PONE-D-21-08715

Comparing non-breeding distribution and behavior of red-legged kittiwakes from two geographically distant colonies

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Reviewers' comments:

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Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Partly

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This study documents the non-breeding distribution and activity patterns of red-legged kittiwakes tracked in two different colonies of the Bering Sea. It reveals that individual activity patterns are similar among the two colonies but distributions differ most of the non-breeding season to eventually overlap during late winter.

The manuscript is well-written and clear but I have concerns about the context in which the study is placed and some speculative interpretations.

1) In the abstract and introduction, a lot of emphasis is given on the threats that seabirds could face during the non-breeding season and which may ultimately impact their annual survival. However, the methods applied (individual tracking) cannot provide results directly related to survival and thus, I feel the introduction is not completely appropriate to present the importance and goals of the study.

For example, when reading in the introduction p.4 L.75-77 that adult survival has declined since the 90’s, we have not value and no reference provided. Then, p.4 L.84-87, it is stated that the two study populations are increasing or stable. Therefore, by reading these two contradictory information, it is easy to give more weight to the fact that the study populations are not declining (which is the focus of the manuscript) rather than being worried about the general but unproven annual survival decline of the whole species.

Overall, the introduction sounds dramatic and leads the reader to think that the species is highly threatened, while it doesn’t seem to be the case based on the information given, at least for the study populations.

2) In the introduction, the paragraph stating that seabirds may converge to the same non-breeding area and thus may represent even greater risk for the study populations is biased. Basically, we know that the distributions of seabirds during the non-breeding season is highly variable, with individuals from the same colony wintering in different areas and individuals from different colonies wintering in different areas or in the same areas. All strategies are possible and without previous tracking studies on the study species or closely-related species which may behave the same, it is hard to favour one hypothesis (overlap in winter distribution) over the other (separate distributions). So ok, you have an a priori hypothesis assuming that birds from the 2 colonies may share some wintering areas but given the information available for the hypothesis, I would rather argue that wintering strategies of seabirds are highly variable within and between populations of a same species, that nothing is really known about the winter distribution of red-legged kittiwakes (stable isotopes are only indirect methods) and because of this gap, you want to document it. From the study carried out, your goal is not to study distribution and activity patterns to explain annual survival but rather to compare wintering strategies between two colonies.

So overall, I would encourage to change the introduction into a less dramatic introduction, highlighting that seabird migration is diverse and that you want to know how your species is behaving in winter depending on its breeding colony. And only after presenting the goal of the study, you can present your hypothesis about shared wintering areas. That will definitely improve the introduction. You can discuss the implications of shared overwintering areas for the species but only in the discussion and with a lot of caution.

3) The discussion section about individual condition sounds very speculative. If weight was not measured close to/after chick fledging, it is likely that the weight may not be representative of the final state of individuals at the end of the breeding season. Therefore, I would present first the discussion on the environmental conditions (from p.18 L.367) which are more likely to affect individual distribution, and then, present hypotheses about individual condition but with a lot of caution not to sound too speculative, especially because of the small sample sizes.

Other comments :

Abstract : the abstract is very unbalanced. Based on the previous comments, the introduction is a little bit out of scope and the discussion results are barely presented. I would try to balance a little bit more the abstract to outline its importance and its implications (because the study still brings new valuable knowledge on a seabird species !).

p.2 L 25-26 : First sentence is about adult survival so we expect the paper to be about survival somehow. Nevertheless, the paper never directly relates to it, as individual/population distributions are not real proxies of survival. So the general aim has to be modified.

Second sentence unclear

Methods

p.9 L.198 : replace "species" by « individuals ».

Discussion

p.17 L.336. : Remove the expression « our study was the FIRST ». You don’t need to specify that « you were the first » to stress the importance of your study ! Moreover, it is not the fact that you simultaneously tracked birds from two colonies which has to be highlighted… It is more about the knowledge gap which is filled thanks to your data…

p.19 from L.385 : acknowledge that you still have small sample sizes for some years and colonies and that further studies are needed.

p.20 L.401-403 : this is contradictory to what is stated in the introduction that both colonies are stable or increasing (see also my general comment 1). I would remove the section trying to find some causes of decline in the species (p.20 403-421), it is way too speculative based on the given evidence in the manuscript that the species is declining.

p.21 L.428-429 : I don’t agree with this sentence. Kittiwakes have the same activity patterns, meaning they need the same amount of time spent foraging and thus have the same energy requirements. But it doesn’t mean that they feed on the same prey species.

Conclusion: I miss a last paragraph summarizing the take-home message and the wider implications of the study.

Reviewer #2: This is an interesting descriptive paper that compares winter distributions in two colonies of a very interesting and unusual small gull, the red-legged kittiwake. The field and analytical methods are sound and the results have important conservation implications. The paper is generally well written, and of appropriate length. There are quite a few Figures, but most or all are warranted. I have a few concerns, related to small sample sizes, especially at Buldir, although the results are probably robust. I also thought there was a bit too much speculation in the Discussion, which should focus less on the weaker components of the paper (activity patterns, body condition), and more on the strengths of the paper (distribution.)

A few specific comments:

Introduction

Line 53: Could maybe replace “are crucial to” with “can have marked effects on” or something similar

Line 56: Given most seabirds spend all of winter at sea, you could delete “when the majority of time is spent at sea”

Line 58-61: I might reword to say “widely distributed, habitat generalists may be more resilient to environmental change than more narrowly distributed habitat specialists” or something like that.

Line 64: Could change to “during the winters of 2007 to 2010”

Line 69: Could change to “colonial, cliff-nesting” to provide more information.

Line 76: Could name the two colonies here.

Line 83: Tell us what years you tracked the birds here.

Line 91 to 93: I like the specific predictions, but you should justify why you predicted that behavior would be invariant among individuals.

Methods

Line 106-107: From above, below, or both?

Line 114: Perhaps report on the range in masses of tagged birds here?

Line 129-131: What is the clutch size in these birds?

Line 157-158: No need to repeat the sample sizes here

Line 182-183: Because sample sizes for the different components of the study vary a bit, you might consider putting the information in a Table to avoid repeatedly breaking up the text.

Line 183: “a bird that spends”. There are other occasions in the paper as well.

Line 192-200: Use of these ‘condition indices” derived by regressing mass on linear morphometrics has received harsh criticism, for good reason. For these birds, do males and females differ in size? And would mass not vary considerably depending (for example) on how long the bird had spent on the nest prior to capture, and the time in the season when it was captured?

Results

Fig. 2, Lines 237-238: You might spell this out in more detail, so that the Figure plus caption can stand on its own, separate from the rest of the paper.

Line 248: Should be “was higher”

Table 3: It looks like almost no birds laid eggs in 2017 at either colony, and none hatched eggs. Numbers in 2016 at St George were also very low. So would a valid intepretation of these results be that breeding birds actively maintain higher mass than non-breeding birds? I sense that your interpretation is that causation works in the other direction, i.e., that birds fail because they are light in mass = in poor condition.

Discussion

Line 354-366: This paragraph encapsulates the issue with condition indices, and their intepretation. This frankly comes off as a lot of arm-waving, and in my opinion, is not a strength of the paper. I would eliminate these sections related to condition unless there is a more clear and compelling interpretation available.

A fair bit of the material in the Discussion is speculative, especially related to seasonal and colony differences in condition and activity patterns. Again, I do not think these are the strengths of the paper. I would probably try to keep the Discussion focused more on the distributions – where, when and (as much as possible), why.

Would it be possible to make at least brief comparisons with other non-Larus, small, northern gulls like Sabine’s, Ross’s, and Ivory? I think those species have all been tracked with GLS tags now.

Reviewer #3: Overall, this study provides a well written description of the broad behaviour and non-breeding season distribution of red-legged kittiwakes from two breeding colonies. The study focuses on a globally important population and therefore has a broad interest to the scientific field. The work also manages to culminate in several concise and interesting conclusions.

While the study has two consecutive years of data, the sample sizes across years is relatively small. The authors do a good job in acknowledging this and none of the conclusions made by the study overstep the bounds of the limited sample size. The study is also fortunate in that there has been work done in the area previously, to help put their results into perspective.

The methods and analysis used are relatively typical in this field and I therefore have no concerns.

I am happy to make a recommendation for publication of this paper, but I have one major comment on the final paragraph (starting line 428) of the discussion, which I discuss below and believe needs to be addressed.

The data obtained from the immersion sensor on the GLS tags is not enough to reach the conclusion that individuals from both colonies are “feeding on similar prey items throughout the winter period”. Additionally, the activity around sunrise and sunset alone is not enough to conclude that the birds are “mainly consuming diel vertically migrating prey”, as foraging activity during these periods does not preclude the consumption of other prey types, without evidence to verify the prey consumed. Finally, the comment that this study adds evidence to the birds being specialist foragers is far too speculative given the data and results from this study. I would suggest rewriting this paragraph to focus on the results obtained in this study, mainly: diel foraging patterns and other behavioral differences and similarities, while avoiding making speculative points on wider foraging behaviour and prey consumption.

Other than these broad comments, I have added a few minor and specific points below:

A comment on the degree of accuracy of geolocators could be helpful, along with a comment in the discussion mentioning how the error in location estimates could also affect the degree of overlap between colonies and the distance travelled. As is stated in the paper, probGLS should minimize these errors and they are very unlikely to affect your results, but I think it is important to mention and quantify (using a value from the literature) them regardless.

Line 27: Suggest you change “widespread impact on the population” to “widespread impact on multiple breeding populations”, as this better conveys your point that changes in a multi-colony non-breeding ground site could have repercussions on multiple breeding populations.

Line 74&75: There is an open “[“ which ends with a “)”. Suggest a change of the “)” to a “]”

Line 97: Include the dimensions of the tag.

Line 104: Include the units for the value of conductivity.

Line 106: It is worth noting in this paragraph whether birds were retagged or not i.e. “Birds were not retagged upon retrieval of GLS devices”

Line 144&145: This sentence is not immediately clear what the “respectively” refers to, as there are three values and two overlaps. I suggest a slight rewording of this sentence just to improve clarity, as this is an important point which puts a lot of the results in context.

Line 189: Include the citation for twGeos (ref 50)

Line 347: In this paragraph it would be nice to know how often complete colony failure happens. Is this a rare event or a relatively common occurrence?

Lines 358 – 360: A reference is needed here which demonstrates the birds have flexibility in molt timings and that timing can be driven by reproductive success, otherwise this point is too speculative.

Lines 395-398: A good point, but the sentence reads a bit awkwardly at the moment, perhaps reword slightly.

Line 404-405: A reference linking food reliability to survival is needed here.

Line 424: Need a reference to the fact the colonies represent 85% of the global population.

Figure 4: Showing the BA50 between the north and south sides of St. George colony on this map would be helpful

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Reviewer #1: Yes: Dr Aurore Ponchon

Reviewer #2: No

Reviewer #3: No

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Comparing non-breeding distribution and behavior of red-legged kittiwakes from two geographically distant colonies

PONE-D-21-08715R1

Dear Dr. Drummond,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Vitor Hugo Rodrigues Paiva, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #3: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #3: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #3: Yes

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4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #3: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I am pleased to see that the authors have dealt satisfactorily with the criticisms that other reviewers and I had raised regarding the original version of this manuscript dealing with the winter migration of red-legged kittiwakes breeding in two different colonies of the Bering Sea. The revised version has been substantially improved, notably by the more appropriate context the study is replaced in and the less speculative discussion describing more closely the results of the study. The manuscript is easier to read and we now understand the importance of this work in the right context.

Reviewer #3: (No Response)

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

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Reviewer #1: Yes: Dr Aurore Ponchon

Reviewer #3: No