birds initiate searching behaviour.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes L206-208: It is unclear which values you report. L225: Does this mean your sample size was 90 or less then 90? L227: Moving (Mardia 1970) to just before the comma would make this sentence easier to read.
L236: Since some individuals clearly foraged locally (at a known latitude), how does this correction factor influence your results? This needs to be explained and justified. You could be correcting for a combined penguin, seal, burrowing petrel signal?
L246: It seems a bit backwards to report your guess for the sexes and then confirm them with molecular sexing. Why not say first that your molecular sexing confirmed your grouping and then report? L252: You should provide some indication of how long birds were tracked for. 1 day, 5days, 2 weeks?? L253: You don't provide any quantitative methods for how you determined these two foraging strategies. These need to be included. Why do the females fit into this group if they are the outliers? It seems like ~90km is too far to be foraging on penguin/seal resources or in-shore waters around Marion?
L253: Were birds that were categorized in the second group tracked for longer? Ie. Long enough to make both the short trips observed in the first group AND a long trip? L281: Figure 3 is hard to interpret since the points are plotted on top each other.
L288: It isn't surprising that at the 50% Utilization distribution scale that the males wouldn't be segregated. Without knowing the unit of the smoothing factor h it is hard to know how "smoothed' these distributions are relative to the data, but from your point data it seems like they might be very smoothed. Did you see finer scale clustering and separation between the males in the GPS data or was your GPS data too low temporal resolution to tell? L328: I am not sure that you explicitly demonstrate this with your data. L329/L332 The statements "exploiting the same resources" and "males were isotopically segregated" seem contradictory.
L338 replace "utilized" with "used" L348-351: You included oceanographic habitat analysis in this paper. How do your results support this statement? L354 Integrating the observations of feeding with the first paragraph would be useful.
L359 Is this because the SGP go further south or are just foraging on other species that range further south? Since you are looking at plasma it doesn't seem like the first would fit with your tracking data results. Clarify and integrate the tracking into this discussion.
L364: I think you methods may only resolve "large scale spatial overlap", ie. you show that they are both foraging on the same island. In Figure 1 the NGPs that made short trips seem to have a much more extensive at sea distribution than the SGPs. They also seem to visit different beaches that the SGPs (that were tracked) didn't visit.
L365: More details (and analysis?) are needed to explain how the timing of spatial overlap is important in order support these statements.
L373-383 Since you can't directly address this, shortening this explanation and integrating it into the prior paragraph would be helpful. Do you know if there were ample carcasses available during your study years?
L385 You Supplemental Figure 7 shows that you are missing the prey with the elevated N. Without options (and data) for your high N prey items your prey paragraph feels very speculative. I am skeptical about burrowing petrels being a high component of the diet of male giant petrels -do you have any observations from your study years?
L405-407 The Gonzalis-Solis and Granroth-Wilding study were both from Bird Island, South Georgia so it seems odd to contrast them to each other like they were from different places. It seems likely that annual variation plays a role as well. Did your study find any variation between the three years? L410: Both males and females forage at sea during the winter, right? Perhaps males exclude females from easily accessed carrion resources, but are able to supplement with at-sea resources as needed.
L419 The reference at the end of this sentence seems out of place. Perhaps a review would make more sense.
L417-432 This paragraph is unexpected and out of place given how the study was framed and the hypothesis presented in the introduction. Omit or reframe to include anthropogenic threats in the introduction.
L435 While it may be true "for the first time" doesn't really speak to the interesting or informative parts of this study. Omit.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept with minor revision (please list in comments) Comments to the Author(s) studying intra and interspecific segregation in seabirds is key to understand theoretical aspects of coexistence in sympatric sibling species but also in terms of species conservation.
This study investigated segregation in two sympatric sibling species using tracking, environmental and stable isotope data. The study is well executed and well written. I suggest only minor changes to clarify certain aspects. 1. in the conclusion the authors suggest very nicely how this intra and inter specific segregation arose in evolutionary times. however, at the beginning and within the introduction they focused on the common definition of competition to address segregation, which implies limited resources which is not prove neither imply in this study. I suggest adding other theories for segregation within the introduction that may be influencing in the pattern found in the study spp (i.e. life history theory). 2. I found very interesting exploring habitat use using random forest analysis, I suggest audience may be benefit if further details are given for the analyses. Also, more details in the results section regarding this will be useful. 3. Lines 349-351 what do the authors mean this this statement? How is the interpretation of oceanography influencing the foraging strategy? diet preference could influence foraging strategy and thus oceanography as prey rely on certain aspects of the ocean, please clarify.

Decision letter (RSOS-200649.R0)
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Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) Overall this paper is well-written and coherent. Conclusions followed well from the evidence in the text and were appropriately drawn in the context segregation of foraging and habitat use segregation as a consequence of competition avoidance. Support from the literature was sufficient to bolster claims and references were appropriately selected. However I have some methodological question descried below.
See comments below: L31: You use characterise here and along the text characterize. Please, try to be coherent on this.
L49-57: Why do you do not show also studies on seabirds to demonstrate the segregation both in space and time for sympatric species? It would be good to see some examples on seabirds.
L127: The default conclusion of non-overlap in seabird studies is most often intra-specific competition, thereby perpetuating an assumption that has little or no support in my opinion. One must demonstrate resource limitation and competitive exclusion to confirm competition, which is rarely possible with seabird studies (especially from remote tracking). There are other equally plausible explanations to spatial and habitat segregation in seabirds that you could explore here. In this way, the mean ARS behaviour for all the foraging trips in each year could be used separately as h factor and you could also use the average ARS for each individual per year as a response variable. Perhaps females show higher ARS scale as a response to the longer and widespread oceanic trips? L177-179: Moreover, one thing that I noticed is that you used all the positions to generate utilisation distributions and do not filtered the positions where birds engage in ARS or searching behaviour. Do you considered filtering the positions based on speed thresholds or to use clustering analysis (e.g. EMBC -Expectation-Maximization Binary Clustering) or Hidden Markov Models to select the positions indicative of searching behaviour and measure the kernel overlap over these positions only? -I really think that this study would benefit from investigating the habitat use by measuring the spatial overlap only in the foraging locations where birds initiate searching behaviour.

Reviewer: 2
Comments to the Author(s) This manuscript provides a good description of how northern and southern giant petrels partition trophic resources and spatially at Marion Island. The authors present three years of tracking data and associated stable isotope values from plasma (short-term concurrent time period). They find that females of the two species have distinct distributions at sea, while the males forage closer to shore or on land. All four categories had low overlap in their isotopic niches, but when just considering birds that made at-sea distant trips male and female NGPs isotopic niches overlap. I have numerous questions about interpretation, structure, and clarity, but I found the study informative.

Main comments
An indication of how "good" your study years would be helpful in informing the discussion. Potentially this could be a reproductive success, breeding number, or a metric of carcass numbers. While not perfect this could help provide context for how abundant prey was, the degree of competition, and how pressing the need might be to exploit other resources -this seems most relevant for the males.
Annual differences or similarities are not addressed. As annual variation is often a strong driver of seabird diets and distributions please provide some justification for why you decided to pool the years.
In the introduction the following hypothesis is given "Given the species' recent divergence and marked sexual dimorphism, we predict that segregation between sexes should be greater than that between species, but that the two species should also show some segregation along one or more niche-axes, which has developed in the 0.2 million years since their divergence." This seems to generally hold true, but the very distinct geographic distributions of the females of the two species seem more distinct than the male-female segregation particularly for the NGPs. Returning to this in the discussion would be helpful.
While great for completeness, this manuscript relies heavily on supplemental figures and tables and at times that is distracting for the reader. The stable isotope analysis section has three tables and two figures devoted to it in the main text (not supplemental). Likewise the two figures with maps in the main text seem somewhat redundant. I suggest balancing the data shown in the main manuscript and reducing the reliance on supplemental materials.
In line Comments: L50 prey type, size, or? L51 remove internal parenthesis -issue throughout the paper L58 "structures" L45-59 The examples in this paragraph are interesting, but they feel like a list of disparate examples of the many permutations of differences in diet/space/time between similar species. A better description of the linking concepts is needed, particularly to understand why the authors chose to highlight these examples. The first sentence and the concluding sentence of the paragraph seem to be expressing the same concept.
L61-68 Are we choosing between the 'forage selection hypothesis' and the 'activity budget hypothesis'? "Among the explanations" implies that there are other explanations not discussed here. This paragraph needs editing to clearly highlight how males and females may or may not partition resources.
L70-86 These seem like on paragraph rather than two.
L112: given the large body of work on this topic, "for the first time" seems like an overstatement and is not a strong or informative justification of this study. L119: reference needed for "0.2 million years…" L123-24: It would be useful to introduce stable isotopes earlier in the introduction.
L126: Given the number of tracking/stable isotope studies of just seabirds, "novel" seems out of place here.
L138: Since your study site is on Marion Island it would be useful to know how many of these breeding pairs are on that island.
L143: This implies that if they had a nesting location and phenology similar to NGP, but a bill tip of SGP they would be classed as NGP?
L148: Handling times would be a useful thing to report. L187: I guess these variables are not for the locations on land and just for the at-sea distributions? Specify "at-sea habitat." Potentially incorporating land variables would be a useful approach for the male giant petrels. L225: Does this mean your sample size was 90 or less then 90? L227: Moving (Mardia 1970) to just before the comma would make this sentence easier to read. L236: Since some individuals clearly foraged locally (at a known latitude), how does this correction factor influence your results? This needs to be explained and justified. You could be correcting for a combined penguin, seal, burrowing petrel signal?
L246: It seems a bit backwards to report your guess for the sexes and then confirm them with molecular sexing. Why not say first that your molecular sexing confirmed your grouping and then report? L252: You should provide some indication of how long birds were tracked for. 1 day, 5days, 2 weeks??
L253: You don't provide any quantitative methods for how you determined these two foraging strategies. These need to be included. Why do the females fit into this group if they are the outliers? It seems like ~90km is too far to be foraging on penguin/seal resources or in-shore waters around Marion?
L253: Were birds that were categorized in the second group tracked for longer? Ie. Long enough to make both the short trips observed in the first group AND a long trip? L281: Figure 3 is hard to interpret since the points are plotted on top each other.
L288: It isn't surprising that at the 50% Utilization distribution scale that the males wouldn't be segregated. Without knowing the unit of the smoothing factor h it is hard to know how "smoothed' these distributions are relative to the data, but from your point data it seems like they might be very smoothed. Did you see finer scale clustering and separation between the males in the GPS data or was your GPS data too low temporal resolution to tell? L328: I am not sure that you explicitly demonstrate this with your data. L329/L332 The statements "exploiting the same resources" and "males were isotopically segregated" seem contradictory.
L338 replace "utilized" with "used" L348-351: You included oceanographic habitat analysis in this paper. How do your results support this statement? L354 Integrating the observations of feeding with the first paragraph would be useful.
L359 Is this because the SGP go further south or are just foraging on other species that range further south? Since you are looking at plasma it doesn't seem like the first would fit with your tracking data results. Clarify and integrate the tracking into this discussion.
L364: I think you methods may only resolve "large scale spatial overlap", ie. you show that they are both foraging on the same island. In Figure 1 the NGPs that made short trips seem to have a much more extensive at sea distribution than the SGPs. They also seem to visit different beaches that the SGPs (that were tracked) didn't visit.
L365: More details (and analysis?) are needed to explain how the timing of spatial overlap is important in order support these statements.
L373-383 Since you can't directly address this, shortening this explanation and integrating it into the prior paragraph would be helpful. Do you know if there were ample carcasses available during your study years?
L385 You Supplemental Figure 7 shows that you are missing the prey with the elevated N. Without options (and data) for your high N prey items your prey paragraph feels very speculative. I am skeptical about burrowing petrels being a high component of the diet of male giant petrels -do you have any observations from your study years?
L405-407 The Gonzalis-Solis and Granroth-Wilding study were both from Bird Island, South Georgia so it seems odd to contrast them to each other like they were from different places. It seems likely that annual variation plays a role as well. Did your study find any variation between the three years? L410: Both males and females forage at sea during the winter, right? Perhaps males exclude females from easily accessed carrion resources, but are able to supplement with at-sea resources as needed.
L419 The reference at the end of this sentence seems out of place. Perhaps a review would make more sense.
L417-432 This paragraph is unexpected and out of place given how the study was framed and the hypothesis presented in the introduction. Omit or reframe to include anthropogenic threats in the introduction.

RSOS-200649.R1 (Revision)
Review form: Reviewer 2 Is the manuscript scientifically sound in its present form? Yes

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Recommendation? Accept with minor revision (please list in comments)
Comments to the Author(s) I found the manuscript much improved. Thank you for your detailed replies to the reviewer comments.
The authors have addressed the potential for annual differences in the permutation analysis, but it is unclear how annual differences were accounted for (or not) in the at-sea habitat use analysis or the stable isotope statistical analysis. If years were not accounted for this should be clearly stated in the text. Otherwise, my comments, listed below are very minor. L62 insert "same" three axes… L68 consider commas rather than parenthesis L123 "high resolution tracks" at hourly locations this "high-resolution" statement seems like an oversight. Omit Replace utilize with use throughout the manuscript L126: Rephrase: The conclusion of your introduction reads like a result "we show segregation in their habitat use and foraging ecology that may have evolved as a competition avoidance mechanism. Consider omitting this last statement L132 Rephrase (missing comma?) "are located on a rise in water ~5000 m deep" L132 Add the word "habitat" before variability L154: Handling time should be reported as a value ± standard deviation. L184: This sentence may not be needed: "We tested other class classification methods, and bivariate k-means, but these results were not ecologically sensible." L260: Rephrase for grammar: "as far as possible" L320-322: Rewrite to avoid 1 sentence paragraphs. L321: Reverse order here for consistency: NGPs and SGPs L336: Reiterate that this is during incubation.

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept as is

Comments to the Author(s)
Authors has revised and added all the suggestions in the first round, I believe the manuscript is sound and sum to the knowledge on sibling species competition and segregation

Decision letter (RSOS-200649.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Reisinger, On behalf of the Editors, we are pleased to inform you that your Manuscript RSOS-200649.R1 "Foraging behaviour and habitat-use drives niche segregation in sibling seabird species" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referees' reports. Please find the referees' comments along with any feedback from the Editors below my signature.
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Reviewer comments to Author: Reviewer: 2 Comments to the Author(s) I found the manuscript much improved. Thank you for your detailed replies to the reviewer comments.
The authors have addressed the potential for annual differences in the permutation analysis, but it is unclear how annual differences were accounted for (or not) in the at-sea habitat use analysis or the stable isotope statistical analysis. If years were not accounted for this should be clearly stated in the text. Otherwise, my comments, listed below are very minor. L62 insert "same" three axes… L68 consider commas rather than parenthesis L123 "high resolution tracks" at hourly locations this "high-resolution" statement seems like an oversight. Omit Replace utilize with use throughout the manuscript L126: Rephrase: The conclusion of your introduction reads like a result "we show segregation in their habitat use and foraging ecology that may have evolved as a competition avoidance mechanism. Consider omitting this last statement L132 Rephrase (missing comma?) "are located on a rise in water ~5000 m deep" L132 Add the word "habitat" before variability L154: Handling time should be reported as a value ± standard deviation. L184: This sentence may not be needed: "We tested other class classification methods, and bivariate k-means, but these results were not ecologically sensible." L260: Rephrase for grammar: "as far as possible" L320-322: Rewrite to avoid 1 sentence paragraphs. L321: Reverse order here for consistency: NGPs and SGPs L336: Reiterate that this is during incubation.
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Decision letter (RSOS-200649.R2)
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Dear Dr Reisinger,
It is a pleasure to accept your manuscript entitled "Foraging behaviour and habitat-use drives niche segregation in sibling seabird species" in its current form for publication in Royal Society Open Science. The comments of the reviewer(s) who reviewed your manuscript are included at the foot of this letter.
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Associate Editor's comments:
Given the commentary supplied by the reviewers, the Editors would like you to fully address the referees' concerns. Bear in mind that the journal does not generally permit multiple rounds of revision, and if you are not able to satisfy the critical reviewers that the paper is ready for publication on receipt of the revision, we may not be able to proceed further, so please do your best to engage critically with the comments and remember to supply a pointby-point response as well as a 'tracked changes' version of the revised paper. We'll look forward to receiving your revision in due course.

RESPONSE: We thank the Associate Editor and Reviewers for their thoughtful remarks. We address each of the reviewer's comments in detail below, but broadly we have made the following major changes:
We focus less on current competition as the force driving segregation, acknowledging that other mechanisms, mentioned in the introduction now, can result in the observed segregation. However, we note that a different causal mechanism does not negate the patterns that can be observed in our data.
We have updated the methods substantially in response to the reviewers' suggestions. At the same time, we provide more details on the methodology, as requested. Particularly our revisions focussed on the recalculation of the utilisation distributions. We note that this has not qualitatively changed our results.

Finally, we allow for the effect of potential inter-annual effects by constraining our permutation tests of spatial and isotopic overlap by year. A full analysis relating any year-to-year variations in segregation that there may be (which are not obvious to us from exploratory analysis or the current analysis) is beyond the scope of the work, since the required data (year-to-year population size information and prey abundance information) is not available to us, and in our opinion would be difficult to collect
properly. However, we briefly discuss this question in the Discussion and recommend such work in the Conclusion.
As well as the revised document, we have attached a marked-up document showing the changes (compared with references unformatted, for simplicity). Line numbers below refer to the clean, reference-formatted text, but we quote the full references for readability.

Reviewers' Comments to Author: Reviewer: 1
Overall this paper is well-written and coherent. Conclusions followed well from the evidence in the text and were appropriately drawn in the context segregation of foraging and habitat use segregation as a consequence of competition avoidance. Support from the literature was sufficient to bolster claims and references were appropriately selected. However I have some methodological question descried below.

See comments below:
L31: You use characterise here and along the text characterize. Please, try to be coherent on this.

RESPONSE: We have made sure to use British spelling throughout.
Appendix A L49-57: Why do you do not show also studies on seabirds to demonstrate the segregation both in space and time for sympatric species? It would be good to see some examples on seabirds.

RESPONSE: We note that Reviewer 2 found the diverse examples in the introduction
interesting. To maintain the broader interest of the paper and not be too taxonomically restrictive, we have changed only one of the examples (allochrony, or segregation along a time axis) for a seabird example. However, we now state at the beginning of paragraph three, that inter-and intra-specific segregation is frequently described in seabirds, with example references (see response to Reviewer 2 below), as a link to the brief introduction to the tools we used in this study.
L127: The default conclusion of non-overlap in seabird studies is most often intra-specific competition, thereby perpetuating an assumption that has little or no support in my opinion.
One must demonstrate resource limitation and competitive exclusion to confirm competition, which is rarely possible with seabird studies (especially from remote tracking). There are other equally plausible explanations to spatial and habitat segregation in seabirds that you could explore here.

RESPONSE: We agree that it is often not possible to demonstrate resource limitation and competitive exclusion, as is the case in our study, and there may be alternative explanations for segregation. We have thus included this caveat in our introduction: "Competition (whether current or in species' evolutionary history) is frequently invoked as an explanation for niche segregation, but segregation may result from other factors including non-competitive evolutionary adaptation and stochastic processes (Begon et al. 2006)." [L66-68]
Further, our revised statement introduction about the hypothesis for sexual segregation makes it clearer that segregation can be due to intrinsic constraints rather than competition avoidance: "The 'specialisation hypothesis' suggests that males and females segregate habitats and/or diets due to different requirements, opportunities and constraints related to their morphology, physiology and reproductive roles ( Regarding an analysis relating population size to foraging effort, this would require estimating maps of habitat use and then converting these to density maps or spatially-explicit abundance estimate for each species. This is not necessarily a straightforward conversion according to population size (which data was not available to us for the study years anyway). Given this, and that the overlap and segregation patterns are consistent across years (see response to one of Reviewer 2's main comments), we believe such an analysis would not be informative with respect to the stated goals of this manuscript, although it would certainly be interesting to explore with additional data in a future, more detailed manuscript. However, we have added a very brief discussion of this issue (see response to Reviewers 2). Regarding using ARS values as a response variable, our current random forest classification uses the group (sex by species combinations) as the response variable, to investigate how environmental conditions are different along the at-sea tracks of each group. The advantage of this approach is that all four groups are considered together in the same model (multi-class classification; we explain this further in response to Reviewer 3's request for more information about the random forest approach). Another analysis relating ARS to environmental conditions (presumably separately for each group) is not in line with the stated aims of our current manuscript, since it would deal more descriptively with the relationship between movement behaviour and environmental conditions for each group, rather than discerning the habitat differences among groups, which is our aim here.
L177-179: Moreover, one thing that I noticed is that you used all the positions to generate utilisation distributions and do not filtered the positions where birds engage in ARS or searching behaviour. Do you considered filtering the positions based on speed thresholds or to use clustering analysis (e.g. EMBC -Expectation-Maximization Binary Clustering) or Hidden Markov Models to select the positions indicative of searching behaviour and measure the kernel overlap over these positions only? -I really think that this study would benefit from investigating the habitat use by measuring the spatial overlap only in the foraging locations where birds initiate searching behaviour.

RESPONSE: During our revisions we explored using EMbC (Garriga et al. 2016, DOI:
10.1371/journal.pone.0151984) to classify the behaviour at locations along the track. EMbC relies on clustering locations according to speed and turning angle. We found that the resulting four behavioural modes classification did not deal well with the different movement modes present in our data (for example, very restricted movement of males versus extensive movement of females and some males). Specifically, we found that the clustering on turning angle appeared to be the less informative part of the classification. In our experience, this was illustrative of the difficulties of detecting behaviour modes in seabird tracking data that is common across many methods. However, to decrease any bias that may result from including locations where birds were not foraging, we calculated a speed threshold (by calculating Fisher-Jenks breaks on the speed values), and excluded locations where speed was greater than 7.2 m/s. This threshold was more conservative than that suggested by EMbC analysis. The rationale is that high-speed movements are unlikely to represent locations at which birds are foraging. We have modified the text thus: "Before calculating utilisation distributions, we excluded locations where the preceding displacement required a speed >7.2m/s, with the rationale that these locations were unlikely to represent foraging and would thus bias the utilisation distributions. The speed threshold was calculated using the Fisher-Jenks algorithm in the classInt package (Bivand 2020)." [L190-193] Reviewer: 2 This manuscript provides a good description of how northern and southern giant petrels partition trophic resources and spatially at Marion Island. The authors present three years of tracking data and associated stable isotope values from plasma (short-term concurrent time period). They find that females of the two species have distinct distributions at sea, while the males forage closer to shore or on land. All four categories had low overlap in their isotopic niches, but when just considering birds that made at-sea distant trips male and female NGPs isotopic niches overlap. I have numerous questions about interpretation, structure, and clarity, but I found the study informative.

Main comments
An indication of how "good" your study years would be helpful in informing the discussion. Potentially this could be a reproductive success, breeding number, or a metric of carcass numbers. While not perfect this could help provide context for how abundant prey was, the degree of competition, and how pressing the need might be to exploit other resources -this seems most relevant for the males.

RESPONSE: See next response.
Annual differences or similarities are not addressed. As annual variation is often a strong driver of seabird diets and distributions please provide some justification for why you decided to pool the years.

RESPONSE:
Our exploratory analyses indicated no significant differences in space-use between years. Nonetheless, in our reanalysis we constrained permutations by year to minimise the potential effect of any interannual effects on spatial distribution (that is, labels could only be swapped within years). This did not qualitatively alter our results (and made only very minor quantitative differences).
"Since interannual variation in resource availability and environmental conditions may influence foraging behaviour, we constrained the permutations by year". [L198-199] In terms of how 'good' each year was, the number of breeding pairs per year was not available to us. Unfortunately, we do not have good information on prey availability each year either. The most reliable information -annual number of southern elephant seal pups born at the island -did not vary significantly among the study years (552, 557 and 623 in 2015-2017, respectively; University of Pretoria, unpublished data). However, this is not an ideal proxy since the number of carcasses should be counted to better assess food availability ashore. In the revised Discussion we mention the potential role of interannual variation in carcass availability, and in our conclusion we stress that interannual variation should be investigated further: "Future work should also address the role of interannual variation in resource availability on segregation patterns, particularly in males ashore." [L434-436] However, we do note that an earlier study at Marion Island found no clear relationship between land-based prey availability (the numbers of various seal and penguin species) and breeding parameters of giant petrels (De Bruyn et al. 2007). It may be that birds compensate for resource variability through their foraging behaviour such that the variability is not observed in their breeding parameters, or that minor changes in resource availability do not observably influence foraging behaviour. That study concludes: "Our data suggest no appreciable overlap whereby the same land-based resource singularly regulates either giant petrel's population. A complex suite of factors related to diet and foraging (both marine and land-based), and other factors, such as the availability of appropriate nest sites, most likely interplay to regulate populations of these sibling species."

De Bruyn PJN, Cooper J, Bester MN, Tosh CA (2007) The importance of land-based prey for sympatrically breeding giant petrels at sub-Antarctic Marion Island. Antarct Sci 19:25-30
In the introduction the following hypothesis is given "Given the species' recent divergence and marked sexual dimorphism, we predict that segregation between sexes should be greater than that between species, but that the two species should also show some segregation along one or more niche-axes, which has developed in the 0.2 million years since their divergence." This seems to generally hold true, but the very distinct geographic distributions of the females of the two species seem more distinct than the male-female segregation particularly for the NGPs. Returning to this in the discussion would be helpful.

RESPONSE:
We return to this issue in our revised conclusion, tying the results back to a potential eco-evolutionary scenario: "The intraspecific segregation between males and females, which may be driven by competitive exclusion of the smaller females from carrion resource ashore, is more marked than interspecific segregation. However, the geographic segregation of northern and southern giant petrels at sea is striking. As suggested by Hunter (Hunter 1987), the parallel pattern of sexual segregation in the two species indicates that sexual segregation arose before speciation in the giant petrels 0.2 mya (Techow et al. 2010), and the use of different geographic foraging areas at sea may represent the foraging preferences of ancestral populations that have persisted after secondary contact (Hunter 1987). These foraging preferences may have arisen in sympatry, not necessarily due to competition, or evolved in allopatry." [L424-432] While great for completeness, this manuscript relies heavily on supplemental figures and tables and at times that is distracting for the reader. The stable isotope analysis section has three tables and two figures devoted to it in the main text (not supplemental). Likewise the two figures with maps in the main text seem somewhat redundant. I suggest balancing the data shown in the main manuscript and reducing the reliance on supplemental materials.

RESPONSE:
We retained Figure 1 since it maps the main feature found to differentiate northern and southern giant petrel habitats -sea surface temperature -while showing the GPS tracking locations (which are not shown in Figure 2). We replaced Figure 3 with supplementary Figure S5, which has seven in-text mentions. We deleted Supplementary Figure S4, S6 and S7. There are now only two supplementary tables (both too large for the main text) and three supplementary figures (which we deem less important than those in the text).

In line Comments:
L50 prey type, size, or? RESPONSE: Prey type and size. Changed to "Sympatric jaguar (Panthera onca) and puma (Puma concolor), for example, had highly overlapping space and time-use, but differed in the prey type and size they selected". [L49-51] L51 remove internal parenthesis -issue throughout the paper RESPONSE: We have now formatted the references as required by RSOS. Note that the responses here use the original full format, for readability.

L45-59
The examples in this paragraph are interesting, but they feel like a list of disparate examples of the many permutations of differences in diet/space/time between similar species. A better description of the linking concepts is needed, particularly to understand why the authors chose to highlight these examples. The first sentence and the concluding sentence of the paragraph seem to be expressing the same concept.

RESPONSE:
We have modified the paragraph to clarify that resource segregation typically occurs along three axes (diet, space and time) and that interacting species may only segregate along one axis, while overlapping on others (thus clarifying why we considered these particular 'permutations', although we tried to use taxonomically diverse examples) -this is important for our paper, since we consider resource use along the axes introduced in these examples. We now also clearly state which of these segregation axes each example illustrates and we have removed one example (fine scale resource partitioning) to make the examples less disparate. We note that Reviewer 1 requested seabird examples in the introduction -we added references elsewhere, but here we replaced the allochrony example with a seabird one and stated as a segue at the beginning of paragraph three that inter-and intra-specific segregation is frequently described in seabirds. We deleted the last sentence (but added the reference to the first paragraph) as it does indeed paraphrase the first sentence. The revised text reads: "It follows that resource use must be partitioned in some manner to alleviate similarity, and this segregation typically occurs along three axes: diet, space (habitat) or time (Pianka 1969, Schoener 1974). Competitors may segregate along one axis while overlapping along others. Sympatric jaguar (Panthera onca) and puma (Puma concolor), for example, had highly overlapping space and time-use, but segregated along the dietary axis, differing in the prey type and size they selected (Scognamillo et al. 2003). Alternatively, species may mitigate resource overlap by segregating habitatuse. For example, a sibling pair of Rhinolophus bat species used different foraging habitat types when they occurred in sympatry, but this habitat preference disappeared in allopatry (Salsamendi et al. 2012). Even when species use the same resources and the same habitats, segregation along the temporal axis can allow species to exploit similar resources without direct competition. For example, a slight difference in the phenology of Adélie (Pygoscelis adeliae) and chinstrap (P. antarcticus) penguins means that the two species substantially reduced spatial overlap by foraging in similar areas a few weeks apart (Clewlow et al.

2019)". [L47-58]
L61-68 Are we choosing between the 'forage selection hypothesis' and the 'activity budget hypothesis'? "Among the explanations" implies that there are other explanations not discussed here. This paragraph needs editing to clearly highlight how males and females may or may not partition resources. L70-86 These seem like on paragraph rather than two.

RESPONSE: We have made this a single paragraph.
L112: given the large body of work on this topic, "for the first time" seems like an overstatement and is not a strong or informative justification of this study. L123-24: It would be useful to introduce stable isotopes earlier in the introduction.

RESPONSE:
We moved parts of this section to an earlier, slightly expanded paragraph that introduces tracking and stable isotope analysis as two complementary sources of information to study segregation. The new paragraph reads: "Niche segregation among related species, and between the sexes, has often been found in seabirds (e.g., The reduced text at the end of the Introduction reads: "To test this hypothesis, we use high resolution tracks from animal-borne GPS devices, and the carbon and nitrogen stable isotope ratios (δ 13 C and δ 15 N) in the blood plasma of the tracked, and some untracked, individuals. Integrating these data types for sympatric-breeding Macronectes populations, we show segregation in their habitat use and foraging ecology that may have evolved as a competition avoidance mechanism". [L121-125] L126: Given the number of tracking/stable isotope studies of just seabirds, "novel" seems out of place here.