PONE-D-21-04869

No evidence of host-specific egg mimicry in Asian koels Eudynamys scolopaceus

PLOS ONE

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Reviewer #1: Comments on PONE-D-21-04869

Title: No evidence of host-specific egg mimicry in Asian koels Eudynamys scolopaceus

In my opinion, this manuscript was well written and made a good contribution to research area of avian brood parasitism.

Thus I have only very minor comments.

1. title:

I suggest “No evidence of host-specific egg mimicry in Asian koels” as title, as most titles do not necessarily have a Latin name.

2. L80:

delete [Nahid et al submitted,], as [56] is ok.

The same as L125-126.

3. L81:

Delete “and may be the first brood parasite mentioned in ancient literature, as it” and changed to “which was documented…”

4. L93:

Pacific koel; and check throughout your text for this.

5. L94-95:

Please use “Asian koels” throughout your text.

6. L275, 330:

Table 1 and 2 should be in three-line tables.

7. L385-386:

In addition to ref. 104-106, here, Yang et al. (2020) should be cited.

8. L450-453:

Please use “see”, not “see-”

9. L478:

What about “Md.”? Perhaps Md. Yousuf should follow “Monoronjon Baroi”

Reviewer #2: In this study, the authors test whether the Asian koel has evolved host-specific egg mimicry in three common host species. They combined several methods to evaluate mimicry, including measurements of egg size and shape, egg spotting patterns, and egg color and luminance using spectrophotometry and modeling on both UVS and VS avian visual systems. They did find significant differences in Asian koel eggs laid in different host nests, however, these differences were the opposite of what would be expected from host mimicry. Although this paper would be strengthened by genetic data from koel eggs to determine maternity and individual identity, this paper is well written, the methodology is clear, and it expands the knowledge on koel host use. I have a few comments that the authors might consider incorporating into their manuscript:

1. I would expand the paragraph in the introduction about cryptic eggs (line 72) to introduce the reader more on the idea of why it would be advantageous for parasitic eggs to be harder for other parasites to see, and explain competition between female parasites more in-depth.

2. You mention in the general methodology (line 154-155) that measurements of host eggs in volume, shape, spotting pattern, and color from the same nest were either averaged or that nest identity was added as a random intercept. You only mention using the random intercepts later in the paper in the statistical methods. I would recommend keeping it consistent and only using random intercepts, so as not to obscure variation in these characteristics that might occur within the same nest.

3. In the discussion, you suggest that larger koel eggs might be harder for shrikes to reject, which may explain why koels lay larger eggs into the nests of shrikes. Is it possible that this pattern could also be driven by competition between koels? As stated in the paper, they have the highest parasitism rates out of the three hosts and also have high multiple parasitism rates. Perhaps larger eggs are an adaptation for koel nestlings to outcompete other koel nestlings? It might be useful to mention survival of parasitic nestlings in multiply parasitized nests.

Reviewer #3: Overall, this is a straightforward manuscript examining for evidence of host-specific egg mimicry by the brood parasitic Asian koel. The authors undertake what is a rather thorough and perhaps even exhaustive approach to explore any convergence in size, shape, reflectance/luminance or egg patterns. I am convinced that there is little evidence of egg mimicry. The authors have explored multiple possible dimensions of mimicry and to me, there are no compelling patterns.

I do confess that I was a bit perplexed as to why such an exhaustive and extensive analysis was deemed necessary. I don’t mean to be overly critical and I appreciate the carefulness but I’m wondering if there is some ongoing debate or question that required such a thorough evaluation. A number of previous references were listed indicating that Asian koel do exhibit egg mimicry of their host species but several of those references seem to be somewhat general or anecdotal reports so perhaps this is really just the focused quantitative analysis? Is there a reason why this might be a contentious issue? If there is a more compelling rationale, that needs to be developed in the introduction. If not, then perhaps the manuscript might be shortened to get straight to the point – no evidence of mimicry. I was impressed that the author list on this paper included individuals who were cited previously as having suggested egg-mimicry by Asian koels and I applaud their willingness to re-examine some of their earlier conclusions or assumptions.

There are a few small concerns to mention, but none that are not particularly critical. For example, the authors reported that they were unable to obtain spectral reflectance data of Asian koel eggs laid in house crow nests yet this would seem to be an important comparison given that the house crow is one species that the Asian koel was previously reported to mimic (and with a long history of having parasitized). The authors simply state that due to logistic reasons (not described) they were unable to analyze those eggs but I think this is a bit of a missing gap in the analysis. Again, given the lack of any evidence of mimicry in any of the other comparisons (Asian koel to the house crow eggs collected elsewhere, or comparisons of Asian koel eggs to eggs of other hosts or koel eggs in other host nests) I’m not sure that this missing piece is critical. Nice but perhaps not absolutely necessary.

More generally, it does seem that one might (snarkily) suggest that perhaps ‘much ado is being made about nothing’ given that house crows and common mynas are not ejector species and Asian koel do not evict host eggs, further lowering the cost to the host. Accordingly, why would we expect mimicry to have evolved – there would have been little selection for host-specific mimicry to evolve in these species? That long-tailed strikes do seem to reject model (non-cryptic eggs) and are – at least currently – a frequently-used host is more interesting but again little evidence of mimicry was found. The large size of the Asian koel eggs laid in nests of long-tailed strikes (which have the smallest eggs of the 3 host species) may well be an adaptation to reduce eggs rejection by the hosts, as the authors note, although this is still speculation.

The authors raise the possibility (lines 58 to 62) that perhaps “moderate mimicry” might be maintained in generalist parasites such that parasite eggs do not accurately match those of any single host but do so ‘moderately” allowing the parasite at least some opportunities to parasitize a broader variety of hosts. The question for me , then, is – what constitutes ‘moderate’ mimicry? I don’t have an operational sense of that and it seems to be a rather open ended and vague concept. How would one reject that hypothesis?

My only other comment is simply that the discussion seems rather long (almost 6 pages) given that there was no evidence of mimicry. I realize the authors are being thorough in discussing a myriad of possible reasons why mimicry has not evolved (e.g., including speculations about the mafia hypothesis, imperfect adaptation, predation rather than parasitism being a driving selection force, or that parasitic Asian koels may remove other koel eggs, etc.). These are all interesting and valid, but quite speculative; perhaps too much discussion is devoted to possible reasons why egg-mimicry does not exist, but for which there is little evidence. The discussion could be shortened by a couple of pages without expensive speculation.

In sum, I did not find any major problems with this analysis. The statistics seem to be adequate although I would suggest that perhaps not every koel egg be treated as an independent sample but rather be “nested” within each nest. I realize nest identity was included as a random intercept but I was not sure that would account for non-independence of each koel egg. Again, not a critical issue.

Overall, I think this manuscript will be a useful contribution, not only to clarify that this species does not exhibit host-specific egg mimicry – as apparently has been claimed – but perhaps even more so as an example of a thorough effort to explore many dimensions of egg mimicry for an obligate brood parasite and several of its host species.

Reviewer #4: This study explores variations in brood parasitic (Asian koel) eggs laid in three different host species. The aim of the study is to determine whether the Asian koel, to some extent, mimics the eggs of its host species. Based on analyses of egg volume, shape, spotting patterns and colour (in the context of avian color vision), the authors found little evidence on the existence of egg mimicry in Asian koel eggs. Furthermore, some characteristics of koel eggs (e.g., egg volume) appear to mismatch the phenotype of host eggs.

Overall, the manuscript is well written and provides quite a lot of details about the study system. The Methods section is clear and provides a complete overview on the general procedures and analyses. The Introduction section is a bit long and I feel that authors go overboard with speculative arguments to explain the lack of egg mimicry in Asian koel eggs. The evolution of mimicry in host-parasite systems is closely related to the costs imposed by parasitism and host rejection abilities. In my opinion, authors should focus their discussion on this point and reduce (not completely eliminate) speculative explanations (eg., tolerance and critical eggs), unless they have evidence that these mechanisms work in their systems. Some of my concerns have been addressed by the authors in their responses to reviewers' comments made to the previous version.

Other comments:

Line 48: “parasite mimicry”. Please change to “egg mimicry”.

Line 48-51: Although factors such as "climatic variables" may play a relatively important role in certain systems, it should be clearly emphasized that the evolution of egg mimicry is closely related to the evolution of host defenses (i.e., egg rejection).

Line 78: “koel chick tolerates…”. Do the authors have evidence of tolerance mechanisms in the Asian koel?

Line 70-71: While examination by spectrophotometry often reveals important information on egg mimicry, I wouldn't say that great-spotted cuckoo eggs are "quite different” from magpie eggs (in many cases they may be almost indistinguishable to the human eye). Please soften this statement.

Lines 72-76: Any mention to cryptic eggs should be integrated in the paragraph about egg mimicry. I think such a paragraph is a bit out of place in the line of argument of introduction.

Line 387: Indeed, there is experimental evidence that eggs volume is an important factor determining egg rejection (Soler at al., Relationships between egg-recognition and egg-ejection in a grasp-ejector species. 2017. Plos One 12(2)).

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: Yes: John M. Eadie

Reviewer #4: No

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No evidence of host-specific egg mimicry in Asian koels

PONE-D-21-04869R1

Dear Dr. Nahid,

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