Host personality predicts cuckoo egg rejection in Daurian redstarts Phoenicurus auroreus

In species that are subject to brood parasitism, individuals often vary in their responses to parasitic eggs, with some rejecting the eggs while others do not. While some factors, such as host age (breeding experience), the degree of egg matching and the level of perceived risk of brood parasitism have been shown to influence host decisions, much of the variation remains unexplained. The host personality hypothesis suggests that personality traits of the host influence its response to parasitic eggs, but few studies have tested this. We investigated the relationship between two personality traits (exploration and neophobia) and a physiological trait (breathing rate) of the host, and egg-rejection behaviour in a population of Daurian redstarts Phoenicurus auroreus in northeast China. We first show that exploratory behaviour and the response to a novel object are repeatable for individual females and strongly covary, indicating distinct personality types. We then show that fast-exploring and less neophobic hosts were more likely to reject parasitic eggs than slow-exploring and more neophobic hosts. Variation in breathing rate—a measure of the stress-response—did not affect rejection behaviour. Our results demonstrate that host personality, along the bold-shy continuum, predicts the responses to parasitic eggs in Daurian redstarts, with bold hosts being more likely to reject parasitic eggs.

personality traits of females is insufficient. 6.
Line 239: did this negative relationship still hold for exploratory behaviour and return latency in the second novel object trial? Similarly for other analyses in which return latency in first novel object trial was a variable. 7.
Line 249: See my main comment above. This is likely due to comparing models based on R^2. 8.

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

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Comments to the Author This is a very well written paper I enjoyed reading. The stats are thorough and robust. This is an impressive field study with good sample sizes. I do have a couple of concerns.
My four main comments are: Line 126: You painted the eggs the same colour, to your eyes, but are the birds seeing the same colour as you are seeing? We know of course that birds see colour different to us. How was the colour matched appropriately within an avian visual sensory capacity?
Line 152: Ten mins of acclimation seems no time at all for the personality studies? Where birds always instantly retrieved and then have the 10-minute period, or had some birds been waiting longer?
Line 252: The models overall don't explain much of what was observed. This is not alluded too enough. There needs to be more transparency regarding this. There is a lot going on that isn't explained.

Minor comments
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Best wishes, Dr Sasha Dall mailto: proceedingsb@royalsociety.org Associate Editor Board Member: 1 Comments to Author: This well-written paper presents experimental and correlative data showing that personality traits are associated with the likelihood of cuckoo egg rejection in a host species, the Daurian redstart. This is a nice study system to test hypotheses about inter-individual variation in rejection, since both rejection and acceptance are frequent in the naturally parasitized population. The combination of field experiments using artificial parasitism and tests of exploratory behavior are also impressive. Overall, the results seem solid and both I and the reviewers thought that the paper will make a valuable contribution to the literature. However, there are a number of important concerns that should be addressed in a revision. Reviewer 1 and I both note that using R2 estimates to select a single best-fit model after AIC selection is not statistically sound, and offer alternatives. Although this should not change your main results, it may change the model estimates presented in table 1. Both reviewers also ask for clarifications on important issues, detailed in their comments. My own line-by-line comments are as follows: 17: here and elsewhere, consider rephrasing to avoid convoluted sentences, e.g. "Individuals often vary in their responses…" instead of "there is typically variation in individuals' responses" Paragraph beginning line 44: the two hypotheses (evolutionary lag vs. equilibrium) are nicely laid out, but the following sentence (beginning "Furthermore") leaves the reader wondering whether these pieces of evidence are meant to support one of those two hypotheses, or whether they suggest additional ones. It seems that some of these factors suggest that the ability to reject eggs is constrained by learning or experience, since older/more experienced birds, and those with perceived high risk of parasitism, are more likely to reject eggs. Is this meant to be interpreted as evidence for a cost/benefit tradeoff where the cost is the risk of making an error? 56-57: here and elsewhere, additional explanation would be useful as to whether, or under what conditions, the host personality hypothesis predicts that variation in egg ejection is adaptive (birds with certain personality traits benefit from rejection whereas those with other traits do not) or not adaptive (perhaps linked to personality traits that are adaptive in other contexts). 85-86: "Being more successful at earlier lines of host defense may decrease selection on later lines of defense." That statement certainly makes sense at the level of the population, but I am not sure that it translates to a tradeoff within individuals, as suggested by the next sentence. It's not clear to me why bold individuals who are better at driving away parasites would necessarily be worse at discriminating parasitic eggs if they are parasitized, unless learning and experience plays a major role in the ability to discriminate? 112-117: are blue egg-laying and pink egg-laying hosts equally likely to be parasitized by cuckoos in this population, or is the risk of parasitism different? 185-191: I am a little concerned about model overfitting for a fairly small sample size in a linear mixed model with one random effect and 5 fixed effects. Did you check for overfitting? 219: Like reviewer 1, I am concerned about using this R2 to select the model with largest explanatory power. These R2 estimates are biased towards models with more parameters, and should not be used in conjunction with an AIC selection approach. You might want to check out Arnold (2010) J. Wildlife Management, which essentially argues that of models within 2 AIC units of the top model, the one with the smallest number of parameters should be the "best fit;" or use model averaging as suggested by Reviewer 1. This should not change your overall conclusions but it might change the estimates of the effect sizes. 245-248: just a comment -how great to have high frequencies of both acceptor and rejectors in the population! Great opportunity to ask this question, perhaps worth emphasizing in the introduction. Discussion: it is a bit surprising to me that in your models, the personality metrics explain more variation in rejection behavior than egg color does. I would have expected differences between host and egg color to be more salient. Related to this, there is not much explanation of the host egg color polymorphism in the introduction or discussion, which would be helpful since egg color polymorphisms are rare and often interpreted as the result of selection against parasitism. Could you perhaps speculate on this briefly? Table 1: also please note in the table caption that these represent best-fit models as chosen by AIC, and list all of the parameters that were included in the full models (if they are not all represented here).
Reviewer(s)' Comments to Author: Referee: 1 Comments to the Author(s) This is a very nice study showing that personality traits (exploration and neophobia) predict some variation in egg rejection in Daurian redstarts. The finding that bold hosts are more likely to reject parasitic/experimental eggs is interesting and suggests that intra-individual variation in hosts requires further study. The paper is very well written, and conclusions are well-supported. It is a valuable contribution to the field. My main comment is about the statistics. Models with delta AIC<2 are compared using Nagelkerke R^2 values (paragraph beginning line 213). The problem with this is that (virtually) always, the addition of a term to a model will improve % variation predicted by the model. Therefore, comparing nested models based on R^2 will result in the least parsimonious model being chosen. Such full models will likely include variables which cannot be generalised beyond your sample (i.e. they will essentially overfit). I therefore strongly recommend that you compare models differently. Either use a model averaging approach, or select the most parsimonious model from the group of models where deltaic<2, or use likelihood ratio tests to compare nested models. Looking at the supplementary table of all models, all of these options should be reasonable. Another concern is that you do not distinguish between parasitism causing bold behaviour, or boldness causing increased egg rejection -see comment 4. Further comments: 1. Abstract: Perhaps specify that the 'parasitic' eggs used in experiments were model eggs.
2. Lines 38-40: Slightly odd choices of references. Several of the references 2-7 would better support the following sentence where 8-10 are referenced (and to a lesser extent vice versa), in my opinion. Please double check that the general references apply well to the points you wish to make. 3. Lines 57-8 and 68-9: Slight contradiction here: is direct evidence absent or scarce? Please clarify. If scarce, cite the relevant study/studies. 4. Methods and Results: As in understand it, all personality assays were conducted after an egg rejection experiment (i.e. when birds had 4-day old chicks or 9-11 days into incubation for exploration and neophobia respectively). It would therefore be worth discussing the direction of causation. Is it not possible that experimental or actual parasitism causes hosts to behave more boldly if they 'know' that they have been parasitised, rather than bold hosts being more likely to reject eggs? How can you tell the difference? This requires some explanation in the discussion section. For example, you may argue that the consistency across years of exploratory behaviour suggests that personality is not labile due to perceived parasitism (but you could not state this for neophobia). 5. Line 124: Do males not reject eggs? Just checking, because if they do, then measuring personality traits of females is insufficient.
6. Line 239: did this negative relationship still hold for exploratory behaviour and return latency in the second novel object trial? Similarly for other analyses in which return latency in first novel object trial was a variable. 7. Line 249: See my main comment above. This is likely due to comparing models based on R^2. 8. Line 344: Lovely concluding paragraph! Referee: 2 Comments to the Author(s) This is a very well written paper I enjoyed reading. The stats are thorough and robust. This is an impressive field study with good sample sizes. I do have a couple of concerns.
My four main comments are: Line 126: You painted the eggs the same colour, to your eyes, but are the birds seeing the same colour as you are seeing? We know of course that birds see colour different to us. How was the colour matched appropriately within an avian visual sensory capacity?
Line 152: Ten mins of acclimation seems no time at all for the personality studies? Where birds always instantly retrieved and then have the 10-minute period, or had some birds been waiting longer?
Line 252: The models overall don't explain much of what was observed. This is not alluded too enough. There needs to be more transparency regarding this. There is a lot going on that isn't explained.

17-May-2021
Dear Professor Deng I am pleased to inform you that your manuscript RSPB-2021-0228.R1 entitled "Host personality predicts cuckoo egg rejection in Daurian redstarts Phoenicurus auroreus" has been accepted for publication in Proceedings B.
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5) Data accessibility section and data citation
It is a condition of publication that data supporting your paper are made available either in the electronic supplementary material or through an appropriate repository.
In order to ensure effective and robust dissemination and appropriate credit to authors the dataset(s) used should be fully cited. To ensure archived data are available to readers, authors should include a 'data accessibility' section immediately after the acknowledgements section. Thanks to the authors for nicely revising their manuscript to take my and the reviewers' suggestions into account. The resulting manuscript reads nicely and the statistical analyses are more straightforward. I have only one very small remaining comment, which could perhaps be incorporated at the proofs stage.
I had previously asked whether blue egg-laying and pink egg-laying hosts are equally likely to be parasitized by cuckoos in this population. You provide a good response in the response to referees, but I think it would be useful to include a very brief explanation (one sentence or less) in the main text as well. This was your reply: "In our study population, redstarts laying blue eggs suffered higher risk of parasitism than hosts laying pink eggs. However, hosts laying pink eggs are also more likely to reject parasitic eggs, and it is therefore not necessarily easy to obtain unbiased information on parasitizing rates (as hosts may have ejected the parasitic egg before we detected it). We therefore do not yet know for sure whether cuckoos preferentially target blue clutches. This is currently subject of further investigation." Otherwise, I have no further comments on this nice study.

19-May-2021
Dear Professor Deng I am pleased to inform you that your manuscript entitled "Host personality predicts cuckoo egg rejection in Daurian redstarts Phoenicurus auroreus" has been accepted for publication in Proceedings B.
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Re revision MS RSPB-2021-0228
Dear Prof. Dall, Thank you very much for giving us the opportunity to revise our manuscript. We have read the comments carefully and have revised the manuscript as suggested by the associate editor and reviewers. Please find herewith the revised manuscript and our detailed responses to all the comments. Line numbers in the response document refer to the highlighted version of the manuscript. We hope the associate editor and reviewers will find that the manuscript has been significantly improved.

Reply:
We now clarify that variation in egg-rejection behaviour between individuals with different personality types can be adaptive if birds with different personalities also differ in the risk of being parasitized (lines 73-74).
85-86: "Being more successful at earlier lines of host defense may decrease selection on later lines of defense." That statement certainly makes sense at the level of the population, but I am not sure that it translates to a tradeoff within individuals, as suggested by the next sentence. It's not clear to me why bold individuals who are better at driving away parasites would necessarily be worse at discriminating parasitic eggs if they are parasitized, unless learning and experience plays a major role in the ability to discriminate?

Reply:
The idea is that bold individuals may be less efficient at discriminating and rejecting the parasitic eggs, because of the higher success in driving away adult parasites, and therefore the reduced opportunity to learn to reject eggs. We have now clarified this (line 109).
112-117: are blue egg-laying and pink egg-laying hosts equally likely to be parasitized by cuckoos in this population, or is the risk of parasitism different?

Reply:
In our study population, redstarts laying blue eggs suffered higher risk of parasitism than hosts laying pink eggs. However, hosts laying pink eggs are also more likely to reject parasitic eggs, and it is therefore not necessarily easy to obtain unbiased information on parasitizing rates (as hosts may have ejected the parasitic egg before we detected it). We therefore do not yet know for sure whether cuckoos preferentially target blue clutches. This is currently subject of further investigation. [185][186][187][188][189][190][191]: I am a little concerned about model overfitting for a fairly small sample size in a linear mixed model with one random effect and 5 fixed effects. Did you check for overfitting?

Reply:
Thank you for pointing out this potential issue. To assess whether overfitting is a problem, we first ran LMMs to determine which variables explained significant variation in each personality trait. We then reran the repeatability models only including the variables that explained significant variation in each personality trait. For example, only the interval between two tests and the date of the test explained significant variation in exploratory behaviour (see supplementary table S1), and we therefore only included these two variables when calculating repeatability of exploratory behaviour. Because this did not qualitatively affect the repeatability estimates, we kept the original estimates in the main text, but referred to the estimates with the reduced models in the supplement. We have revised all relevant sections of the manuscript (lines 228-234). Note that we are not interested in the significance of the fixed effects, but rather want to control for variables that might obscure the "true" repeatability.
219: Like reviewer 1, I am concerned about using this R2 to select the model with largest explanatory power. These R2 estimates are biased towards models with more parameters, and should not be used in conjunction with an AIC selection approach. You might want to check out Arnold (2010) J. Wildlife Management, which essentially argues that of models within 2 AIC units of the top model, the one with the smallest number of parameters should be the "best fit;" or use model averaging as suggested by Reviewer 1. This should not change your overall conclusions but it might change the estimates of the effect sizes.

Reply:
We greatly appreciate this comment. We now follow the suggestion to always select the most parsimonious model (i.e. the model with the smallest number of parameters) among those with a ΔAICc lower than 2 (see below and lines 255-259).
245-248: just a commenthow great to have high frequencies of both acceptor and rejectors in the population! Great opportunity to ask this question, perhaps worth emphasizing in the introduction.

Reply:
Thank you very much. We have added a sentence in the introduction to emphasize this point (lines 116-118).
Discussion: it is a bit surprising to me that in your models, the personality metrics explain more variation in rejection behavior than egg color does. I would have expected differences between host and egg color to be more salient. Related to this, there is not much explanation of the host egg color polymorphism in the introduction or discussion, which would be helpful since egg color polymorphisms are rare and often interpreted as the result of selection against parasitism. Could you perhaps speculate on this briefly?

Reply:
We are grateful for this excellent suggestion. Because we now adopt a different model selection approach, clutch colour is not in the final model examining return latency anymore. In the other two models, clutch colour is retained, and in these models it actually explains more variation than any of the other variables. We have added a section in the introduction about the role of the egg colour polymorphism in egg-rejection behaviour in Daurian redstarts (lines 119-122). Table 1: also please note in the table caption that these represent best-fit models as chosen by AIC, and list all of the parameters that were included in the full models (if they are not all represented here).

Reviewer(s)' Comments to Author:
Referee: 1 Comments to the Author(s) This is a very nice study showing that personality traits (exploration and neophobia) predict some variation in egg rejection in Daurian redstarts. The finding that bold hosts are more likely to reject parasitic/experimental eggs is interesting and suggests that intra-individual variation in hosts requires further study. The paper is very well written, and conclusions are well-supported. It is a valuable contribution to the field. My main comment is about the statistics. Models with delta AIC<2 are compared using Nagelkerke R^2 values (paragraph beginning line 213). The problem with this is that (virtually) always, the addition of a term to a model will improve % variation predicted by the model. Therefore, comparing nested models based on R^2 will result in the least parsimonious model being chosen. Such full models will likely include variables which cannot be generalised beyond your sample (i.e. they will essentially overfit). I therefore strongly recommend that you compare models differently. Either use a model averaging approach, or select the most parsimonious model from the group of models where deltaic<2, or use likelihood ratio tests to compare nested models. Looking at the supplementary table of all models, all of these options should be reasonable.

Reply:
Thank you very much for the positive appraisal of our study and for the helpful comments. We agree with your point regarding the model selection, and followed your suggestion to always select the most parsimonious model (i.e. the model with the smallest number of parameters) with a ΔAICc lower than 2 (lines 255-259). We have revised all relevant sections of the manuscript accordingly (lines 290-300, table 1).