PONE-D-22-11340Counting young birds: a simple tool for the determination of avian population parametersPLOS ONE

Dear Dr. Rinas,

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Reviewers' comments:

Reviewer's Responses to Questions

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

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

Reviewer #1: Yes

Reviewer #2: No

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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

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4. 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 #2: 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: The manuscript titled “Counting young birds: a simple tool for the determination of avian population parameters” is an interesting approach to the issue of obtaining valuable demographic parameters from limited count data. The basic premise of using age-differentiated count data rather than mark-recapture data to calculate things like age-specific life expectancy or the age distribution of a population is potentially very useful for conservation managers.

I did not have any concerns about the actual mathematical approach, but I do provide several general comments about the manuscript. Specifically, I was concerned about how the approach is laid out and how it is differentiated from other methods to obtain similar results. There were also a few aspects of the data used in the analysis that I believe the authors may want to expand upon in their discussion. I describe those general comments first, and then end with several minor comments about specific lines of text in the manuscript that could be improved.

The general analytical approach laid out in the manuscript seems good, however there are important limitations to the method that should be more clearly explained, particularly in the Abstract and Introduction. Most importantly, if I understand correctly, the major age-specific results displayed in the manuscript’s figures are derived from a few inputs that are not age-specific (i.e., multi-year averages of the ratio of young:adult birds and population growth rate, as well as a single assumed maximum life span), which are then run through a deterministic exponential function to calculate the parameters of interest. The authors compare their method to a mark-recapture approach, but a mark-recapture approach provides age-specific data (and therefore estimates of mortality that actually reflect age-specific information), whereas the authors’ method creates age-specific results from lifetime averages. The authors allude to the fact that a mark-recapture approach is different in the first line of the discussion but are vague about exactly how. I believe that many readers will see the age-specific results in the figures without appreciating that the differences among years are not a result of data, but rather simply reflect output from a deterministic function. Of course, this does not mean that the authors’ method does not provide useful results (especially if age-specific data are not available), but I feel that there should be more detail—certainly in the discussion, but perhaps also in the Introduction—describing how the results from this method use a function to calculate estimates of these age-specific demographic parameters, rather than obtaining such estimates from age-specific data.

When reading the manuscript, one initial question I had was: why are the majority of the methods summarized in the manuscript on L70 as “some additional mathematics”, and then laid out in detail in a supplement, rather than included in the main manuscript as a methods section? I suspect that this may relate to the description of the manuscript as a “communication” on L 42, perhaps suggesting that the manuscript was originally intended as a shorter-format paper in a different journal. I recommend moving much of S1 to a methods section in the manuscript, perhaps leaving some ancillary sections (such as 3.2.2 through 3.2.3, 3.3.1 through 3.3.2, and 3.4) in a supplement.

Within those methods (S1), I struggled to determine what part of the material came from existing literature on more traditional life table calculations, and what part came from novel work by the authors. Part of my struggle could have been due to differences in the notation. It may be helpful for the authors to cite some existing literature for steps/equations that were known prior to this work (and perhaps explain when notation differs from those sources), and then explicitly describe which steps in the mathematical logic were developed for this manuscript. This is important, I believe, because some of the equations are difficult to understand based solely on those that come before, without also having access to the more comprehensive theory of life table mathematics.

My final general comments concern the nature of the bird data that the authors suggest may be used with their methods by others, particularly the use of the term “citizen science”. I am not very familiar with the protocols for ornitho.de data, however citizen science programs are extremely variable in both precision and accuracy of bird count data, largely contingent on what effort data are collected, the amount of volunteer training, and the procedures for data verification. Observers will almost invariably have less-than-perfect detection for birds they are counting (although that applies to professional researchers as well). The current manuscript would benefit from at least a discussion of potential bias from differences in detection of young vs. adult birds, as uncertainty in that ratio was mentioned as having an important effect on the results. Even if the counts of species used as examples here are believed to have relatively little difference in the detection of young vs. adult birds, such differences may limit the use of this method for other species or with data from other citizen science programs.

Minor comments:

L39: The phrase “insects up to mammals” implies a natural hierarchy of importance among animals, which may exacerbate existing biases for the study of charismatic species. I recommend changing the sentence to read: “…can be marked, such as insects and mammals.”

L46: This is the first use of the word “respectively” in the text, which I believe the authors are using incorrectly. I believe what they intended to mean with the word is “that is” (or “i.e.”).

L58-66: Although I did appreciate the analogy of a tank of water, it seemed odd to introduce the analogy before simply explaining the relationship in direct terms. It is also simple enough that I don’t know if the analogy is actually necessary (perhaps a diagram could do the same job?). I think that many biologists reading that paragraph will, as I first did, begin to think of all the reasons why a bird population will not behave as a tank of water, even though that is one of the points of the approach: to make several simplifying assumptions about the birds, allowing us to estimate the demographic parameters with only age-ratio data.

S1; 3.0: The authors state that “…predation, shooting, extreme weather, accidents and infections. These causes usually strike the birds independent of their age, so that each year a certain proportion of birds dies from all age groups.” I would disagree; in many species (likely including the example species), susceptibility to all of these factors could easily be dependent on a bird’s age. However, most of those factors will likely increase in importance as a bird ages, so as long as the exponential model provides a good fit to the data, the authors’ approach should still be appropriate. This is probably one of the biggest reasons why, as the authors suggest, their method is not able to completely replace a mark-recapture study, which could provide data to address those factors.

Reviewer #2: The proposed method is creative, novel, and potentially applicable in many other studies and situations. The example of the water tank is simple but pedagogic. I found no problems either in the system of equations or the rationale behind it. However, some aspects can be improved. In conclusion, the study is worth to be published after some major changes.

INTRODUCTION

1) The authors should consider modifying the manuscript to target a broader audience. For example, regarding the mark/recapture method, they simply mention that “Although this method is certainly the gold standard it has also some drawbacks. Most obviously, it depends on animal marking and their recaptures/resightings” (lines 39-41) which is not necessarily an evident problem for many readers.

2) The authors simply mention the “citizen science databases”, without explaining what these databases are or citing examples where they have been successfully applied. Both aspects, including the drawbacks associated with this source of information, must be commented/discussed at some point.

3) The authors mention that “The main prerequisites of this method encompass that young and adult birds are easily distinguishable in the field and the existence of large data sets” (lines 39-41). However, and keeping in mind the water tank example (and their statemen in S1 that “counting year is usually not the first of January but the month when migration starts”), a question comes in mind: can this method be applied to tropical species (that is, to the largest bulk of the bird biodiversity) with much less synchronized reproductive events?

4) Kindly remove “Sweden and Finland” (line 94).

5) Kindly include composed photos or pictures illustrating the age-related differences of each one of the species studied.

MATHEMATICAL BACKGROUND

6) The authors include some confidence measurements in the S2, but these are omitted in the main text. Confidence intervals are important to avoid imprecisions such as “there is also need to have access to sufficiently large data sets” (lines 285-286) or “the time period investigated should encompass several years” (lines 298-299). At this point, it’s impossible to know what “sufficiently large” or “several years” represents with precision.

7) Mortality may be affected by many different factors across species, for which I would expect differences in the probability distributions of the respective mortalities among the species studied (and, of course, divergencies from the modified geometric one used in the present study). Considering this and given that (a) the inclusion of confidence intervals improves the life tables and (b) the mathematics involved in the present study are not complicated, I would strongly suggest using bootstrap to approximate the shape of the sampling distribution and calculate the tables at each run. This approach is computationally more intensive, but the modern computers and the widely available resources, such as Python or R, should be easily up to the task.

DISCUSSION

8) The authors highlight the limitations regarding the information available measured in the field for the species considered in the study (lines 324-325). However, they could enrich the discussion by comparing their results against other studies on phylogenetically related species of cranes, swans, gulls and chats.

9) The authors should highlight the validity and relevance of using indirect estimators for populations parameters by discussing how this approach has successfully been used in other studies, including situations that otherwise could not be resolved, such as fossils (https://doi.org/10.1139/E10-051)

10) Figures should be improved, by reducing the space among plots and highlight the letters and terms referring to some parameter.

SUPPLEMENTARY MATERIALS

11) Please, kindly highlight the letters referring to parameters in the text (by italicizing them or using an alternative font). Sentences such as “The residual life expectancy of a bird that has reach already age a…” can be misleading.

12) In the previously cited sentence, please change “has reach” by “has reached”.

13) Please, clearly define the formula terms the first time they appear. For example, “k” in formula F8.

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Reviewer #1: Yes: Michael Schrimpf

Reviewer #2: No

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Counting young birds: a simple tool for the determination of avian population parameters

PONE-D-22-11340R1

Dear Dr. Rinas,

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,

M. Andreína Pacheco, Ph.D.

Academic Editor

PLOS ONE

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 #2: 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 #2: Yes

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

Reviewer #2: 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 #2: 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 #2: Yes

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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 #2: After carefully reading this second version of the manuscript "Counting young birds: a simple tool for the determination of avian population parameters. (PONE-D-22-11340R1) as well as the authors' responses to my comments, I conclude that the authors have adequately responded to my observations to the previous version, so I consider that this manuscript is now publishable.

I thank the authors for having included all the observations that I consider fundamental and for having satisfactorily explained to me those that were not included in this version.

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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.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: Yes: Paolo Ramoni Perazzi

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