The limits of egg recognition: testing acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest

Some hosts of avian brood parasites reduce or eliminate the costs of parasitism by removing foreign eggs from the nest (rejecter hosts). In turn, even acceptor hosts typically remove most non-egg-shaped objects from the nest, including broken shells, fallen leaves and other detritus. In search for the evolutionary origins and sensory mechanisms of egg rejection, we assessed where the potential threshold between egg recognition and nest hygiene may lie when it comes to stimulus shape. Most previous studies applied comparisons of egg-sized objects with non-continuous variation in shape. Here, instead, we used two series of three-dimensional-printed objects, designed a priori to increasingly diverge from natural eggs along two axes (width or angularity) of shape variation. As predicted, we detected transitions from mostly acceptance to mostly rejection in the nests of American robins Turdus migratorius along each of the two axes. Our methods parallel previous innovations in egg-rejection studies through the use of continuous variation in egg coloration and maculation contrast, to better understand the sensory limits and thresholds of variation in egg recognition and rejection in diverse hosts of avian brood parasites.

strong link between presenting birds with weird looking objects in the nest and the presence/absence of egg rejection behaviour in brood parasite hosts.
How about: Egg rejection varies within and among brood parasite hosts. Thought to be mediated by costs. But an outstanding question remains -how does egg rejection initially arise? Thought to be a continuation of nest hygiene behaviour which is common across birds, but previous studies show these traits do not covary. Why might that be? Then explain why we need to look at variation in egg shape, from 'normal' to 'nest detritus' continuously.
(2) The methods lack key details (a) The rationale for the two axes of variation is very unclear and currently requires careful reading of a different paper to understand what is being done here. The authors of the current manuscript also then label their use of the traits differently, and nowhere is it explained why or what significance this has. (b) Similarly the model types are referred to by the name of the manufacturer rather than the biological trait of interest. It would be much easier to follow if you labelled the cowbird-like eggs as such and not 'Shapeway models', for example. On page 7, line 6 we are told that they will be referred to as 'controls' but in the next paragraph they go back to being Shapeways eggs. (c) At what breeding stage were the eggs added to the nest? Presumably during laying and/or incubation, but this is not stated. This is critical as the timing of egg-laying by brood parasites can have large effects on the likelihood of egg rejection. How consistent was this across treatments? (d) There are also details missing with the statistical methods and presentation of results. For example, how was experimental order treated in the models? This is critical because from previous work with a range of hosts, we know that repeated presentations of stimuli can affect rejection responses. Indeed, in this study there is a marginally non-significant effect of experiment order on the width model series (although the authors rather strongly state that it did not have an effect, which is a bit strong given the p-values were 0.056 and 0.057). There are no tables of results, or degrees of freedom stated with the chi-square statistics, so it is unclear whether this variable was continuous or categorical.
(3) Further analyses would be helpful to place these results in context. The results of this study are very interesting in that they appear to indicate that the two axes of variation produce different shaped response curves. It was disappointing, however, that these features were not explored in more depth. For example, you could actually report the estimated inflection points (± error) for these curves and then use a z-test or similar to assess whether they are statistically different. Alternatively, by centering and scaling the x-axes of each plot, you could combine them into one analysis and use an interaction term to determine if the response curves really are different. This would provide more robust evidence and mean that you are not left to qualitatively compare the responses in the discussion.
(4) The discussion lacks depth. The discussion is disappointingly brief (3 paragraphs) and lacks evaluation of what the results might mean for our understanding of how egg rejection behaviour evolves, for example, or for our understanding of perceptual limits. Instead, the discussion (i) reviews the response curve results, (ii) discusses potential confounds about egg weight and volume (with no conclusion as to whether the authors think this means their results are useful or not) and then includes (iii) a paragraph telling us that increasing numbers of studies are now looking at continuous egg characteristics and the present study is one of these. Surprisingly, there is no evaluation of whether the response curves here are similar or not to the results of Yang et al. 2019 that this study replicates, and the reader is left with no clear take-home message other than that this study uses a similar methodology to others.
Other more specific comments: Introduction Page 4, Line 40 -45: this is phrased a little oddly as recognition errors are also costly if they reduce fitness. Could this be phrased differently? i.e. recognition costs can arise incidentally by damaging own eggs, or because of recognition errors. It's also pretty surprising to not see some of the classic literature on egg recognition costs referred to here (e.g. Davies et al. 1996 Proc B "Recognition errors and probability of parasitism determine whether reed warblers should accept or reject mimetic cuckoo eggs"; or Stokke 2002 Behaviour "Costs associated with recognition and rejection of parasitic eggs in two European passerines").

Methods
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 Hauber
The Editors assigned to your paper RSOS-201615 "The limits of egg recognition: Testing the acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest" have now received comments from reviewers and would like you to revise the paper in accordance with the reviewer comments and any comments from the Editors. Please note this decision does not guarantee eventual acceptance.
We invite you to respond to the comments supplied below and revise your manuscript. Below the referees' and Editors' comments (where applicable) we provide additional requirements. Final acceptance of your manuscript is dependent on these requirements being met. We provide guidance below to help you prepare your revision.
We do not generally allow multiple rounds of revision so we urge you to make every effort to fully address all of the comments at this stage. If deemed necessary by the Editors, your manuscript will be sent back to one or more of the original reviewers for assessment. If the original reviewers are not available, we may invite new reviewers.
Please submit your revised manuscript and required files (see below) no later than 21 days from today's (ie 23-Nov-2020) date. Note: the ScholarOne system will 'lock' if submission of the revision is attempted 21 or more days after the deadline. If you do not think you will be able to meet this deadline please contact the editorial office immediately.
Please note article processing charges apply to papers accepted for publication in Royal Society Open Science (https://royalsocietypublishing.org/rsos/charges). Charges will also apply to papers transferred to the journal from other Royal Society Publishing journals, as well as papers submitted as part of our collaboration with the Royal Society of Chemistry (https://royalsocietypublishing.org/rsos/chemistry). Fee waivers are available but must be requested when you submit your revision (https://royalsocietypublishing.org/rsos/waivers). Your manuscript has been evaluated by two referees who are both experts in the subject area. Both referees agreed that this study addresses an important and unresolved question in brood parasitism, and believe that with appropriate revisions, the study would be suitable for publication in Royal Society Open Access.
Please revise the manuscript following the comments/suggestions from the referees, which were both thoughtful and constructive. Both referees point out that aspects of the methods require greater detail, including the statistical analyses. Both referees also pointed out the lack of discussion of results whose p values were barely > 0.05. I would recommend discussing these (and other results) with reference not only to the p-values, but also the estimated effect size and potential biological significance. Referee #2 also provides excellent suggestions for follow-up analyses, and points out that the discussion in its current form is quite superficial.
Please be sure to provide a point-by-point response to the referees comments indicating how each have been addressed in the revised version of the manuscript.
Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) This very interesting study tackles fundamental questions about discerning between nest hygiene and foreign egg rejection behavior in birds. The authors partly replicated methodologies of Igic et al. (2015) and Yang et al. (2019) by manipulating shape of 3D printed models on continuous scale from egg-shaped to non-egg like objects. Using the original analysis approach, they were able to detect transitions in behavioral responses according to decreasing egg-shape similarity. I am very glad to see a new study on this important topic. I find the results interesting but I am unclear about data processing and analyses and raised few points, which should be carefully addressed.
Page 5, line 20: Next to Luro and Hauber (2017), I would add study of Su et al (2018 Avian Research) -nest sanitation in Pycnonotus xanthorrhous. P. 7, first sentence: Dimensions measured by software or manually using a calliper? P. 7, l. 15: Add a reference of Croston and Hauber (2015 PLoS ONE) to refer on other details, such as natural egg colour measurements (e.g. colour measurements taken from active or abandoned clutches). P. 7, l. 24: Not very clear, if these sourced data are presented as "control treatment" in this manuscript? I think it is so but when checking the control treatment in Fig. 2, I see two rejections. Is then the statement "1 rejection out of n=15 trials" correct? P. 7, l. 34: "and/or (ii) robin eggs" change to "and/or (iii) robin eggs" P. 8, l. 3: Is the number 1-5 treatments per nest correct? Dataset suggests 1-4. P. 8, l. 6-8: I am missing more details on sample size. P. 8, l. 35: Predictor "experimental order" was modelled as continuous or categorical? P. 9, Results: After a look on dataset, I am unclear about coding the predictor order of experiment. I expect continuous ordering starting from 1 for each nest but there many cases starting from 2 or 3. In several other cases there are gaps in ordering sequence. Further, both experiments on the nest C-022 were noted as first experiment (experiment order = 1). I also notice marginally non-significant effect of experiment order on p. 9, l. 35. This surely deserves more attention and should be briefly discussed. For example, I see 35 observations for dependent variable in each test using data from season 2020 and thus is it possible that the power of the test is too low to detect significance? I did a quick GLMM tests in R with the same model structure and using data provided by the authors. Interestingly, I found non-significant effects for both predictors, order of experiment and model metric (panel length or width). The model outputs were relatively similar to those in manuscript only after excluding nest ID as a random effect. Am I missing something? In any case, please re-check data thoroughly and re-run analyses. The terms "Shapeways eggs", "Shapeways 'cow bird' models", "Shapeways model egg", "Shapeways-made model cowbird eggs", "model cow bird egg", "control[s]" are used interchangeably and a unifying term would help here.
Is not the person named D. Hanley in acknowledgements one of the co-authors?
Reviewer: 2 Comments to the Author(s) This manuscript presents data regarding an interesting question for the field: what is the relationship between nest sanitation and egg rejection in brood-parasite hosts? This is an important topic as it still remains unclear how egg rejection defences arise in response to invasion by a brood-parasite. The authors use model eggs that vary from 'egg-like' to 'nest detritus-like' along two perceptual axes of variation and test rejection responses of American robins to derive response curves. This study could make a nice contribution to this growing field of examining host-defences from a more perceptual-perspective; however I found several key issues that would benefit from clarification: (1) The rationale could be presented more convincingly. Currently the authors introduce (i) nest hygiene behaviour and then (ii) egg rejection by brood parasite hosts, and the errors and costs associated with it. They then (iii) mention very briefly that egg rejection and detritus removal are independent behavioural traits that do not covary before (iv) stating the key question is about how birds recognise foreign eggs.
I think a more intuitive angle would be to state this question a little differently: How does egg rejection evolve? Perceptual problems are likely to be a key constraint here, and this study is a great contribution to our growing knowledge in this field. However, at the moment there is not a strong link between presenting birds with weird looking objects in the nest and the presence/absence of egg rejection behaviour in brood parasite hosts.
How about: Egg rejection varies within and among brood parasite hosts. Thought to be mediated by costs. But an outstanding question remains -how does egg rejection initially arise? Thought to be a continuation of nest hygiene behaviour which is common across birds, but previous studies show these traits do not covary. Why might that be? Then explain why we need to look at variation in egg shape, from 'normal' to 'nest detritus' continuously.
(2) The methods lack key details (a) The rationale for the two axes of variation is very unclear and currently requires careful reading of a different paper to understand what is being done here. The authors of the current manuscript also then label their use of the traits differently, and nowhere is it explained why or what significance this has. (b) Similarly the model types are referred to by the name of the manufacturer rather than the biological trait of interest. It would be much easier to follow if you labelled the cowbird-like eggs as such and not 'Shapeway models', for example. On page 7, line 6 we are told that they will be referred to as 'controls' but in the next paragraph they go back to being Shapeways eggs. (c) At what breeding stage were the eggs added to the nest? Presumably during laying and/or incubation, but this is not stated. This is critical as the timing of egg-laying by brood parasites can have large effects on the likelihood of egg rejection. How consistent was this across treatments? (d) There are also details missing with the statistical methods and presentation of results. For example, how was experimental order treated in the models? This is critical because from previous work with a range of hosts, we know that repeated presentations of stimuli can affect rejection responses. Indeed, in this study there is a marginally non-significant effect of experiment order on the width model series (although the authors rather strongly state that it did not have an effect, which is a bit strong given the p-values were 0.056 and 0.057). There are no tables of results, or degrees of freedom stated with the chi-square statistics, so it is unclear whether this variable was continuous or categorical.
(3) Further analyses would be helpful to place these results in context. The results of this study are very interesting in that they appear to indicate that the two axes of variation produce different shaped response curves. It was disappointing, however, that these features were not explored in more depth. For example, you could actually report the estimated inflection points (± error) for these curves and then use a z-test or similar to assess whether they are statistically different. Alternatively, by centering and scaling the x-axes of each plot, you could combine them into one analysis and use an interaction term to determine if the response curves really are different. This would provide more robust evidence and mean that you are not left to qualitatively compare the responses in the discussion.
(4) The discussion lacks depth. The discussion is disappointingly brief (3 paragraphs) and lacks evaluation of what the results might mean for our understanding of how egg rejection behaviour evolves, for example, or for our understanding of perceptual limits. Instead, the discussion (i) reviews the response curve results, (ii) discusses potential confounds about egg weight and volume (with no conclusion as to whether the authors think this means their results are useful or not) and then includes (iii) a paragraph telling us that increasing numbers of studies are now looking at continuous egg characteristics and the present study is one of these. Surprisingly, there is no evaluation of whether the response curves here are similar or not to the results of Yang et al. 2019 that this study replicates, and the reader is left with no clear take-home message other than that this study uses a similar methodology to others.
Other more specific comments:

Introduction
Page 4, Line 40 -45: this is phrased a little oddly as recognition errors are also costly if they reduce fitness. Could this be phrased differently? i.e. recognition costs can arise incidentally by damaging own eggs, or because of recognition errors. It's also pretty surprising to not see some of the classic literature on egg recognition costs referred to here (e.g. Davies et al. 1996 Proc B "Recognition errors and probability of parasitism determine whether reed warblers should accept or reject mimetic cuckoo eggs"; or Stokke 2002 Behaviour "Costs associated with recognition and rejection of parasitic eggs in two European passerines").
Page 5, line 3: this should be either "their nests" or "the nest" Methods Page 6, line 35: I appreciate that we can look in Igic et al. but could you be a bit more specific here about what 'natural plastic' you used, for replication purposes? In experiments with 3D printed stimuli, we've found that this can have major effects on rejection outcomes.
Page 6, line 40 -55: this is a bit confusing for anyone not familiar with Yang et al. 2019. or what these terms mean. Could you perhaps introduce these dimensions a little more in the introduction? What is the significance of the 'surface-edge' variable and stereoscopic structure? Why did you label them differently in this study?
Page 8, lines 3 -6: at what stage were these nests? Page 8, line 35: how did you enter experimental order in your models? Is this categorical or continuous? If categorical, did you enter it as an ordered factor? Discussion Page 10, line 30: typo "may explain also explain" ===PREPARING YOUR MANUSCRIPT=== Your revised paper should include the changes requested by the referees and Editors of your manuscript. You should provide two versions of this manuscript and both versions must be provided in an editable format: one version identifying all the changes that have been made (for instance, in coloured highlight, in bold text, or tracked changes); a 'clean' version of the new manuscript that incorporates the changes made, but does not highlight them. This version will be used for typesetting if your manuscript is accepted. Please ensure that any equations included in the paper are editable text and not embedded images.
Please ensure that you include an acknowledgements' section before your reference list/bibliography. This should acknowledge anyone who assisted with your work, but does not qualify as an author per the guidelines at https://royalsociety.org/journals/ethicspolicies/openness/.
While not essential, it will speed up the preparation of your manuscript proof if accepted if you format your references/bibliography in Vancouver style (please see https://royalsociety.org/journals/authors/author-guidelines/#formatting). You should include DOIs for as many of the references as possible.
If you have been asked to revise the written English in your submission as a condition of publication, you must do so, and you are expected to provide evidence that you have received language editing support. The journal would prefer that you use a professional language editing service and provide a certificate of editing, but a signed letter from a colleague who is a native speaker of English is acceptable. Note the journal has arranged a number of discounts for authors using professional language editing services (https://royalsociety.org/journals/authors/benefits/language-editing/).

===PREPARING YOUR REVISION IN SCHOLARONE===
To revise your manuscript, log into https://mc.manuscriptcentral.com/rsos and enter your Author Centre -this may be accessed by clicking on "Author" in the dark toolbar at the top of the page (just below the journal name). You will find your manuscript listed under "Manuscripts with Decisions". Under "Actions", click on "Create a Revision".
Attach your point-by-point response to referees and Editors at Step 1 'View and respond to decision letter'. This document should be uploaded in an editable file type (.doc or .docx are preferred). This is essential.
Please ensure that you include a summary of your paper at Step 2 'Type, Title, & Abstract'. This should be no more than 100 words to explain to a non-scientific audience the key findings of your research. This will be included in a weekly highlights email circulated by the Royal Society press office to national UK, international, and scientific news outlets to promote your work.

At
Step 3 'File upload' you should include the following files: --Your revised manuscript in editable file format (.doc, .docx, or .tex preferred). You should upload two versions: 1) One version identifying all the changes that have been made (for instance, in coloured highlight, in bold text, or tracked changes); 2) A 'clean' version of the new manuscript that incorporates the changes made, but does not highlight them. --If you are requesting a discretionary waiver for the article processing charge, the waiver form must be included at this step.
--If you are providing image files for potential cover images, please upload these at this step, and inform the editorial office you have done so. You must hold the copyright to any image provided.
--A copy of your point-by-point response to referees and Editors. This will expedite the preparation of your proof.

At
Step 6 'Details & comments', you should review and respond to the queries on the electronic submission form. In particular, we would ask that you do the following: --Ensure that your data access statement meets the requirements at https://royalsociety.org/journals/authors/author-guidelines/#data. You should ensure that you cite the dataset in your reference list. If you have deposited data etc in the Dryad repository, please include both the 'For publication' link and 'For review' link at this stage.
--If you are requesting an article processing charge waiver, you must select the relevant waiver option (if requesting a discretionary waiver, the form should have been uploaded at Step 3 'File upload' above).
--If you have uploaded ESM files, please ensure you follow the guidance at https://royalsociety.org/journals/authors/author-guidelines/#supplementary-material to include a suitable title and informative caption. An example of appropriate titling and captioning may be found at https://figshare.com/articles/Table_S2_from_Is_there_a_trade-off_between_peak_performance_and_performance_breadth_across_temperatures_for_aerobic_sc ope_in_teleost_fishes_/3843624.

At
Step 7 'Review & submit', you must view the PDF proof of the manuscript before you will be able to submit the revision. Note: if any parts of the electronic submission form have not been completed, these will be noted by red message boxes.

Decision letter (RSOS-201615.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 Hauber
On behalf of the Editors, we are pleased to inform you that your Manuscript RSOS-201615.R1 "The limits of egg recognition: Testing acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referees' reports. Please find the feedback from the Editors below my signature.
We invite you to respond to the comments and revise your manuscript. Below the Editors' comments (where applicable) we provide additional requirements. Final acceptance of your manuscript is dependent on these requirements being met. We provide guidance below to help you prepare your revision.
Please submit your revised manuscript and required files (see below) no later than 7 days from today's (ie 11-Dec-2020) date. Note: the ScholarOne system will 'lock' if submission of the revision is attempted 7 or more days after the deadline. If you do not think you will be able to meet this deadline please contact the editorial office immediately.
Please note article processing charges apply to papers accepted for publication in Royal Society Open Science (https://royalsocietypublishing.org/rsos/charges). Charges will also apply to papers transferred to the journal from other Royal Society Publishing journals, as well as papers submitted as part of our collaboration with the Royal Society of Chemistry (https://royalsocietypublishing.org/rsos/chemistry). Fee waivers are available but must be requested when you submit your revision (https://royalsocietypublishing.org/rsos/waivers). Thank you for submitting your revised manuscript. I feel that you have done a thorough job addressing the comments received from the referees in the initial review of the manuscript. I have just one additional revision I would like to request, which has to do with the use of the term "treatment order". When you have a series of up to 4 treatments per nest (A, B, C, and D), treatment order implies a categorical variable such as ABCD, ABDC, ACDB, etc.). Can you instead refer to "treatment number" throughout the manuscript, and explicitly state in the methods that treatment number refers to the sequence with which the treatment occurred at a given nest.

===PREPARING YOUR MANUSCRIPT===
Your revised paper should include the changes requested by the referees and Editors of your manuscript. You should provide two versions of this manuscript and both versions must be provided in an editable format: one version identifying all the changes that have been made (for instance, in coloured highlight, in bold text, or tracked changes); a 'clean' version of the new manuscript that incorporates the changes made, but does not highlight them. This version will be used for typesetting. Please ensure that any equations included in the paper are editable text and not embedded images.
Please ensure that you include an acknowledgements' section before your reference list/bibliography. This should acknowledge anyone who assisted with your work, but does not qualify as an author per the guidelines at https://royalsociety.org/journals/ethicspolicies/openness/.
While not essential, it will speed up the preparation of your manuscript proof if you format your references/bibliography in Vancouver style (please see https://royalsociety.org/journals/authors/author-guidelines/#formatting). You should include DOIs for as many of the references as possible.
If you have been asked to revise the written English in your submission as a condition of publication, you must do so, and you are expected to provide evidence that you have received language editing support. The journal would prefer that you use a professional language editing service and provide a certificate of editing, but a signed letter from a colleague who is a native speaker of English is acceptable. Note the journal has arranged a number of discounts for authors using professional language editing services (https://royalsociety.org/journals/authors/benefits/language-editing/).

===PREPARING YOUR REVISION IN SCHOLARONE===
To revise your manuscript, log into https://mc.manuscriptcentral.com/rsos and enter your Author Centre -this may be accessed by clicking on "Author" in the dark toolbar at the top of the page (just below the journal name). You will find your manuscript listed under "Manuscripts with Decisions". Under "Actions", click on "Create a Revision".
Attach your point-by-point response to referees and Editors at Step 1 'View and respond to decision letter'. This document should be uploaded in an editable file type (.doc or .docx are preferred). This is essential.
Please ensure that you include a summary of your paper at Step 2 'Type, Title, & Abstract'. This should be no more than 100 words to explain to a non-scientific audience the key findings of your research. This will be included in a weekly highlights email circulated by the Royal Society press office to national UK, international, and scientific news outlets to promote your work.

At
Step 3 'File upload' you should include the following files: --Your revised manuscript in editable file format (.doc, .docx, or .tex preferred). You should upload two versions: 1) One version identifying all the changes that have been made (for instance, in coloured highlight, in bold text, or tracked changes); 2) A 'clean' version of the new manuscript that incorporates the changes made, but does not highlight them. --If you are requesting a discretionary waiver for the article processing charge, the waiver form must be included at this step.
--If you are providing image files for potential cover images, please upload these at this step, and inform the editorial office you have done so. You must hold the copyright to any image provided.
--A copy of your point-by-point response to referees and Editors. This will expedite the preparation of your proof.

At
Step 6 'Details & comments', you should review and respond to the queries on the electronic submission form. In particular, we would ask that you do the following: --Ensure that your data access statement meets the requirements at https://royalsociety.org/journals/authors/author-guidelines/#data. You should ensure that you cite the dataset in your reference list. If you have deposited data etc in the Dryad repository, please only include the 'For publication' link at this stage. You should remove the 'For review' link.
--If you are requesting an article processing charge waiver, you must select the relevant waiver option (if requesting a discretionary waiver, the form should have been uploaded at Step 3 'File upload' above).
--If you have uploaded ESM files, please ensure you follow the guidance at https://royalsociety.org/journals/authors/author-guidelines/#supplementary-material to include a suitable title and informative caption. An example of appropriate titling and captioning may be found at https://figshare.com/articles/Table_S2_from_Is_there_a_trade-off_between_peak_performance_and_performance_breadth_across_temperatures_for_aerobic_sc ope_in_teleost_fishes_/3843624.

At
Step 7 'Review & submit', you must view the PDF proof of the manuscript before you will be able to submit the revision. Note: if any parts of the electronic submission form have not been completed, these will be noted by red message boxes.

Decision letter (RSOS-201615.R2)
This year has been very difficult for everyone, and we want to take the opportunity to thank you for your continued support in 2020.
The Royal Society Open Science editorial office will be closed from the evening of Friday 18 December 2020 until Monday 4 January 2021. We will not be responding during this time. If you have received a deadline within this time period, please contact us as soon as possible to allow us to extend the deadline. If you receive any automated messages during this time asking you to meet a deadline, we offer apologies and invite you to respond after the festive period or during normal working hours.
With our best for a peaceful festive period and New Year, and we look forward to working with you in 2021.

Dear Dr Hauber,
It is a pleasure to accept your manuscript entitled "The limits of egg recognition: Testing acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest" in its current form for publication in Royal Society Open Science.
Please ensure that you send to the editorial office an editable version of your accepted manuscript, and individual files for each figure and table included in your manuscript. You can send these in a zip folder if more convenient. Failure to provide these files may delay the processing of your proof. You may disregard this request if you have already provided these files to the editorial office.
You can expect to receive a proof of your article in the near future. Please contact the editorial office (openscience@royalsociety.org) and the production office (openscience_proofs@royalsociety.org) to let us know if you are likely to be away from e-mail contact --if you are going to be away, please nominate a co-author (if available) to manage the proofing process, and ensure they are copied into your email to the journal.
Due to rapid publication and an extremely tight schedule, if comments are not received, your paper may experience a delay in publication.
Please see the Royal Society Publishing guidance on how you may share your accepted author manuscript at https://royalsociety.org/journals/ethics-policies/media-embargo/. Thank you for your continued interest in our manuscript and for the referees' constructive comments on its content.
Below we explain how we revised our text according to the criticism and we trust that the new draft will be suitable for consideration to publish in Royal Society Open Science.
Thank you, sincerely, Mark Hauber and coauthors.

Dear Dr Hauber
The Editors assigned to your paper RSOS-201615 "The limits of egg recognition: Testing the acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest" have now received comments from reviewers and would like you to revise the paper in accordance with the reviewer comments and any comments from the Editors. Please note this decision does not guarantee eventual acceptance.
We invite you to respond to the comments supplied below and revise your manuscript. Below the referees' and Editors' comments (where applicable) we provide additional requirements. Final acceptance of your manuscript is dependent on these requirements being met. We provide guidance below to help you prepare your revision.
We do not generally allow multiple rounds of revision so we urge you to make every effort to fully address all of the comments at this stage. If deemed necessary by the Editors, your manuscript will be sent back to one or more of the original reviewers for assessment. If the original reviewers are not available, we may invite new reviewers.
OUR RESPONSE: We have revised the ms in detail in light of the comments and explain how we did so point-by-point. Your manuscript has been evaluated by two referees who are both experts in the subject area. Both referees agreed that this study addresses an important and unresolved question in brood parasitism, and believe that with appropriate revisions, the study would be suitable for publication in Royal Society Open Access.
Our Response: Thank you for your positive assessment.
Please revise the manuscript following the comments/suggestions from the referees, which were both thoughtful and constructive. Both referees point out that aspects of the methods require greater detail, including the statistical analyses.
Our Response: Please note that we have detected a fatal flaw in JMP 12.0 Software during our revision. Namely, we discovered that JMP 12.0 appears to conduct random-effects binomial tests but in fact, it simply drops the random effect from the model. Therefore, we re-did all of our statistics in the R statistical environment. We lost one significant result but the rest of the results and patterns remained statistically the same.
Both referees also pointed out the lack of discussion of results whose p values were barely > 0.05. I would recommend discussing these (and other results) with reference not only to the p-values, but also the estimated effect size and potential biological significance.
Our Response: We now fully discuss potential biological significance of both marginally significant and non-significant results. We also present odds ratios for each response variable for each dataset to allow for an assessment of relative effect sizes and their biological impact.
Referee #2 also provides excellent suggestions for follow-up analyses, and points out that the discussion in its current form is quite superficial.
Our Response: To address these concerns, the discussion is now expanded; also please see below.
Please be sure to provide a point-by-point response to the referees comments indicating how each have been addressed in the revised version of the manuscript.
Our Response: We now explain our changes and responses as per below.
Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) This very interesting study tackles fundamental questions about discerning between nest hygiene and foreign egg rejection behavior in birds. The authors partly replicated methodologies of Igic et al. (2015) and Yang et al. (2019) by manipulating shape of 3D printed models on continuous scale from egg-shaped to non-egg like objects. Using the original analysis approach, they were able to detect transitions in behavioral responses according to decreasing egg-shape similarity.
I am very glad to see a new study on this important topic. I find the results interesting but I am unclear about data processing and analyses and raised few points, which should be carefully addressed.
Our Response: Thank you very much for your interests and for the opportunity to clarify our methods and results. Our Response: Thank you, replaced. P. 7, first sentence: Dimensions measured by software or manually using a calliper?
Our Response: Explained now that it was done by a caliper to the nearest 0.1 mm. P. 7, l. 15: Add a reference of Croston and Hauber (2015 PLoS ONE) to refer on other details, such as natural egg colour measurements (e.g. colour measurements taken from active or abandoned clutches).
Our Response: We added Hauber et al. 2020b which is based on our most recent measurements of natural robin eggs. P. 7, l. 24: Not very clear, if these sourced data are presented as "control treatment" in this manuscript? I think it is so but when checking the control treatment in Fig. 2, I see two rejections. Is then the statement "1 rejection out of n=15 trials" correct?
Our Response: Corrected. P. 8, l. 6-8: I am missing more details on sample size. Fig. 2 provides some information but figure points are sometimes difficult to read. I would explicitly indicate number of experiments performed for each model type separately (perhaps at each model in Fig. 1B or Fig. 2 is a good place).
Our Response: Done, sample sizes per treatments are specified in the Fig. 2AB legend. P. 8, l. 35: Predictor "experimental order" was modelled as continuous or categorical?
Our Response: Now stated as continuous variable. P. 9, Results: After a look on dataset, I am unclear about coding the predictor order of experiment. I expect continuous ordering starting from 1 for each nest but there many cases starting from 2 or 3. In several other cases there are gaps in ordering sequence.
Our Response: Please note that nests with predation (n=6) or abandonment (n=8) and, thus, no accept/reject outcome were also present in the original dataset's ordering, but these are not included in the final data set as nest predation is stochastic whereas nest abandonment is not a response by American Robins to experimental brood parasitism (Croston and Hauber 2014).
Further, both experiments on the nest C-022 were noted as first experiment (experiment order = 1).
Our Response: This is corrected now.
I also notice marginally non-significant effect of experiment order on p. 9, l. 35. This surely deserves more attention and should be briefly discussed. For example, I see 35 observations for dependent variable in each test using data from season 2020 and thus is it possible that the power of the test is too low to detect significance? I did a quick GLMM tests in R with the same model structure and using data provided by the authors. Interestingly, I found non-significant effects for both predictors, order of experiment and model metric (panel length or width). The model outputs were relatively similar to those in manuscript only after excluding nest ID as a random effect. Am I missing something? In any case, please re-check data thoroughly and re-run analyses.
Our Response: Thank you. As stated above, please note that we have detected a fatal flaw in JMP 12.0 Software during our revision. Namely, we discovered that JMP 12.0 appears to conduct random-effects binomial tests but in fact, it simply drops the random effect from the model output (which is why you also found what we found when you excluded the random effects). Therefore, we re-did all of our statistics in the R statistical environment. We only lost one significant result but the rest of the results and patterns remained statistically the same. Nonetheless, we now also discuss marginally non-significant findings and their potential biological significance in light of effect sizes. The terms "Shapeways eggs", "Shapeways 'cow bird' models", "Shapeways model egg", "Shapeways-made model cowbird eggs", "model cow bird egg", "control[s]" are used interchangeably and a unifying term would help here.