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Referee comments: Referee 1

Posted by PLOS_ONE_Group on 03 Mar 2008 at 12:02 GMT

Referee 1's review:

1. Are the experiments, statistics, and other analyses performed to a sufficient technical standard? Yes.

2. Are the conclusions presented in an appropriate fashion with speculations and hypotheses identified as such? Yes

3. Have the techniques used been documented in sufficient detail to allow replication? Yes

4. Is the report presented in an intelligible fashion and written in English? Yes

5. Have the exact results reported been published elsewhere? No.

6. Does the research meet all applicable standards with regard to the ethics of human / animal experimentation, participants' consent, and research integrity? Please contact us if you have any concerns. Yes

7. Has the report adhered to the relevant community standards for research conduct, presentation of results, and deposition of data, where appropriate? Yes

General Comments:
This is a novel and exciting study. The results, that environmental contaminants enhance male ornaments while simultaneously reducing immune function is new and of basic and applied interest.

The study was well designed and carried out, and all methods used appropriate. Conclusions are in line with the data.

Minor suggestions for improvement:
p. 3 "many songbirds are THOUGHT to show seasonal development"
this has been definitively shown both within and between subjects so "thought" should be removed.
p. 3 reference 15 is outdated. It would be better to refer to Tramontin and Brenowitz TINS review.
Results. Why were other song nuclei (Area X, RA) or other brain regions (hippocampus) or the telencephalon as a whole not measured?

p. 6, 3rd last line. Ref 14 is about seasonal changes, not development.

p. 7, "suggesting that this region is likely to be one of the most susceptible to the effects of EDCs, as our study has shown". The study has not shown this, since only data on HVC are reported we don't know if the effect is on HVC or on overall brain size.

p. 10 further details on housing would be useful. We don't find out they are housed in trios until 2 pages later. Indoor or outdoor housing?

p. 11 please supply more detail on the haemagglutination test or at least a reference.

p. 12 provide details on the date the birds were killed and brains collected.

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N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication the manuscript has been revised in light of these comments and to address other editorial requirements.